Room Acoustics and Treatment White Paper


 
Rooms have a major impact on the performance of loudspeakers.  I get many requests for information on what is an ideal room and how best to set up my electrostatic speakers in rooms.  This white paper is designed to provide some insight and suggestions that will help.  Most of this information applies to all speakers, not just my electrostats.

My electrostats are very tolerant of poor rooms.  But of course, they will work best in rooms that have reasonable acoustics.  In general this means that "slap-echo" should be suppressed.  In most cases, this means that there should be sufficient damping in each of the three room dimensions that you do not hear slap-echo.  

Slap-echo is when you slap your hands together and can hear an echo shortly after doing so.  You should suppress this.

Generally, this means that 15 - 30% of the surface area of each dimension needs to consist of some sort of soft damping material that will absorb sound energy.  For example, in most rooms the floor-ceiling dimension is damped by carpet or rugs.  The horizontal dimensions are usually damped by heavy window drapes and soft furniture.  Sometimes additional damping is required in the form of "Echo Busters" or similar commercial room treatment products.  

Note that you do not have to buy expensive "audiophile" products.  Any soft material will work, such as foam rubber, cork tiles, cloth, etc  .

My speakers don't spray treble all over the room like conventional, wide-dispersion speakers.  So it is not necessary to specifically damp the side wall reflections as is necessary with other speakers.  You can put the damping materials most anywhere that is convenient to eliminate slap-echo.

The wall behind the speaker may be damped or not.  It just depends on your personal preferences and the situation in which you plan to use the speakers.  Here is what to consider:

Are you only going to listen to the speakers at the sweet spot -- or do you want to hear them well everywhere in the room?  For most home use, listeners want them to sound excellent everywhere.  So it is best to have the rear radiation from the speaker bounce off a hard surface like a bare wall or windows.  This will assure excellent high frequency response when you are out of the sweet spot and listening casually to music elsewhere in the room.

This rear radiation will not have any significant effect on the sound at the sweet spot due to the Precedence Effect.  This is where the rear radiation is so delayed that our ears only pay attention to the direct sound.  This is the key to the superior imaging, transient response, and frequency response of my electrostats.  

However, there are exceptions to this.  For example, in very small rooms, the sound from the rear wave may not be delayed enough for the Precedence Effect to be effective.  In that case, putting damping material on the wall directly behind the speakers can really help make the sound more clear.   We do this routinely in recording studios.  I suggest you try it both ways and see what you like best.

Note that damping only works for treble frequencies.  It has no significant effect on the bass response because base wave lengths are too long and energetic to be absorbed effectively.  Therefore, rooms will always have a few significant bass resonances, with which you must live.

Dealing with the bass is best done by trying to achieve the ideal bass frequency response, which would be provided by having an infinite number of infinitely small bass resonances.  Of course, this cannot be achieved in the real world.  But you can certainly help by designing your listening room to have many small resonances instead of a few large ones.  

Consider the acoustical techniques used by the engineers of fine concert halls.  They do three things:
  1. They use room dimensions that are 1/3 octave ratios.  This assures that each dimension will have different resonances.  By comparison, a room that was in the shape of a cube would have the worse bass performance because all three dimensions would be the same, thereby producing only one resonance at triple the magnitude of three different resonances.

  2. They make the walls non-parallel.  This breaks up the single large resonance in that dimension into a nearly infinite number of tiny resonances.  Observe any good concert hall and you will see that the walls taper outward from the stage, specifically for this reason.

  3. Large diffusers are used to further break up bass resonances.  Look up near the ceiling in a good concert hall and you will see very large, irregular, blockish structures that are designed for this purpose.


Now I understand that you probably can't make your own listening room using concert hall techniques (although I have and it worked extremely well).  But you can help by picking a room with close to the ideal 1/3 octave ratios, using diffusers (usually large pieces of irregular-shaped furniture), and using suitable speaker placement.  

Speaker placement is the biggest issue.  Most audiophiles use symmetrical placement where the speakers are the same distance from walls and corners.  This is the worst placement because both speakers will produce the same few resonances and they will be at double the magnitude than if the speakers were placed randomly in the room.  

These audiophiles also usually put the speakers on the short wall, which tends to force the speakers into corners, which also aggravates resonances and their magnitude.   Putting the speakers on the long wall at different distances from corners is better than using the short wall.

In extreme conditions, like we have in the hotel rooms we manufacturers must use at shows, I place the speakers on adjacent walls, straddling a corner.  I assure that the speakers are at different distances from the walls and corners.  This produces the most resonances at the smallest magnitudes and gives surprisingly good bass frequency response -- I often win the award for the "Best Sound at the Show."   

Of course, setting up the speakers to straddle a corner this means that listeners must face diagonally in the room.  But there is nothing wrong with that -- it just looks unusual.

You can see photos of such setups on my website under "show coverage."  Scroll down through the show coverage and you will see several photos.  Here is a direct link: http://www.sanderssoundsystems.com/show-coverage

Finally, you should make every effort to avoid placing your sweet spot directly against a wall.  Doing so will produce a short reflection from the wall behind you, which will cause smearing of the transients and a poor image.  It is much better if you are out in the room and well away from the wall.  

It is also best if the wall is at an angle to you instead of perpendicular to your sweet spot.  When it is at an angle, the reflections off that wall will bounce away from you rather than directly back to you.  This is another reason that diagonal placement of speakers is better.

If you must sit directly against a wall, then you should place some damping material on the wall behind your head.  This could simply be a small pillow.  This will greatly help to reduce the intensity of the reflected sound and produce clearer sound.


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