I have been reading a fairly comprehensive and
sensible article on room acoustics.This article from Linkwitz Lab is divided into several sections.Rather than trying to digest all the information at one go,can we take it up one section at a time,and discuss the views expressed by the author?
Room Acoustics
INTRODUCTION
The best rooms ...
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.
Actually, the loudspeaker is the problem
The room is usually considered to be the problem when a loudspeaker does not sound right. Actually, the loudspeaker is the problem, because it illuminates the room unevenly with sound at different frequencies. The room merely talks back and the listener's brain cannot withdraw attention from it. Room correction will make the loudspeaker sound different but it cannot fix its off-axis frequency response, which is heard via the room.
Below you will find a lot of theory that you can safely ignore, because your room is most likely not one of those ideal cases that can be described mathematically. No one can tell you the right room proportions, though many have and are trying. Listening rooms in the home are much more difficult to understand and describe than concert halls, because their acoustic size varies from being small compared to a 56 foot wavelength at 20 Hz, to being very large at 10 kHz with 1.3 inch wavelength. Concert halls are acoustically large even at the lowest frequencies and thus easier to analyze and they have been studied extensively. Even so, concert hall design is still a blend of art and science. For your listening/living room design and layout follow the simple guidelines above and forget what you read about 1/3rd rules, costly room treatment products, magic wood blocks, etc. and use appropriate loudspeakers.
Much has been written in the popular and professional audio press about the acoustic treatment of rooms. The purpose of such treatment is to allow us to hear more of the loudspeaker and less of the room. I am convinced that a properly designed sound system can perform well in a great variety of rooms and requires only a minimum of room treatment if any at all.
To understand this claim let's look at the typical acoustic behavior of domestic size listening rooms, which have linear dimensions that are small compared to the 17 m wavelength of a 20 Hz bass tone, but are acoustically large when compared to a 200 Hz or 1.7 m wavelength midrange tone (G1 on the piano keyboard).
Below 200 Hz the acoustics of different locations in the room are dominated by discrete resonances. Above 200 Hz these resonances become so tightly packed in frequency and space that the room behaves quite uniformly and is best described by its reverberation time RT60 (Ref. 1).
Room treatment can be very effective above 200 Hz, but the same result may be obtained more aesthetically with ordinary furnishings, wall decoration, rugs on the floor and the variety of stuff we like to surround ourselves with. How much treatment is needed, or how short the reverberation time should be, depends on the polar radiation characteristics of the loudspeaker. For my open baffle speaker designs a room becomes too dead when its RT60 falls below 500 ms.
We can think of sound as propagating like a light ray. Thus, we can use a mirror to find the region on the side wall or ceiling where sound from the speaker might be reflected towards the preferred listening location. It depends on driver, crossover and baffle design, i.e. the polar radiation pattern, whether the region so found is illuminated by sound to any significant degree. If so, then a variety of commercial surface coverings are available to scatter and/or absorb the offending reflection.
The acoustically most problematic frequency range is below 200 Hz, because of the spatially and frequency wise irregular distribution of room resonances. Many computer programs have been written that calculate the resonant modes of a given room and recommend optimum loudspeaker and listener placements. Usually, real rooms are much more complex than the calculated models. Walls are not infinitely stiff, rooms have windows, doors, openings, suspended floors or ceilings, etc. In addition, it is the polar pattern and the acoustic source impedance of the given loudspeaker that determines which of the potential room modes are actually excited and to which degree. The usefulness of such programs is marginal at best. Likewise, recommended proportions for room length, width and height should not be taken more seriously than other proportions that may be based on visual aesthetics.
The conventional closed or vented box design, that is used for the majority of loudspeakers on the market, contributes significantly to the room problems below 200 Hz. These designs are omni-directional radiators and they tend to excite a maximum number of room resonances, particularly when located in room corners. While this adds to the perceived bass output at certain frequencies, it can lead to a falsification of the recorded material, namely when the room resonance decays more slowly than the original sound. In general, the low frequency response of omni-directional speakers in small rooms is quite non-uniform. Attempts to treat the room with absorbers will make only marginal differences unless very many absorbers or large absorbing surfaces are used. It is best to attenuate peaks in the bass response with parametric equalization. Holes in the response cannot be filled in (Ref. 1).
By far the perceptually most uniform response in the range below 200 Hz is obtained with an open-baffle, dipole or figure-of-eight radiating source. Because of its directionality, the dipole excites far fewer room resonances than an omni-directional source. The measured room response is not necessarily any smoother than that for an omni-directional source. But the perceived difference in bass reproduction is startling at first, because we are so used to hearing the irregular and booming bass of the typical box speaker in acoustically small rooms. Quickly one learns to recognize the distortion of this combination and it becomes intolerable.
For evaluating a given room and loudspeaker combination a CD is available. It contains unique sound tracks to identify room resonances and their effect upon the clarity of sound reproduction. Many of the tests require no instrumentation other than your ears.
