What drive units will fit my loudspeakers?
POPE Professional will endeavour to keep supplies of replacement drive units for our loudspeakers for as long as this is possible or viable. In cases where we can no longer supply a replacement, there may be an alternative part which will function to some degree; however, POPE Professional Products does not offer any opinion on the comparative performance of drive units when they are used in loudspeakers other than those for which they were originally intended. While the use of alternative drive units may be an authorized procedure, POPE Professional does not make any recommendations or otherwise as to whether or not the performance of the speaker will be improved as a result. Where a loudspeaker has been directly replaced by a newer model, it is not possible to fully convert an old version to new – drive unit replacement is the only aspect that may be suggested.
We recommend that the replacement of drive units should only ever be carried out by an authorized POPE Professional retailer. Any customer who wishes to replace the drive units in their loudspeakers in this manner, does so at their own risk, accepting all the above conditions.
What is Active?
An Active system refers to a POPE Professional audio system where the loudspeaker’s internal passive crossover is bypassed and an active crossover is used. In a POPE Professional system, these active crossovers take the form of modules which are fitted to the amplifiers internally and allow the signal to be filtered immediately prior to the amplification stage. An Active system is significantly more efficient than a passive system and results in more accurate amplitude, phase, and time response. The flexibility of an Active system also enables the system to be tuned to suit the room. The end result of an upgrade to Active will be, quite simply, a breathtaking improvement in sound quality.
Please note – only an authorized POPE Professional retailer should convert your system to Active - inexpert conversion could result in damage to the amplifiers and/or loudspeakers.
What difference will bi-wiring my loudspeakers make?
Bi-wiring is, quite simply, connecting two sets of speaker cables between the power amplifier and the loudspeaker instead of one – i.e. one set for the woofer and one for the tweeter. Bi-wiring is used for two-way speakers and tri-wiring is used for three-way speakers – one set of cables each for bass, midrange and treble. It is important to remember that not all loudspeakers can be bi-wired/tri-wired. A loudspeaker must have been designed to incorporate this facility. Most, but not all, POPE Professional loudspeakers can be multi-wired – check the Owner's Manual or consult your POPE Professional retailer to see if your loudspeakers include this facility.
When multi-wired, the loudspeaker’s crossover network is divided into electrically separate sections (usually by the removal of jumpers or links on the rear of the loudspeaker). Separate speaker cables are then run from the amplifier to each of the loudspeakers. One of these wires connects to the treble section of the loudspeaker’s crossover, and the other connects to the bass section of the crossover and for three-way loudspeakers, another is run to the midrange.
What should I listen to when evaluating speakers?
The most important thing is to listen to recordings that you *know*. Any good salesman will play you recordings that highlight that particular speaker. Do not be embarrassed about bringing a stack of CDs with you to the hi-fi shop.
Do not spend your valuable listening time switching between a dozen pairs every 3 seconds. If you are shopping at a quality store, the dealer will, from the description of your room, your size requirements, your musical tastes, and your budget, be able to show you a couple of pairs that will be close to what you want. Spend several minutes listening to each. When you think you're close, don't be embarrassed about spending half an hour or more listening to the speakers. You're going to have them in your home for a lot longer, and many speakers will cause "listening fatigue" after a short time. Make sure you really like them before you hand over money. One thing to try is well recorded "Spoken Word" records; most people have a very good ability to tell when a speaking voice sounds unnatural, even if they've never heard the person speaking live. If you play an acoustic instrument, find something that features that instrument solo, or in a small group; make sure it really sounds like it should. Almost everyone has heard a live piano. Piano can be very revealing.
Blues, jazz, folk, or 'easy listening' music with simple instruments and a female vocalist is also revealing. Well done female singing voices provide a very good test of a system's response. Try something simple and soft, which will let you hear any noises coming from the system; and something complex, with lots of instruments all happening at once, to make sure the system doesn't go muddy when things get complicated. And, of course, try a few of your favorites, and see if you like what happens with them.
If a sales person suggests some music to listen to, the odds are that it isn't the most revealing. Sales people tend to suggest things which sound great. Anything you own and like is good, because you know it and are happy to listen to it carefully. No matter how good the recording, if you don't like Opera, you won't listen to it as carefully as your favorite, scratchy, 1940's rhythm and blues.
Most important is to listen to something you are familiar with. Even if a recording is flawed (and what ones aren't?), how is it different from your normal setup? Some of the most important differences are "Gee, I never heard that instrument before!"
What should I listen for when evaluating speakers?
When comparing two speakers side-by-side, doing an AB comparison, be extremely careful to match the levels before evaluating. A slight level difference can make one speaker sound better, even though the difference may not be perceived as a level difference. Some claim that you will be influenced by a difference of less than 1/2 dB!
First and foremost, the sound should be natural. If you listen to vocals, close your eyes and try to picture someone singing in the same room with you. Does it sound realistic? Likewise with instruments. You selected recordings of instruments that you like and have heard live. Do they sound like what you remember them sounding like live?
Your very first impression should be something like "what nice sound". If your initial gut reaction is "gosh, what a lot of detail", the system is likely to be heavy in the treble (often interpreted by beginners as "more detailed") and you'll probably find that annoying after a while. If your first reaction is "hey, what powerful bass", then the system is probably bass-heavy, rather than ideal. The most common mistake for beginners is to buy a system with REALLY powerful bass, because it sounds "impressive" at first. After a while, though, you'll get tired of being thumped on the head by your music.
Not to say that good bass and treble aren't important. But your first realization should be that the music is all there, and that it comes together as good music, without one particular part trying to dominate it. Sit back and listen to it for a bit. You should be able to pick out the individual instruments if you want. They shouldn't force themselves on you, and you should also be able to hear the music as a single piece, the sum of its parts, without feeling like each of the instruments is trying to grab your attention away from the others.
You should check how things sound with the amp turned up, and also with it turned down to a fairly low volume level. Some speakers which sound very nice at low levels begin to sound confused, like they can't cope, when turned up. On the other hand, some sound nice loud, but sound thin and bodiless when you turn them down a bit. With the spoken word or female vocalist, listen for "sibilance", a pronounced 'hiss' at the end of 's' and 'z' sounds. It shouldn't be there. Most planar speakers just can't play very loud. Whatever you hear, do some auditioning at the maximum volume you anticipate ever wanting.
It is acceptable and sometimes desirable to switch the stereo to mono to evaluate naturalness. Mono is a good test of both the room and the speakers. The image should be rock-solid dead center, and not move with signal or level. If it isn't perfect mono, it will be nearly impossible to create a good stereo.
A speaker in a large box is capable of producing low frequencies at higher volumes with more efficiency than a small box, but that doesn't mean that a small box can't have great bass, it just won't be as efficient and can't play as loud.
Good speakers can "recreate a natural stereo sound stage", placing some instruments to the left of the left speaker, some sounds in the middle, and some to the right of the right speaker. Poorer speakers make it harder to localize voices.
Why use a subwoofer? Will it help? One or two?
One reason to get a subwoofer is to add bass to a feeble system. A second reason is to move the lowest frequencies to a separate driver, and thereby reduce a particular kind of distortion caused by the nonlinear mixing of different sounds, called "intermodulation distortion". A third is to increase the power handling ability of the system and the overall reliability. All are valid reasons, but it isn't so simple.
To improve the sound of a good speaker system, a subwoofer must "integrate smoothly" into the system, extending the bass without causing peaks or dips. Many subwoofers have a crossover that goes between your amp and your main speaker which sends the lows to the subwoofer and sends the higher frequency signals to the main speakers. This may damage the perfect sound of a good system, it may sound similar, or it may sound better.
Most good small speaker systems have a bass peak at resonance, which attempts to compensate for the absence of lower bass. Like it or not, this is the only way to make a small system sound realistic. If the small system is done well, the improvement you will get from a subwoofer will be small, but still real and, to many, significant.
Correctly done, a good subwoofer will enhance the sound of a good small-box system. Done wrong or haphazardly, anything is possible. Even a fine large speaker system might benefit from careful addition of a subwoofer. However, the better the original system, the more likely it will be that a modest subwoofer will do more harm than good.
Low frequencies travel less directionally than high frequencies, so many people say that only one subwoofer is required for good sound. This is true to some extent, but not completely true.
There are a few reasons for getting two subwoofers. Some feel that you need two subwoofers to accurately reproduce the stereo image, no matter how little low-frequency stereo information there is. Others feel that two subwoofers are much easier to set up in a room, less likely to excite standing waves in the room, and give smoother sound.
A third reason is that two subwoofers can produce twice the sound of one. Finally, even though subwoofers produce very low frequency sound and very low frequency sound is non-directional, subwoofers also have output at 100 Hz, and sound at 100 Hz is directional, so two subwoofers will give a slightly better stereo image than one. Assuming, of course, that the two are separated by at least two feet.
Finally, even though original source signals rarely contain any music with stereo components below 50Hz, there may be some noise component with low-frequency out-of-phase noise. This unusual noise might add a sense of space to a recording if it is reproduced by a system in which the woofers are very far apart.
It is still true that a single good subwoofer, correctly added to a system will help the sound but two will probably help more.
How do you connect a subwoofer to a stereo?
Many subwoofers contain their own amplifier and crossover. For these, take the preamp output and feed it into the subwoofer amp input and also into the main amplifier.
For other subwoofers, just run them in parallel with your main speakers, or combine them into your system with your own bass amplifier and crossover.
Some A/V receivers contain a splitter specifically for use with subwoofers. If you have one of these, you will either want a separate amplifier for your subwoofer or an amplified subwoofer.
Consult the manual which comes with the subwoofer.
What do I need for surround sound?
"Surround Sound" has referred to a number of different products over the years. Many mass-fi receivers have "Surround Sound" buttons that do little more than muck up the imaging.
In recent years the term "Surround Sound" has become synonymous with the surround systems produced by Dolby Laboratories. Dolby Surround comes in several flavors, such as passive surround (which simply decodes the phase information and sends it to the rear speakers) and the more advanced system called Pro Logic. Pro Logic system uses computer circuitry to route directional information to the appropriate speakers.
How can I improve the sound of my speakers?
The best way to change the sound of your speakers is to change where you put them. Ideally, the speakers should be located at ear level, in front of you, squared off between you. It's then a matter of fiddling with a) the angles, b) the distance apart, c) the distance from you, and d) the distance from the wall. Just moving the speakers around in the room or putting them onto stands can make a major difference.
Other than that, speaker modifications can be a can of worms, or can produce very subtle changes, which you might prefer. For example, you might improve a speaker by adding some cross braces of 1"x1" wood from left to right and from front to back. This will stiffen the cabinet and reduce speaker cabinet wall vibrations, which probably hurt sound quality. Alas, this will be most effective with lower-cost and poorly built speakers.
Along similar lines, some claim success putting lead wire or epoxy putty on thin parts of the speaker to damp out resonances. You can try doing this to the thinner parts of the speaker "basket" or frame, or to the front "baffle" or supporting panel.
Still another "tweak" is to add sound deadening felt pads to the inside walls of the speaker. Instead of felt pads some advocate sand-filled latex coatings on the inside walls of speakers. Others advocate ceramic tiles held in place with "thinset". Still others rave about commercial products like AC Glop, Acoustic Magic, and Bostik Sheet. However, the people who rave about these products tend to be the same people who sell them.
Any change along the lines of adding felt, cross-bracing, or putty will have subtle effects on the sound.
For the brave at heart, you can replace old or cheap drivers with better ones, but the results of this one change can be very dissatisfying if you happen to get the wrong type of driver for that application, and may never sound right, even if you use a similar driver. Speaker system design is still somewhat of a science and somewhat of an art. Throwing paint on a canvas often makes a mess.
Whatever change you try, don't "burn your bridge" home. Be sure that you can undo whatever change you did, just in case. Many tweaks to good speakers, no matter how well thought through, will correct for one flaw, but create others, or correct a flaw that the designer had cleverly used to his advantage.
POPE Professional will endeavour to keep supplies of replacement drive units for our loudspeakers for as long as this is possible or viable. In cases where we can no longer supply a replacement, there may be an alternative part which will function to some degree; however, POPE Professional Products does not offer any opinion on the comparative performance of drive units when they are used in loudspeakers other than those for which they were originally intended. While the use of alternative drive units may be an authorized procedure, POPE Professional does not make any recommendations or otherwise as to whether or not the performance of the speaker will be improved as a result. Where a loudspeaker has been directly replaced by a newer model, it is not possible to fully convert an old version to new – drive unit replacement is the only aspect that may be suggested.
We recommend that the replacement of drive units should only ever be carried out by an authorized POPE Professional retailer. Any customer who wishes to replace the drive units in their loudspeakers in this manner, does so at their own risk, accepting all the above conditions.
What is Active?
An Active system refers to a POPE Professional audio system where the loudspeaker’s internal passive crossover is bypassed and an active crossover is used. In a POPE Professional system, these active crossovers take the form of modules which are fitted to the amplifiers internally and allow the signal to be filtered immediately prior to the amplification stage. An Active system is significantly more efficient than a passive system and results in more accurate amplitude, phase, and time response. The flexibility of an Active system also enables the system to be tuned to suit the room. The end result of an upgrade to Active will be, quite simply, a breathtaking improvement in sound quality.
Please note – only an authorized POPE Professional retailer should convert your system to Active - inexpert conversion could result in damage to the amplifiers and/or loudspeakers.
What difference will bi-wiring my loudspeakers make?
Bi-wiring is, quite simply, connecting two sets of speaker cables between the power amplifier and the loudspeaker instead of one – i.e. one set for the woofer and one for the tweeter. Bi-wiring is used for two-way speakers and tri-wiring is used for three-way speakers – one set of cables each for bass, midrange and treble. It is important to remember that not all loudspeakers can be bi-wired/tri-wired. A loudspeaker must have been designed to incorporate this facility. Most, but not all, POPE Professional loudspeakers can be multi-wired – check the Owner's Manual or consult your POPE Professional retailer to see if your loudspeakers include this facility.
When multi-wired, the loudspeaker’s crossover network is divided into electrically separate sections (usually by the removal of jumpers or links on the rear of the loudspeaker). Separate speaker cables are then run from the amplifier to each of the loudspeakers. One of these wires connects to the treble section of the loudspeaker’s crossover, and the other connects to the bass section of the crossover and for three-way loudspeakers, another is run to the midrange.
What should I listen to when evaluating speakers?
The most important thing is to listen to recordings that you *know*. Any good salesman will play you recordings that highlight that particular speaker. Do not be embarrassed about bringing a stack of CDs with you to the hi-fi shop.
Do not spend your valuable listening time switching between a dozen pairs every 3 seconds. If you are shopping at a quality store, the dealer will, from the description of your room, your size requirements, your musical tastes, and your budget, be able to show you a couple of pairs that will be close to what you want. Spend several minutes listening to each. When you think you're close, don't be embarrassed about spending half an hour or more listening to the speakers. You're going to have them in your home for a lot longer, and many speakers will cause "listening fatigue" after a short time. Make sure you really like them before you hand over money. One thing to try is well recorded "Spoken Word" records; most people have a very good ability to tell when a speaking voice sounds unnatural, even if they've never heard the person speaking live. If you play an acoustic instrument, find something that features that instrument solo, or in a small group; make sure it really sounds like it should. Almost everyone has heard a live piano. Piano can be very revealing.
Blues, jazz, folk, or 'easy listening' music with simple instruments and a female vocalist is also revealing. Well done female singing voices provide a very good test of a system's response. Try something simple and soft, which will let you hear any noises coming from the system; and something complex, with lots of instruments all happening at once, to make sure the system doesn't go muddy when things get complicated. And, of course, try a few of your favorites, and see if you like what happens with them.
If a sales person suggests some music to listen to, the odds are that it isn't the most revealing. Sales people tend to suggest things which sound great. Anything you own and like is good, because you know it and are happy to listen to it carefully. No matter how good the recording, if you don't like Opera, you won't listen to it as carefully as your favorite, scratchy, 1940's rhythm and blues.
Most important is to listen to something you are familiar with. Even if a recording is flawed (and what ones aren't?), how is it different from your normal setup? Some of the most important differences are "Gee, I never heard that instrument before!"
What should I listen for when evaluating speakers?
When comparing two speakers side-by-side, doing an AB comparison, be extremely careful to match the levels before evaluating. A slight level difference can make one speaker sound better, even though the difference may not be perceived as a level difference. Some claim that you will be influenced by a difference of less than 1/2 dB!
First and foremost, the sound should be natural. If you listen to vocals, close your eyes and try to picture someone singing in the same room with you. Does it sound realistic? Likewise with instruments. You selected recordings of instruments that you like and have heard live. Do they sound like what you remember them sounding like live?
Your very first impression should be something like "what nice sound". If your initial gut reaction is "gosh, what a lot of detail", the system is likely to be heavy in the treble (often interpreted by beginners as "more detailed") and you'll probably find that annoying after a while. If your first reaction is "hey, what powerful bass", then the system is probably bass-heavy, rather than ideal. The most common mistake for beginners is to buy a system with REALLY powerful bass, because it sounds "impressive" at first. After a while, though, you'll get tired of being thumped on the head by your music.
Not to say that good bass and treble aren't important. But your first realization should be that the music is all there, and that it comes together as good music, without one particular part trying to dominate it. Sit back and listen to it for a bit. You should be able to pick out the individual instruments if you want. They shouldn't force themselves on you, and you should also be able to hear the music as a single piece, the sum of its parts, without feeling like each of the instruments is trying to grab your attention away from the others.
You should check how things sound with the amp turned up, and also with it turned down to a fairly low volume level. Some speakers which sound very nice at low levels begin to sound confused, like they can't cope, when turned up. On the other hand, some sound nice loud, but sound thin and bodiless when you turn them down a bit. With the spoken word or female vocalist, listen for "sibilance", a pronounced 'hiss' at the end of 's' and 'z' sounds. It shouldn't be there. Most planar speakers just can't play very loud. Whatever you hear, do some auditioning at the maximum volume you anticipate ever wanting.
It is acceptable and sometimes desirable to switch the stereo to mono to evaluate naturalness. Mono is a good test of both the room and the speakers. The image should be rock-solid dead center, and not move with signal or level. If it isn't perfect mono, it will be nearly impossible to create a good stereo.
A speaker in a large box is capable of producing low frequencies at higher volumes with more efficiency than a small box, but that doesn't mean that a small box can't have great bass, it just won't be as efficient and can't play as loud.
Good speakers can "recreate a natural stereo sound stage", placing some instruments to the left of the left speaker, some sounds in the middle, and some to the right of the right speaker. Poorer speakers make it harder to localize voices.
Why use a subwoofer? Will it help? One or two?
One reason to get a subwoofer is to add bass to a feeble system. A second reason is to move the lowest frequencies to a separate driver, and thereby reduce a particular kind of distortion caused by the nonlinear mixing of different sounds, called "intermodulation distortion". A third is to increase the power handling ability of the system and the overall reliability. All are valid reasons, but it isn't so simple.
To improve the sound of a good speaker system, a subwoofer must "integrate smoothly" into the system, extending the bass without causing peaks or dips. Many subwoofers have a crossover that goes between your amp and your main speaker which sends the lows to the subwoofer and sends the higher frequency signals to the main speakers. This may damage the perfect sound of a good system, it may sound similar, or it may sound better.
Most good small speaker systems have a bass peak at resonance, which attempts to compensate for the absence of lower bass. Like it or not, this is the only way to make a small system sound realistic. If the small system is done well, the improvement you will get from a subwoofer will be small, but still real and, to many, significant.
Correctly done, a good subwoofer will enhance the sound of a good small-box system. Done wrong or haphazardly, anything is possible. Even a fine large speaker system might benefit from careful addition of a subwoofer. However, the better the original system, the more likely it will be that a modest subwoofer will do more harm than good.
Low frequencies travel less directionally than high frequencies, so many people say that only one subwoofer is required for good sound. This is true to some extent, but not completely true.
There are a few reasons for getting two subwoofers. Some feel that you need two subwoofers to accurately reproduce the stereo image, no matter how little low-frequency stereo information there is. Others feel that two subwoofers are much easier to set up in a room, less likely to excite standing waves in the room, and give smoother sound.
A third reason is that two subwoofers can produce twice the sound of one. Finally, even though subwoofers produce very low frequency sound and very low frequency sound is non-directional, subwoofers also have output at 100 Hz, and sound at 100 Hz is directional, so two subwoofers will give a slightly better stereo image than one. Assuming, of course, that the two are separated by at least two feet.
Finally, even though original source signals rarely contain any music with stereo components below 50Hz, there may be some noise component with low-frequency out-of-phase noise. This unusual noise might add a sense of space to a recording if it is reproduced by a system in which the woofers are very far apart.
It is still true that a single good subwoofer, correctly added to a system will help the sound but two will probably help more.
How do you connect a subwoofer to a stereo?
Many subwoofers contain their own amplifier and crossover. For these, take the preamp output and feed it into the subwoofer amp input and also into the main amplifier.
For other subwoofers, just run them in parallel with your main speakers, or combine them into your system with your own bass amplifier and crossover.
Some A/V receivers contain a splitter specifically for use with subwoofers. If you have one of these, you will either want a separate amplifier for your subwoofer or an amplified subwoofer.
Consult the manual which comes with the subwoofer.
What do I need for surround sound?
"Surround Sound" has referred to a number of different products over the years. Many mass-fi receivers have "Surround Sound" buttons that do little more than muck up the imaging.
In recent years the term "Surround Sound" has become synonymous with the surround systems produced by Dolby Laboratories. Dolby Surround comes in several flavors, such as passive surround (which simply decodes the phase information and sends it to the rear speakers) and the more advanced system called Pro Logic. Pro Logic system uses computer circuitry to route directional information to the appropriate speakers.
How can I improve the sound of my speakers?
The best way to change the sound of your speakers is to change where you put them. Ideally, the speakers should be located at ear level, in front of you, squared off between you. It's then a matter of fiddling with a) the angles, b) the distance apart, c) the distance from you, and d) the distance from the wall. Just moving the speakers around in the room or putting them onto stands can make a major difference.
Other than that, speaker modifications can be a can of worms, or can produce very subtle changes, which you might prefer. For example, you might improve a speaker by adding some cross braces of 1"x1" wood from left to right and from front to back. This will stiffen the cabinet and reduce speaker cabinet wall vibrations, which probably hurt sound quality. Alas, this will be most effective with lower-cost and poorly built speakers.
Along similar lines, some claim success putting lead wire or epoxy putty on thin parts of the speaker to damp out resonances. You can try doing this to the thinner parts of the speaker "basket" or frame, or to the front "baffle" or supporting panel.
Still another "tweak" is to add sound deadening felt pads to the inside walls of the speaker. Instead of felt pads some advocate sand-filled latex coatings on the inside walls of speakers. Others advocate ceramic tiles held in place with "thinset". Still others rave about commercial products like AC Glop, Acoustic Magic, and Bostik Sheet. However, the people who rave about these products tend to be the same people who sell them.
Any change along the lines of adding felt, cross-bracing, or putty will have subtle effects on the sound.
For the brave at heart, you can replace old or cheap drivers with better ones, but the results of this one change can be very dissatisfying if you happen to get the wrong type of driver for that application, and may never sound right, even if you use a similar driver. Speaker system design is still somewhat of a science and somewhat of an art. Throwing paint on a canvas often makes a mess.
Whatever change you try, don't "burn your bridge" home. Be sure that you can undo whatever change you did, just in case. Many tweaks to good speakers, no matter how well thought through, will correct for one flaw, but create others, or correct a flaw that the designer had cleverly used to his advantage.
What are fluid-filled (fluid-cooled, ferro-fluid) tweeters?
These tweeters are built almost exactly the same as other tweeters. They look and act almost exactly the same, too. The only difference is that they have a small, controlled amount of a special fluid inserted into the gap between the magnet and the voice coil.
One big effect of adding this fluid to a tweeter (or to any speaker) is that it makes the voice coil capable of dissipating more heat. This means that the speaker can have a lighter voice coil, for better performance, or a higher power rating for the same voice coil. The other big effect of this fluid is to add mechanical damping. The frequency response and transient response of the driver will change, possibly for the better.
In addition, this fluid may help center the voice coil, may lubricate the voice coil, and may help keep dirt out of the gap. This fluid will not increase the magnetic field, concentrate the magnetic field or otherwise change the magnetic circuit. Nor will it cushion impact if the voice coil bottoms.
The fluid used for this purpose is often called "ferrofluid". It consists of sub-microscopic particles of magnetic material suspended in special oil. This fluid stays in the gap because of the strong magnetic pull of the magnet. There is some debate over whether these fluids can dry out with time. Manufacturers claim that the oil used is non-volatile.
It is possible to use ferrofluids in mid-range drivers and woofers. However, as tweeters tend to have the most fragile voice coils, tweeters have the most to gain from ferrofluid. There are various different fluids on the market, some of which have characteristics tailored to tweeters, some to woofers, etc.
It is very risky to blindly add fluid to a driver. It may not be compatible with the adhesives used in the driver, may not be practical with the particular driver layout, and is impossible to remove. Permanent driver damage is possible.
Should I use spikes under my speakers? Pennies under the spikes?
| Spikes prevent speakers from rocking. They also couple the speaker directly to the floor. Spikes will pierce carpet. Some spikes will damage carpet. Most will just put a small hole in the carpet which is invisible. Putting a heavy speaker directly on carpet will cause a permanent mark on the carpet. Spikes can prevent this.
If you have a pretty hardwood floor, then spikes will definitely damage the finish. A rigid disc under the spike will distribute the load and lessen the damage. Any coin should work fine. Using a coin will not change the speaker/floor interaction. Do not use a coin with a carpeted floor. Alternatives to spikes for wood floors are Blu-Tack and similar products.
If your floor is extremely rigid, then the spikes will make the speaker more rigid. If the floor is more conventional, such as a suspended floor or a wooden floor over joists, spikes can have a positive or negative effect, depending on the resonant characteristics of the floor/speaker system.
The counterforce resulting from a forward cone motion in a speaker may try to move the speaker backwards, but spikes will have little or no effect on this. Most audible effects from spikes are due to coupling the speaker to the floor, so it will be less likely to resonate on its stand. Some argue that in most cases, spikes will have no audible effect at all. Try it for yourself.
How do you couple speakers to speaker stands?
Ideally, your speakers should sit flat on the speaker stand or floor. They shouldn't see-saw back and forth if nudged.
One good way to accomplish this is to use a small dab of putty under each corner of the speaker. There are a few common putties used for this, but all share the properties of being very elastic and staying flexible indefinitely. These putties are inexpensive, removable, and reusable.
| Try either Blu-Tak, which is available in the UK from office supply stores for cleaning typewriter elements, Faber Castell UHU Hold-It, which is available in the US from office supply stores for holding up pictures, and DAP's Fun-Tak, which is sold in hardware stores for holding up pictures, or Pritt Buddies..
These tweeters are built almost exactly the same as other tweeters. They look and act almost exactly the same, too. The only difference is that they have a small, controlled amount of a special fluid inserted into the gap between the magnet and the voice coil.
One big effect of adding this fluid to a tweeter (or to any speaker) is that it makes the voice coil capable of dissipating more heat. This means that the speaker can have a lighter voice coil, for better performance, or a higher power rating for the same voice coil. The other big effect of this fluid is to add mechanical damping. The frequency response and transient response of the driver will change, possibly for the better.
In addition, this fluid may help center the voice coil, may lubricate the voice coil, and may help keep dirt out of the gap. This fluid will not increase the magnetic field, concentrate the magnetic field or otherwise change the magnetic circuit. Nor will it cushion impact if the voice coil bottoms.
The fluid used for this purpose is often called "ferrofluid". It consists of sub-microscopic particles of magnetic material suspended in special oil. This fluid stays in the gap because of the strong magnetic pull of the magnet. There is some debate over whether these fluids can dry out with time. Manufacturers claim that the oil used is non-volatile.
It is possible to use ferrofluids in mid-range drivers and woofers. However, as tweeters tend to have the most fragile voice coils, tweeters have the most to gain from ferrofluid. There are various different fluids on the market, some of which have characteristics tailored to tweeters, some to woofers, etc.
It is very risky to blindly add fluid to a driver. It may not be compatible with the adhesives used in the driver, may not be practical with the particular driver layout, and is impossible to remove. Permanent driver damage is possible.
Should I use spikes under my speakers? Pennies under the spikes?
| Spikes prevent speakers from rocking. They also couple the speaker directly to the floor. Spikes will pierce carpet. Some spikes will damage carpet. Most will just put a small hole in the carpet which is invisible. Putting a heavy speaker directly on carpet will cause a permanent mark on the carpet. Spikes can prevent this.
If you have a pretty hardwood floor, then spikes will definitely damage the finish. A rigid disc under the spike will distribute the load and lessen the damage. Any coin should work fine. Using a coin will not change the speaker/floor interaction. Do not use a coin with a carpeted floor. Alternatives to spikes for wood floors are Blu-Tack and similar products.
If your floor is extremely rigid, then the spikes will make the speaker more rigid. If the floor is more conventional, such as a suspended floor or a wooden floor over joists, spikes can have a positive or negative effect, depending on the resonant characteristics of the floor/speaker system.
The counterforce resulting from a forward cone motion in a speaker may try to move the speaker backwards, but spikes will have little or no effect on this. Most audible effects from spikes are due to coupling the speaker to the floor, so it will be less likely to resonate on its stand. Some argue that in most cases, spikes will have no audible effect at all. Try it for yourself.
How do you couple speakers to speaker stands?
Ideally, your speakers should sit flat on the speaker stand or floor. They shouldn't see-saw back and forth if nudged.
One good way to accomplish this is to use a small dab of putty under each corner of the speaker. There are a few common putties used for this, but all share the properties of being very elastic and staying flexible indefinitely. These putties are inexpensive, removable, and reusable.
| Try either Blu-Tak, which is available in the UK from office supply stores for cleaning typewriter elements, Faber Castell UHU Hold-It, which is available in the US from office supply stores for holding up pictures, and DAP's Fun-Tak, which is sold in hardware stores for holding up pictures, or Pritt Buddies..
What is a Sealed, Ported, Bass Reflex, Acoustic Suspension, Bandpass, and Coupled Cavity Speaker? Which is better?
All are "direct radiator" enclosures, so called because the sound is produced directly from the driver (the "radiator") without the assistance of a contrivance such as a horn.
SEALED BOX:
The simplest direct-radiator system. The rear of the driver sees a sealed enclosure, and none of the rear output of the driver contributes to the sound output. Depending upon how stiff the mechanical suspension is vs how stiff the enclosed air in the enclosure is (and that's a function of the size of the box), you can have either an Infinite Baffle enclosure, in which the mechanical suspension is the dominant source of system stiffness and the box is large; or an Acoustic Suspension enclosures, where the air in the box is the dominating stiffness, and the box is small.
Sealed boxes tend to be the lowest efficiency systems for a given box size and bass cutoff frequency.
VENTED ENCLOSURES:
Also the same as Bass Reflex, Ported, or Passive Radiator. Here, an aperture in the box provides a means for the rear output of the cone to contribute to the total output of the system. However, it only contributes over a very narrow range of frequencies. In fact, in a properly designed system, the front output of the cone is reduced at the same time the output of port increases, so the port DOES NOT ADD to the output of the woofer, it REPLACES the output of the woofer at these frequencies. This, if done properly, can significantly reduce distortion and increase power handling at very low frequencies, a region that can be difficult for drivers.
Vented systems can be up to 3 dB more efficient than a sealed box system that has the same bass cutoff frequency and size.
BANDPASS:
These are compound systems in that they have at least two enclosures: one on the front and one on the rear of the driver. The enclosure on the front, which looks remarkably like a vented box (because it is), acts as a low pass filter, and, can couple the output of the woofer more efficiently to the outside. They have several useful advantages. For example, the front enclosure can be used as a very effective acoustic crossover, filtering out mechanical noises generated by the woofer, something no electronic crossover can do. For very low frequencies, such an acoustic crossover can be far less expensive and more easily designed than an equivalent electronic crossover.
They are called "bandpass" because the combination of the rear enclosure and the driver form the high pass portion while the front enclosure forms the low pass section. Making the bandwidth of the system narrower raises the efficiency of the system.
COUPLED CAVITY:
A variation of bandpass and vented systems, they are the results of a designers attempt to solve specific problems. They consist of two or more rear enclosures, each coupled to the next by a vent. Each enclosure/vent combination is another resonant system, and the combination is, essentially, a high order, multi-tuned resonant system.
Generally, these systems have quite complex response and are difficult to design. No comprehensive theory on their operation exists like that for sealed, vented and bandpass systems.
What is the best material to make speaker boxes out of? Why?
An ideal speaker cabinet material would be very stiff, so that it would not tend to move with variations in box air pressure. It would also be very well damped, so that if it ever does deflect from air pressure, it will come back to the original position without resonating. It would also have a very high resonant frequency (supersonic), so that low frequency box air pressure would not cause it to resonate. An attractive material is preferred, and additional credit is given for a material which is easy to cut, glue, and finish. A great material would be cheap, too. Finally, it would be nice if the material were light, because we all have to move our speakers sometimes, and it's hard to appreciate good speakers with a sore back.
With all of those attributes, it would seem that no material is perfect. However, there are many materials that have enough of the above good attributes to make excellent speaker cabinets. Yet each has advantages and disadvantages.
In the list of good speaker box materials below, letters are used to indicate which attributes the material possesses.
S = Stiff
D = Damped
H = High Resonance
A = Attractive
M = Machinable
C = Cheap
L = Light
All are "direct radiator" enclosures, so called because the sound is produced directly from the driver (the "radiator") without the assistance of a contrivance such as a horn.
SEALED BOX:
The simplest direct-radiator system. The rear of the driver sees a sealed enclosure, and none of the rear output of the driver contributes to the sound output. Depending upon how stiff the mechanical suspension is vs how stiff the enclosed air in the enclosure is (and that's a function of the size of the box), you can have either an Infinite Baffle enclosure, in which the mechanical suspension is the dominant source of system stiffness and the box is large; or an Acoustic Suspension enclosures, where the air in the box is the dominating stiffness, and the box is small.
Sealed boxes tend to be the lowest efficiency systems for a given box size and bass cutoff frequency.
VENTED ENCLOSURES:
Also the same as Bass Reflex, Ported, or Passive Radiator. Here, an aperture in the box provides a means for the rear output of the cone to contribute to the total output of the system. However, it only contributes over a very narrow range of frequencies. In fact, in a properly designed system, the front output of the cone is reduced at the same time the output of port increases, so the port DOES NOT ADD to the output of the woofer, it REPLACES the output of the woofer at these frequencies. This, if done properly, can significantly reduce distortion and increase power handling at very low frequencies, a region that can be difficult for drivers.
Vented systems can be up to 3 dB more efficient than a sealed box system that has the same bass cutoff frequency and size.
BANDPASS:
These are compound systems in that they have at least two enclosures: one on the front and one on the rear of the driver. The enclosure on the front, which looks remarkably like a vented box (because it is), acts as a low pass filter, and, can couple the output of the woofer more efficiently to the outside. They have several useful advantages. For example, the front enclosure can be used as a very effective acoustic crossover, filtering out mechanical noises generated by the woofer, something no electronic crossover can do. For very low frequencies, such an acoustic crossover can be far less expensive and more easily designed than an equivalent electronic crossover.
They are called "bandpass" because the combination of the rear enclosure and the driver form the high pass portion while the front enclosure forms the low pass section. Making the bandwidth of the system narrower raises the efficiency of the system.
COUPLED CAVITY:
A variation of bandpass and vented systems, they are the results of a designers attempt to solve specific problems. They consist of two or more rear enclosures, each coupled to the next by a vent. Each enclosure/vent combination is another resonant system, and the combination is, essentially, a high order, multi-tuned resonant system.
Generally, these systems have quite complex response and are difficult to design. No comprehensive theory on their operation exists like that for sealed, vented and bandpass systems.
What is the best material to make speaker boxes out of? Why?
An ideal speaker cabinet material would be very stiff, so that it would not tend to move with variations in box air pressure. It would also be very well damped, so that if it ever does deflect from air pressure, it will come back to the original position without resonating. It would also have a very high resonant frequency (supersonic), so that low frequency box air pressure would not cause it to resonate. An attractive material is preferred, and additional credit is given for a material which is easy to cut, glue, and finish. A great material would be cheap, too. Finally, it would be nice if the material were light, because we all have to move our speakers sometimes, and it's hard to appreciate good speakers with a sore back.
With all of those attributes, it would seem that no material is perfect. However, there are many materials that have enough of the above good attributes to make excellent speaker cabinets. Yet each has advantages and disadvantages.
In the list of good speaker box materials below, letters are used to indicate which attributes the material possesses.
S = Stiff
D = Damped
H = High Resonance
A = Attractive
M = Machinable
C = Cheap
L = Light
MEDIUM DENSITY FIBERBOARD (MDF): SDMC This is the most practical material for quality speakers. It is harder to find than plywood, but most lumber yards can special order it. It cuts very nicely and has a smooth surface. It takes veneer very well. However, bring a helper when you pick the stuff up. One sheet is very heavy. MDF is harder on tools than common wood, but easier than particle board. This is the material that many great speaker makers use. US $45 for a 4'x8'x1" sheet. Density: 50 lbs/cu ft.
POLYCARBONATE (LEXAN): DML A clear or solid-color polycarbonate box can look strikingly good. However, this is not a cheap material. To locate it, look in the classified directory under PLASTICS. US $60 for a 1'x1'x1" sheet. Density: 75 lbs/cu ft. Acrylic (Plexiglass) is cheaper than Polycarbonate, but weaker and poorer damped (not recommended).
CORIAN, FOUNTAINHEAD, AVONITE, SURELL, GIBRALTAR: SDA Regardless of the brand, these synthetic countertop materials come in a wide array of colors and look beautiful. They are hard to buy, and harder to work. They take special glue to bond and require wet sanding with very fine paper to finish. You can tap it, but it's too brittle for wood screws. Corian is acrylic mixed with powdered aluminum triwwwte clay filler. Avonite, Gibraltar, and Surell are polyester resin mixed with the same clay filler. US $26 per 1'x1'x0.5". Density: 100 lbs/cu ft.
MARBLE: SDHA One challenge with marble speaker enclosures is cutting holes for the drivers. A carbide bit on a router will work, but it will dull quickly. Marble is also difficult to glue, so bracing is difficult. But it sure is pretty when you're done! US $25 to $45 per 1'x1'x1.25". Density: 160 lbs/cu ft.
PLYWOOD SHEETS SPACED AND FILLED WITH SAND OR LEAD SHOT: SDAMC If you have time on your hands and want a great impractical box, try this. Make a simple box out of common plywood. Then glue cleats on the outside of the box to space the outside plywood from the common plywood. Glue hardwood-veneered plywood to the cleats and pour sand or lead shot into the spaces between the cleats. It won't be light, but with the filler, it will be extremely well damped. In addition, if you use strong cleats and glue well, the box will be extremely stiff. One person used different size Sonotubes as an alternative to plywood, and filled the space between them with sand. Be sure to sterilize the sand in your oven before putting it in the box.
ALUMINUM SHEETS SPACED AND FILLED WITH ALUMINUM HONEYCOMB (Aerolam): SDHL Airplanes use this material for flooring. Next time a plane crashes in your neighborhood, see if you can get the wreckage for your next speaker project. You can't get a better, light-weight material. Celestion has exploited this for some great products. If you're really ambitious, you can make your own sandwich out of high-quality plywood faces and a thick honeycomb core. You will probably need an epoxy to glue the honeycomb to the plywood. A home-brew sandwich is easier to cut and glue than Aerolam.
FORMED CONCRETE: SDHC There are tricks to working concrete, such as to cast braces, rebar, and steel-wire right into the mix. Also, some concrete is better damped than other. Remember to oil your concrete forms so that they can be removed. Most concrete speakers use an MDF front panel, but you can pour one if you use cardboard tubes or plywood rings to mold the concrete into the shape of a speaker cutout. Alternately, you can make a common veneered plywood speaker box and cast concrete inside it for stiffening.
Any box can be improved by making the walls thicker, by bracing the walls, and by stiffening the walls. The stiffness of a material goes up as the cube of the thickness, so a slightly thicker material is much stiffer. A thicker panel will also have a higher resonant frequency because the stiffness goes up faster than the mass.
Consider lining the inside of your speaker with ceramic tile, attached with thinset mortar. You can get tile remnants cheaply. They are easy to apply and can be added as an afterthought to an imperfect box. However, be sure to attach all braces before tiling, because it is hard to attach anything to tile.
Also consider bracing any weak parts of the box. For example, all joints will benefit from a wooden cleat. The back of the box will benefit from stiffeners where the speaker terminals are attached. Most importantly, brace the front panel, or make it out of a double thickness of material.
What is the best "stuff" to fill a speaker cabinet with?
The following discussion will focus on practical facts on speaker cabinet stuffing and on sealed systems. Theory is limited help in selecting speaker stuffing. Vented system do share a few of these same issues and will also be mentioned, but the goals and physics of stuffing a vented box are different than those of a sealed box.
NHT speakers use polyester fill. Some use a Danish polyester that mimics the properties of fiberglas very closely. Excluding this special poly, there are two kinds of polyester available: pillow stuffing, and audio-spec polyester.
Forget pillow fill. It's cheap and easy to get. If you use enough, it will damp the midrange, and that's a lot better than an empty box. But it has little effect on lower frequencies. Some fabric stores sell "Super Good Stuff" from Stearns Technical Textiles. This is a common, inexpensive material that is said to perform as well as audio-spec polyester.
For lining the walls of a vented enclosure to reduce internal reflections, or filling a transmission line to absorb the back wave, highly absorptive wool or fiberglas are ideal. However, these materials do not provide the desired results in a sealed system. They will provide more reflection absorption than polyester, but the latter is quite good in this regard in the critical midrange. In a sealed system you don't want absorption at lower frequencies anyway; you want damping and isothermal conversion. (Author's note: I have tried "all-out" efforts using fiberglas lining and polyester fill to achieve the best of both worlds. I found little practical benefit over polyester alone.)
Most professional designers agree that practical experience, combined with trial and error is the best way to get optimum stuffing material, quantity, and method for a given design. This is why good designers routinely experiment with fill in the development of a new system. If you are designing a system that differs substantially in shape or volume or source impedance (passive crossover) from one of known reference, you will need to experiment to get best performance.
Adjusting the filling is the last step in getting bass right, and is used mostly to fine-tune the system Qtc and resonance. As increasing amounts of polyester are added to a sealed box, the resonance and Q gradually go down. This can be shown mathematically to be due in roughly equal parts to the effects of simple resistive damping and isothermal conversion. At some point, a minimum is reached, and further material reverses the trend by taking up volume. An experienced designer can find the optimum amount of fill in a few trials by monitoring the impedance versus frequency curve as stuffing is added or removed.
Filling also has the important effect of reducing internal reflections, to reduce standing waves and comb filtering. However, the amount of filling has comparatively little effect on this.
What size fuse or circuit breaker should I put in my speaker to protect it from damage?
Most modern speakers consist of a box containing more two or more drivers interconnected through a network of inductors, capacitors, and resistors. One fuse or circuit breaker in series with that array can't possible protect all drivers.
Conventional circuit breakers are a very bad choice for speaker protection. They add series resistance, series inductance, and lousy electrical contacts, all tending to degrade performance. Moreover, breakers have a trip characteristic that does not match the damage mechanisms of speakers.
Fuses are a better choice, but still are not very good. This is because speakers have complex thermal behavior. Loud playing will warm up the voice coil making it more sensitive to damage. No fuse takes this into account correctly. A fuse will do a better job of protecting tweeters, but is still not perfect.
If you want to protect a speaker with a fuse, use the lowest current, fast-blow fuse which will not blow during normal listening. This may trip prematurely in a very loud passage, or may degrade sound quality, but it is your best bet for fuse protection. For a woofer, start with a 1 Amp fuse and work up. For a tweeter, start with 100mA and work up.
There are also cheap tweeter protectors available which contain a light bulb and a resistor potted in a small tube. They work pretty well, and if you reduce the tweeter network's series resistance by a few tenths of an ohm, they are not terrible for the sound. But they are audible and not failsafe.
How a loudspeaker's impedance influences fidelity
Many loudspeaker designers, reviewers, hobbyist, and consumers, fail to recognize some basic points about how a loudspeaker should be measured and what effects the amplifier can have on tests and measures as well as the musical performance, timbre, bandwidth, presence and so on. This FAQ addresses the most basic electrical relations between amplifier and loudspeaker. The Article does not attempt to detail the more complex dynamic behavior of the loudspeaker system, thermal changes at the transducers motor, loudspeaker cable influences, environmental conditions, nor how an audio power amplifier’s design will react to these dynamic impedance variables.
2.8 Volts equal 1 Watt, right? Only for true 8 Ohm loudspeakers.
Ohm’s law, power, and SI units of measure:
V=IR I=V/R R=V/I W=IV I=√W/R R=V2/W Efficiency = output / input
Electric potential is Voltage V, current is Amperes I, resistance is Ohms R, power is Wattage W.
Loudspeakers are generally reactive AC devises. Power factors and impedance differentials between amplifier and loudspeaker must be considered. Solving for power, Watts not Voltage, is essential for understanding relationships. Power in Watts is current times Voltage. Phase angles can be ignored in basic loudspeaker testing but does factor in more complex dynamic behavior modeling. Without the correct understanding of basic power transfer, a complete detailing of the system and device under test, measures and data cannot be accurately correlated into observed fidelity. It should also be pointed out that without a basic understanding of test system, device and procedure, techno marketing can easily manipulate the tests, data, and you.
The amplifier is a major factor in how a loudspeaker system performances, both technically and sonically. If testing the efficiency of a driver in an infinite baffle, correctly mapping the impedance across its bandwidth, then at least distilling the string of points into a nominal number is essential. You cannot simply pump in 2.83 Volts and call it. Data acquired this way has little meaning. Once impedance is known, power and system efficiency can then be tested and the results can be useful.
Application: basic, using a nominal impedance reference. The Zu260FR series of driver measures 12 Ohms nominal as used in the Druid loudspeaker system. Remember, the driver’s measures and performance are in part determined by the acoustic impedance system (box, horn, baffle...) they are used in. To measure efficiency, also referred to as sensitivity, at 1 Watt, we solve for current as Wattage and resistance are known, then Voltage, all measures being at the loudspeaker’s input.
First solve for Amperes; square-root of Wattage over resistance (ignoring phase) which equals 0.289 Amps. Then take Amps times resistance to solve for Voltage and we arrive at 3.47 Volts input. So a 12 Ohm load requires 3.47 Volts at input to reach 1 Watt. An input of 2.83 Volts into a 12 Ohm load yields 2/3 Watts. Nearly all modern tests and measures on loudspeakers simply input 2.83 Volts (assumption of an 8 Ohm standard) which has almost no lay correlation to actual transduction efficiency and power. If all loudspeakers had a nominal impedance of 8 Ohms then a 2.83 Volt input would be fine and does in fact result in a nominal 1 Watt of input power. And with 4 Ohm nominal loudspeakers we get; 2.83 Volts input equals 2 Watts at input. Again, this example is basic but gives a real idea of how much power is being soaked and how much work is being done.
POLYCARBONATE (LEXAN): DML A clear or solid-color polycarbonate box can look strikingly good. However, this is not a cheap material. To locate it, look in the classified directory under PLASTICS. US $60 for a 1'x1'x1" sheet. Density: 75 lbs/cu ft. Acrylic (Plexiglass) is cheaper than Polycarbonate, but weaker and poorer damped (not recommended).
CORIAN, FOUNTAINHEAD, AVONITE, SURELL, GIBRALTAR: SDA Regardless of the brand, these synthetic countertop materials come in a wide array of colors and look beautiful. They are hard to buy, and harder to work. They take special glue to bond and require wet sanding with very fine paper to finish. You can tap it, but it's too brittle for wood screws. Corian is acrylic mixed with powdered aluminum triwwwte clay filler. Avonite, Gibraltar, and Surell are polyester resin mixed with the same clay filler. US $26 per 1'x1'x0.5". Density: 100 lbs/cu ft.
MARBLE: SDHA One challenge with marble speaker enclosures is cutting holes for the drivers. A carbide bit on a router will work, but it will dull quickly. Marble is also difficult to glue, so bracing is difficult. But it sure is pretty when you're done! US $25 to $45 per 1'x1'x1.25". Density: 160 lbs/cu ft.
PLYWOOD SHEETS SPACED AND FILLED WITH SAND OR LEAD SHOT: SDAMC If you have time on your hands and want a great impractical box, try this. Make a simple box out of common plywood. Then glue cleats on the outside of the box to space the outside plywood from the common plywood. Glue hardwood-veneered plywood to the cleats and pour sand or lead shot into the spaces between the cleats. It won't be light, but with the filler, it will be extremely well damped. In addition, if you use strong cleats and glue well, the box will be extremely stiff. One person used different size Sonotubes as an alternative to plywood, and filled the space between them with sand. Be sure to sterilize the sand in your oven before putting it in the box.
ALUMINUM SHEETS SPACED AND FILLED WITH ALUMINUM HONEYCOMB (Aerolam): SDHL Airplanes use this material for flooring. Next time a plane crashes in your neighborhood, see if you can get the wreckage for your next speaker project. You can't get a better, light-weight material. Celestion has exploited this for some great products. If you're really ambitious, you can make your own sandwich out of high-quality plywood faces and a thick honeycomb core. You will probably need an epoxy to glue the honeycomb to the plywood. A home-brew sandwich is easier to cut and glue than Aerolam.
FORMED CONCRETE: SDHC There are tricks to working concrete, such as to cast braces, rebar, and steel-wire right into the mix. Also, some concrete is better damped than other. Remember to oil your concrete forms so that they can be removed. Most concrete speakers use an MDF front panel, but you can pour one if you use cardboard tubes or plywood rings to mold the concrete into the shape of a speaker cutout. Alternately, you can make a common veneered plywood speaker box and cast concrete inside it for stiffening.
Any box can be improved by making the walls thicker, by bracing the walls, and by stiffening the walls. The stiffness of a material goes up as the cube of the thickness, so a slightly thicker material is much stiffer. A thicker panel will also have a higher resonant frequency because the stiffness goes up faster than the mass.
Consider lining the inside of your speaker with ceramic tile, attached with thinset mortar. You can get tile remnants cheaply. They are easy to apply and can be added as an afterthought to an imperfect box. However, be sure to attach all braces before tiling, because it is hard to attach anything to tile.
Also consider bracing any weak parts of the box. For example, all joints will benefit from a wooden cleat. The back of the box will benefit from stiffeners where the speaker terminals are attached. Most importantly, brace the front panel, or make it out of a double thickness of material.
What is the best "stuff" to fill a speaker cabinet with?
The following discussion will focus on practical facts on speaker cabinet stuffing and on sealed systems. Theory is limited help in selecting speaker stuffing. Vented system do share a few of these same issues and will also be mentioned, but the goals and physics of stuffing a vented box are different than those of a sealed box.
NHT speakers use polyester fill. Some use a Danish polyester that mimics the properties of fiberglas very closely. Excluding this special poly, there are two kinds of polyester available: pillow stuffing, and audio-spec polyester.
Forget pillow fill. It's cheap and easy to get. If you use enough, it will damp the midrange, and that's a lot better than an empty box. But it has little effect on lower frequencies. Some fabric stores sell "Super Good Stuff" from Stearns Technical Textiles. This is a common, inexpensive material that is said to perform as well as audio-spec polyester.
For lining the walls of a vented enclosure to reduce internal reflections, or filling a transmission line to absorb the back wave, highly absorptive wool or fiberglas are ideal. However, these materials do not provide the desired results in a sealed system. They will provide more reflection absorption than polyester, but the latter is quite good in this regard in the critical midrange. In a sealed system you don't want absorption at lower frequencies anyway; you want damping and isothermal conversion. (Author's note: I have tried "all-out" efforts using fiberglas lining and polyester fill to achieve the best of both worlds. I found little practical benefit over polyester alone.)
Most professional designers agree that practical experience, combined with trial and error is the best way to get optimum stuffing material, quantity, and method for a given design. This is why good designers routinely experiment with fill in the development of a new system. If you are designing a system that differs substantially in shape or volume or source impedance (passive crossover) from one of known reference, you will need to experiment to get best performance.
Adjusting the filling is the last step in getting bass right, and is used mostly to fine-tune the system Qtc and resonance. As increasing amounts of polyester are added to a sealed box, the resonance and Q gradually go down. This can be shown mathematically to be due in roughly equal parts to the effects of simple resistive damping and isothermal conversion. At some point, a minimum is reached, and further material reverses the trend by taking up volume. An experienced designer can find the optimum amount of fill in a few trials by monitoring the impedance versus frequency curve as stuffing is added or removed.
Filling also has the important effect of reducing internal reflections, to reduce standing waves and comb filtering. However, the amount of filling has comparatively little effect on this.
What size fuse or circuit breaker should I put in my speaker to protect it from damage?
Most modern speakers consist of a box containing more two or more drivers interconnected through a network of inductors, capacitors, and resistors. One fuse or circuit breaker in series with that array can't possible protect all drivers.
Conventional circuit breakers are a very bad choice for speaker protection. They add series resistance, series inductance, and lousy electrical contacts, all tending to degrade performance. Moreover, breakers have a trip characteristic that does not match the damage mechanisms of speakers.
Fuses are a better choice, but still are not very good. This is because speakers have complex thermal behavior. Loud playing will warm up the voice coil making it more sensitive to damage. No fuse takes this into account correctly. A fuse will do a better job of protecting tweeters, but is still not perfect.
If you want to protect a speaker with a fuse, use the lowest current, fast-blow fuse which will not blow during normal listening. This may trip prematurely in a very loud passage, or may degrade sound quality, but it is your best bet for fuse protection. For a woofer, start with a 1 Amp fuse and work up. For a tweeter, start with 100mA and work up.
There are also cheap tweeter protectors available which contain a light bulb and a resistor potted in a small tube. They work pretty well, and if you reduce the tweeter network's series resistance by a few tenths of an ohm, they are not terrible for the sound. But they are audible and not failsafe.
How a loudspeaker's impedance influences fidelity
Many loudspeaker designers, reviewers, hobbyist, and consumers, fail to recognize some basic points about how a loudspeaker should be measured and what effects the amplifier can have on tests and measures as well as the musical performance, timbre, bandwidth, presence and so on. This FAQ addresses the most basic electrical relations between amplifier and loudspeaker. The Article does not attempt to detail the more complex dynamic behavior of the loudspeaker system, thermal changes at the transducers motor, loudspeaker cable influences, environmental conditions, nor how an audio power amplifier’s design will react to these dynamic impedance variables.
2.8 Volts equal 1 Watt, right? Only for true 8 Ohm loudspeakers.
Ohm’s law, power, and SI units of measure:
V=IR I=V/R R=V/I W=IV I=√W/R R=V2/W Efficiency = output / input
Electric potential is Voltage V, current is Amperes I, resistance is Ohms R, power is Wattage W.
Loudspeakers are generally reactive AC devises. Power factors and impedance differentials between amplifier and loudspeaker must be considered. Solving for power, Watts not Voltage, is essential for understanding relationships. Power in Watts is current times Voltage. Phase angles can be ignored in basic loudspeaker testing but does factor in more complex dynamic behavior modeling. Without the correct understanding of basic power transfer, a complete detailing of the system and device under test, measures and data cannot be accurately correlated into observed fidelity. It should also be pointed out that without a basic understanding of test system, device and procedure, techno marketing can easily manipulate the tests, data, and you.
The amplifier is a major factor in how a loudspeaker system performances, both technically and sonically. If testing the efficiency of a driver in an infinite baffle, correctly mapping the impedance across its bandwidth, then at least distilling the string of points into a nominal number is essential. You cannot simply pump in 2.83 Volts and call it. Data acquired this way has little meaning. Once impedance is known, power and system efficiency can then be tested and the results can be useful.
Application: basic, using a nominal impedance reference. The Zu260FR series of driver measures 12 Ohms nominal as used in the Druid loudspeaker system. Remember, the driver’s measures and performance are in part determined by the acoustic impedance system (box, horn, baffle...) they are used in. To measure efficiency, also referred to as sensitivity, at 1 Watt, we solve for current as Wattage and resistance are known, then Voltage, all measures being at the loudspeaker’s input.
First solve for Amperes; square-root of Wattage over resistance (ignoring phase) which equals 0.289 Amps. Then take Amps times resistance to solve for Voltage and we arrive at 3.47 Volts input. So a 12 Ohm load requires 3.47 Volts at input to reach 1 Watt. An input of 2.83 Volts into a 12 Ohm load yields 2/3 Watts. Nearly all modern tests and measures on loudspeakers simply input 2.83 Volts (assumption of an 8 Ohm standard) which has almost no lay correlation to actual transduction efficiency and power. If all loudspeakers had a nominal impedance of 8 Ohms then a 2.83 Volt input would be fine and does in fact result in a nominal 1 Watt of input power. And with 4 Ohm nominal loudspeakers we get; 2.83 Volts input equals 2 Watts at input. Again, this example is basic but gives a real idea of how much power is being soaked and how much work is being done.
Understanding power transfer dynamics will hint to why power amplifiers have such a huge impact on the playback systems timbre, dynamic range, bass response, presence, treble, how loud it sounds, and so on. Remember, the reactivity of a dynamic driver is dramatically effected by the loading model (box, horn, baffle...) and the necessity for measuring the device as a complete loudspeaker. This also reveals how the exaggerated “sensitivity” measures are being generated by the majority of brands. Now that we understand the basic relations between impedances voltage and current we can now approach how a given power amplifier might behave and influence the tone, power, and presence of playback.