Hey Guys,
Haven't posted much, but I read most of the posts. Recently I e-mailed Roger Sanders about room treatment since I am trying to make some changes in my own home, and he sent back an excellent reply. Just thought I would post it here since it pertains to the subject of the thread. Enjoy!
Hi Mark,
I finally have a few spare minutes and will try to answer your questions regarding room treatment. This is a complex subject, but I will give you sound general guidelines that should help you.
There are really two acoustic issues with speakers (high and low frequencies), and each has to be addressed separately. Although your questions have mainly to do with how to handle the higher frequencies, let me address the lows quickly first since all speakers have the same issues with them.
Low frequencies contain a lot of energy that cannot be absorbed as can be done with high frequencies. All rooms have bass resonances that dramatically color the sound. So the object of room treatment for the bass is to minimize room resonances. In other words, the ideal is to have an infinite number of infinitely small resonances instead of just a few very large resonances.
Concert hall designers do this by making the room dimensions into 1/3 octave ratios, making opposing surfaces non-parallel, and by using large irregular objects ("diffusers") to break up the resonant modes. While it is possible to build such listening rooms in a home (I have done so), it is not practical for most home owners, not to mention renters. However, they work extremely well to produce smooth, non-resonant bass.
The best thing you can do in most rooms to minimize bass resonances is to avoid placing woofers in a corner and randomizing speaker placement. By randomizing speaker placement, I mean that your speakers should NOT be symmetrically placed in your room. If they are placed symmetrically, both speakers will produce the same room resonances, which doubles the amplitude of the resonances and greatly increases the problem. If the speakers are deliberately placed in a random (non-symmetrical) way, the each will produce different resonances at half the amplitude that symmetrical placement causes.
Most audiophiles use symmetrical placement, which is very unfortunate for the reasons listed above. To obtain the maximum amount of randomness, I place my speakers on adjacent walls rather than on the same wall. This means that they will straddle a corner and be placed diagonally in the room. Additionally, I am careful to be sure that they are different distances from the corner and from the wall to their rear.
Obviously, my placement is as randomized and radical as I can make it, and it doesn't fit well into the typical woman's idea of how furniture should be placed. In particular, the "sweet spot" is out in the room where it is best, but few women want chairs in the middle of their rooms. They usually want chairs against the wall, which is the worst listening location because it produces a short reflection directly behind the listener.
If you are limited by a woman's decorating ideas, then you may have to compromise from ideal speaker positioning. But at least you should be able to avoid symmetrical placement.
As an aside, sub-woofers are intended to be just that. They are designed to operate below 30 Hz -- usually far below 30 Hz where sound is only felt -- never heard. Sub-woofers are too massive to function will above 30 Hz. So I strongly encourage you to change your sub-woofer crossover frequency from 80 Hz to 24 Hz.
Also, remember that drivers do not simply stop operating at their crossover point. They contribute a significant amount of sound for at least a couple of octaves on either side of the crossover point. So you should use the steepest crossover slopes possible to minimize this problem.
In summary, a sub-woofer of good quality will have a crossover that offers 24 dB/octave slopes (or more) and when this is combined with a crossover point below 30 Hz, it should produce an excellent foundation for good earthquakes and train wrecks -- but it should not be involved in the production of music except for the deepest pedal tones of a pipe organ.
Now turning to your questions about high frequencies. I invented the curved electrostatic panel that Martin Logan is using. I designed it specifically to produce moderately-wide dispersion from a large panel speaker. So your speaker has wide dispersion, not narrow dispersion.
On the other hand, the dispersion of your speaker is more limited (about 30 degrees) than a typical dome speaker (about 140 degrees). By comparison, my flat panel speakers have essentially zero dispersion.
You might ask why I prefer zero dispersion speakers after inventing the wide dispersion ESL. Simply put, I prefer to hear the sound coming directly from my speaker rather than listening to all the bad room acoustics that are caused by spraying the high frequencies all over the room.
When most of the sound you hear is reflected off room surfaces, the sound waves are slightly delayed (because they cover more distance than the direct sound from the speaker). These delayed sounds ruin the magnificent transient response of an ESL.
They also ruin imaging as the accurate production of depth in an image requires accurate phase (timing) information. And the variously delayed reflections caused by the room confuse the phase totally. The delayed sounds also interact with the direct sound to cause non-linear frequency response. So I eliminate all these problems by using highly directional speakers.
With all this in mind, let's see what you should do to acoustically treat your room. In general, Your listening room should not be excessively "live." A gymnasium has awful acoustics. But it shouldn't be "dead" either. A library is a poor acoustic environment.
To this end, most rooms have a carpet, which absorbs a lot of energy in the midrange and highs and tames excessive "liveness." When this is combined with the typical amount of upholstered furniture and window draperies, you get a moderately live room that is a good start.
If your room is excessively live, then you need to analyze why. The usual cause of this is hardwood or tile floors instead of carpet. My current home is like this. I deal with it by putting one inch thick, "egg crate" foam pads on the ceiling. I have about 10 of these panels that are 4' x 4'and I have covered them with thin cloth to make them look attractive. You can make your own very inexpensively.
Two adjacent walls need to have some sound absorbing material on them. By adjacent walls, I'm talking about two walls at right angels to each other. This way you are actually damping the acoustics in each of the two floor dimensions.
Usually draperies work well. But if your room has none, then you need to hang something that will absorb sound on them. Decorative rugs work well and look great. In my dedicated sound room, I glued 1 foot square cork tiles on adjacent walls. Cork is available in 1/2" sheets at home improvement centers for reasonable prices.
As for the room treatment behind your speakers, putting foam directly behind the speakers has the advantage of eliminating a lot of the delayed room reflections that tend to distort the imaging and ruin transient response. However, the disadvantage of using such damping is that it reduces high frequency energy when you are listening out of the sweet spot.
So in general, if most of your listening is at the sweet spot, putting damping behind the speaker is a good idea. But if most of your listening is done casually, out of the sweet spot, you should leave the wall behind the speaker live so you get better highs off-axis.
Also keep in mind that the most critical aspect of the rear energy from a dipole has to do with how it interacts with the panel itself. If the energy from the rear bounces off the wall and back through the panel, it will really mess up the frequency response of the speaker. So you should toe-in the speaker so that the rear reflection bounces away from the speaker rather than coming back through the panel.
The most important place to have sound damping materials is where the direct, forward radiation of the speaker strikes a wall in such a way that that short reflection comes straight at you. In dome speakers, there will always be such a wall position because they have extremely wide dispersion. But in your ML speakers, this may not be a problem because the dispersion is more limited.
You can check this by imagining that your speakers are shooting balls out of their ESL perpendicular to the surface of the speaker. If such a ball would strike a surface or wall such that it would then bounce off and strike you at the sweet spot, then you need to put sound absorbing material on the wall at the point where the ball could hit.
Another option would be to rotate the speakers so that they shoot in a different direction. In any case, your speakers will sound best if they are toed-in and pointed directly at your sweet spot instead of being perpendicular to the wall and parallel with each other.
In closing, I know that I have given you a lot of information that is highly abbreviated and can be hard to understand. But perhaps I have given you some useful information too.
You can always do some testing to find out what works. For example, you could temporarily hang a heavy blanket behind your speakers and listen to see what effect it has. If it helps, you could then install some foam materials semi-permanently on the wall behind the speaker. If you liked the live sound better, then you would just leave things alone.
As for resources, there are many books on acoustics you can get. Naturally some are more readable and understandable than others. But experimenting with listening tests will get you a lot more useful information a lot faster.
Great listening,
-Roger
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