GETTING THE ROOM RIGHT -- II
LAST month I proposed, only half humorously, that you might consider rebuilding your listening room to get it to sound better. Audiophiles have actually been known to do this, throwing up a substantial wall or partition to alter the dimensions of a troublesome room. Admittedly, however, the idea has very limited appeal. Are there any worthwhile alternatives?
First, let's keep in mind that what we are talking about is correcting a situation in which the wavelength of a particular low frequency relates mathematically to a room dimension (or dimensions) in such a way that the frequency is reinforced or exaggerated in strength in various areas of the room to the detriment of uniform frequency response. As I pointed out last month, if two or more of the room's dimensions are identical or very similar, this "standing-wave" problem will be worsened.
The offending reinforcement takes place only because the interior surfaces of the room reflect the sound and bounce it back into the room, where at certain frequencies it is in phase with and augments the strength of succeeding sound waves as they are emitted from the loudspeakers. This gives us a clue to a possible plan of attack. If we could somehow change what seems to be the troublesome reflecting room surface into an absorbing one that is particularly effective at the frequency in question (or perhaps even install a barrier), we would scotch the offending resonance rather well. An example of an acoustically absorbent room surface would be a diaphragmatic absorber-that is, any flimsy wall or wall panel that will flex at the problem frequency, soaking up some of the sound energy in doing the work of flexion. (In addition to this, there will be a general loss of low bass, since much of it will go straight through the wall or panel to adjacent rooms or the out side.) Properly designed diaphragmatic absorbers are occasionally found in recording studios, but if you suspect that they are not very practical in a home setting you are absolutely correct.
There are, of course, various porous or fibrous materials specially designed to absorb sound, and they can be applied to walls and ceilings. However, they are at their best at higher frequencies and usually must be used in heavy, bulky quantities and/or in such a way as to leave an appreciable air space be tween them and the reflective surface before they begin to become effective at frequencies as low as those of typical room-resonance modes. They are certainly worth considering for certain applications, but even a cursory discussion of these materials and their correct use would carry us well beyond this page and probably through the next as well.
What else? Well, these undesired resonances manifest themselves physically as standing waves in the room, and these waves can be prevented from forming if the path they take through the room is interrupted.
However, we are talking of sound waves well over ten feet in length, and a rather bulky interfering object will therefore be needed if it is to do much good. A false wall is sometimes resorted to, and it almost always does the job provided its construction is sufficiently sturdy (brick, cinder block, or heavily braced plywood or wallboard). But this solution is a little extreme. Another approach, not always as effective but much more convenient, is some thing like a large room divider packed solidly with books or, better yet, record albums. The more the divider extends into the room and the closer it approaches the ceiling, the more benefit you can expect from it.
Room dividers and similar barriers can serve two purposes: they help to prevent the formation of standing waves, and they act as low-frequency sound absorbers (the heavier and more porous they are, the more effective). Because of the nature of standing waves, room resonances are not equally audible in all locations in the room. As a rule, the standing waves most likely to be offensively audible to a typically situated listener are those that form between the wall along which the speakers are arrayed and the wall opposite. If this is your case, you have a clue as to where your planned obstruction might be located to best break up a standing wave. If a room divider is used, try to arrange your seating toward the middle of the room so that the divider can be set up as a sort of false wall far enough away from the real (reflecting) wall to do the most good.
A fairly simple mathematical analysis (un fortunately also beyond the scope of this column) can be very helpful in pinning down and curing a room resonance, and for this I direct your attention to How to Correct Your Room Acoustics, No. 1 in STEREO REVIEW'S reprint series ($1 from Stereo Review Information Center, Consumer Service Division, 595 Broadway, New York, N. Y. 10012). In it you will find useful construction information that goes beyond what can be covered here as well as a discussion of acoustic materials.
WHAT you won't find in it are possible solutions to one of the worst acoustic problems of all: not what your room does to your sound, but what the sound coming from your room does to your neighbors. Apartment dwellers know very well what I mean, and they also know that often there simply is no practical remedy other than shutting down early every night. While turning the matter over in my mind some years ago, it occurred to me (a longtime apartment dweller) that my immediate neighbors tended to bother me with their puny TV speakers as much as I bothered them with my often stentorian audio system.
This made no sense at all until it popped into my head that these TV consoles are usually supported by spindly feet or legs exerting considerable pressure on the floor. The pressure made for a good physical coupling, transmitting vibration from the TV speaker into the structure of the building (often an efficient medium of vibration transmission in itself) and thence into my apartment. It was hard to believe how much the insertion of an inexpensive set of rubber pads under the TV console's feet helped to solve the problem, and a bit of elaboration on this scheme for my own audio equipment made my neighbors equally happy.
It seems obvious that anything that distributes the weight of a speaker system over a larger floor area is probably going to make your speakers less audible to your neighbors, especially if it serves to cushion the speaker-floor interface as well. It's mind boggling to think how many tempers, how much sleep and good will have been lost because this simplest of home remedies was not employed.
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