The Listening Room: the Forgotten Factor Page 2

(This is the most common recommendation; other authorities recommend having the 100% at the speaker end and the 50% at the listening end. I prefer the room to be completely deadened, with the addition of an electronic reverb device like the Benchmark ARU to add acoustical space to the listening area. Play the absorption game by ear. An excess of deadening is generally better than a deficiency, but any error is easily reversible if you don't glue things in place.)

What about size? Well, nonparallel room boundaries will yield many different dimensions, but the average dimensions—that is, the distance from the center of each surface to the center of the opposite one—should be in ratios of 1 to 1.25 to 1.6. With these ratios, no frequency can generate a standing wave between more than one pair of surfaces. Thus, the bumps and dips will be evenly distributed across the lower part of the audio range.

The longest dimension should be great enough to support the lowest frequency you wish to reproduce. This can be calculated by dividing the bottom-most frequency of interest into 525. Thus if you want to go for a 20Hz bottom, the average longest dimension should be a little over 26'. For 16Hz, make it 33'. Of course, this will mean that the room's shortest dimension—that is, its floor-to-ceiling height—will have to be 20.5', but then, we are not discussing practicality.

So, let's be practical. About size, first. Making a room "too small" will not kill deep bass, it will merely reduce it in amplitude. With most good bass speakers, you'll have no troub!e supporting a floor-shaking 22Hz tone in a 20'-long room. Secondly, standing waves are not limited to the distance-dependent fundamental; they also occur at multiples of the fundamental. If a room dimension produces a standing wave at 30Hz, it will also produce standing waves of diminishing intensity at 60, 90, 120 and so on. This, you may notice, is the classic harmonic progression.

The reason we choose 1 to 1.25 to 1.6 as the ideal dimension ratios is that no two standing-wave frequencies will coincide. Dips and peaks will occur at regular intervals and so closely spaced that they average out to a fairly flat response. This also means that we can safely play some games with these ratios, without sacrificing smoothness. Doubling any one of them will move its fundamental standing wave upwards by one octave without changing its progression frequencies. Thus, our almost-ideal room can have dimension ratios of 1 to 1.6 to 2.5, Or 1 to 2.5 to 3.2. These variations make it possible to design for a "reasonable" ceiling height.

For example, let's start with a maximum room dimension of 20'. The smallest dimension—the height—should be 20 times the reciprocal of 2.5, or 20 x 0.4, which works out to 8'. How's that for a practical room height? The width will then be 8 times 1.6, or 12.8'. A room this size could. easily be added to any home, and is even likely to be found in a ready-built hoUse.

As for the shape, if you're not building from scratch, you can scratch the non-parallel surfaces. Their absence will not have all that much effect on the sound of the room.

Construction materials? Masonry of some kind is still the ideal, but wood, framing can be made less flexible by the use of thick sheetrock panels on inside walls rather than fiberboard or wood. And diagonal bracing strips between the wall framing will add additional rigidity if nailed to the wall panels. If you want to go after overkill, you can even contrive to fill the wall spaces with pea-sized chunks of that foamylooking volcanic rock that's used extensively out West for driveway and flat-roof surfacing. (Its rough surface prevents it from packing down, as would sand or smooth pebbles.)

Acoustical treatment can be expensive, effective and ugly, or inexpensive, moderately effective and attractive, depending on whether you decide to use those moulded pyramidal foam panels or go with something like cork, perforated acoustical "tiles", or fiberglass instead. The most effective treatments, such as Sonex and the Robac tiles we reported on in Vol.4 No.8, are likely to cost around $5/square foot, but because they are so efficient, you need less complete wall coverage to get the same amount of deadening as less effective panels. No one would say they are the best-looking treatments you can use.

Cork is one alternative, although it is so inefficient that most of the wall surface must be covered with it. Use the 3/8" thickness; thicker panels inflate the total cost tremendously while bestowing no improvement in absorption.

Owens Corning makes a very reasonably priced "Glass Cloth" rigid fiberglass panel which is ideal for acoustical treatment of walls and ceilings. It is visually attractlve (footnote 1), and is also an effective thermal insulator which will help to keep your room cooler in summer and warmer in winter. And, unlike most porous absorptive surfaces, it may be painted (using a special "nonbridging" paint—that is, one that forms beads instead of filling gaps). It is available in 1" or 1.5" thicknesses; the 1.5" is preferable.

Large window areas are a problem that must be coped with in some way. Closed, they provide, almost 1OO% reflectance of sound; open, they give almost 100% absorption. The best solution here is to use heavy lined drapes and keep them shut when listening.

Finally, if you're actually building your dream room from scratch, there are a couple of other non-acoustical details you might look into. One terribly clever innovation, if there is to be no crawl space or basement under the room, is to install underfloor pipes from the equipment area to the loudspeakers, for running speaker cables without putting moletrails across the carpet. The pipes should be nonmetallic—black polyethylene plastic pipe is cheap and ideal. And just so you can easily feed wires through the pipes (and replace them with better cables later on), it's a good idea to run heavy-duty nylon cord (fishing line is perfect) through each pipe before it is (literally) set in concrete. Each cord should be a little more than twice the length of the pipe, so you can pull the wire. ends through from one end to the other without unthreading the cord from the pipe. (Once removed, it will be. difficult to get the cord through the pipe again.)

You might also take this' opportunity to have a really adequate high-current AC supply laid in directly from your fuse box, for powering your system. This should terminate in a row of at least four three-way (grounded) AC outlets. And it's not a bad idea to install a surge protector at the outlet box, just in case of nearby lightning strikes. (These devices, which look like oversized ceramic-disc capacitors, can be obtained for a few dollars from any electronics supply house.)

All details, details, details. But it's details that help to extract the best possible sound from your system that it's capable of. If you don't, you're wasting some of the money you paid for all that equipment. And that' s not only uneconomical, it's dumb.—J. Gordon Holt



Footnote 1: I think some audiophiles (not to mention their spouses, of whichever sex) will disagree with JGH concerning the attractiveness of this fiberglass material: it is covered with white vinyl, and has a definite industrial flavor. Any of the sound-absorbing materials can be attractively covered with burlap, normally available in a variety of colors at a fabric store.—Larry Archibald
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