Room Acoustics - What makes a good room? and Monitor Speaker Placement an ITR acoustic construction.

Room Acoustics - What makes a good room?


The best rooms are at least 15 feet wide, 20 feet long and have a ceiling height of 8 feet or more. This allows the two loudspeakers of a stereo system to be placed symmetrically and with their tweeters at least 3 feet from side and rear walls. With the loudspeaker tweeters 8 feet apart the sweet spot is located on the room symmetry line and at 8 feet from left and right loudspeakers. This leaves more than 9 feet behind the listeners for the sound to travel before it is reflected back. It is very important for balanced phantom image creation that the immediate vicinity around the two loudspeakers is symmetrical.

Rooms can, of course, be much larger than 15 x 20 x 8 feet and with the loudspeakers much further than 3 feet from the walls, but the optimum listening distance for phantom imaging remains equal to the loudspeaker left-right separation or up to 1.5 times that value.

Room construction can vary widely, which tends to affect low frequency reproduction and sound transmission to and from the neighbors. You take what you get and try to correct one or two frequencies if necessary. But, if the room is pleasing to live in, to have a conversation or to relax, is neither a dungeon nor a stuffed pillow, then it is also suited for accurate sound playback. The room should be furnished, have irregular hard surfaces, books and shelves for sound diffusion, rugs, pillows and soft surfaces for sound absorption at higher frequencies. Just keep it lively. The best loudspeakers will make you forget the room, if the room talks back from all directions in the same familiar voice.


Monitor or Loudspeaker and listener placement

It is often assumed that a study of room acoustics can lead to highly specific loudspeaker and listener placement locations, down to within an inch. Other proponents are not as optimistic and recommend a 1/3rd rule (FAQ31). I have come to the conclusion that real rooms are acoustically far too complex to predict the transmission of sound from speaker to listener, where the sound paths are in three dimensions, have direction and frequency dependent attenuation and diffusion, and can excite the inherent resonance modes of a room to unknown degrees.

From practical experience I recommend the following setups as starting points. They are for ORION, a dipole or bi-directional loudspeaker, and for PLUTO, a monopole or omni-directional speaker. Three room sizes are considered. The 180 ft2 (17 m2) room with 8 ft (2.4 m) ceiling would seem like the absolute minimum for quality sound reproduction with the ORION. A 400 ft2 (37 m2) or larger room with 10' (3 m) ceiling should be perfect.

D1 - Dipole setup
ORION separation is 8'. They are slightly towed in. The listener is at the apex of an equilateral triangle. Distance to the wall behind the speakers is 4', and to the side walls 2'.

The listener is only 4' from the wall behind, and this might require some heavy curtains and other absorbing material on that wall. As the room gets larger it expands around this triangular setup and especially behind the listener. Sound should just wash by the listener and disappear.

The wall behind the speakers should be diffusive. The rear radiation from a dipole must not be absorbed or it is no longer a dipole. Similarly, the side walls should not absorb sound at the reflection points but diffuse it. A dipole can even be towed in so that the listener sees the radiation null axis in a wall reflection mirror.


D2 - Monopole setup
PLUTO setup differs from ORION. The listener sits closer to the speakers. The distance to the wall behind the speakers can be slightly less, because of the uniform acoustic illumination of the room. It should not be less than 3' (<6 ms) to separate reflection from direct sound psychoacoustically, and to preserve phantom imaging.

Sidewall reflections should be diffused if treated at all. Absorbing them is like turning down the tweeter. Absorbers are not broadband and ineffective below a few hundred Hz.. Besides, lateral reflections are important for sound scene recognition.

Again, larger rooms expand around the triangle and increase the space more behind the listener than in front of him.





*sources*
http://www.linkwitzlab.com/rooms.htm
http://www.ItchyTastyRecords.com