Behind The Scenes (Jun. 1978)

Since this column is concerned with room acoustics, their effect on sound reproduction, and possible modification of the listening room, it is understandably directed to those who have absolute control of their listening environment. (My utmost sympathies to the rest of you out there, who share your domain with someone who is not enamored of stereo sound systems, who doesn't like the effect those large speaker enclosures have on the living room decor, and who definitely does not like loud listening levels.) Let's assume that you have gone through a number of hi-fi systems and have progressed to the point where you now own a very sophisticated system. You're conversant with slew rates and SID and TIM and all that new fangled stuff, and like all dedicated audiophiles, you zealously pursue all manner and means of updating the quality of your system. In essence, you have become a "purist," and any signal-processing device that would sully the quality of your audio signal is an anathema to you.

Yet the moment you placed your loudspeakers in your listening room, no matter if they were the finest, most expensive units on the market, you created gross distortions of a magnitude far beyond what any processing device would impose on your audio signal. Depending on where you placed the speakers in the room, you would have varying degrees of time-delayed reflections, diffractions, and standing waves which cause ringing modes and all manner of peaks and dips, along with assorted acoustic anomalies. That lovely flat anechoic response of your speaker is now a shambles in the real life situation of your listening room.

What's that? You don't believe me? If you were to perform an acoustic analysis of your listening room with such instruments as the UREI Soni-pulse, the Shure M615AS, or the new Crown real-time analyzer, you would be shocked to find that you had a pretty ragged curve across the frequency spectrum. You may well find that you had a rising bass characteristic from 125 Hz down, with 63 Hz up +3 dB, 50 Hz perhaps up +6 dB, and 31 Hz up as much as 3 or 4 dB. All this is a major contribution to "boomy" bass response. You might be even more shocked with your high-end response, with progressive attenuation beyond 8 kHz, to the point where 12.5 kHz is down by 8 dB, and 15 kHz might be down as much as 11 or 12 dB (don't get panicky ... a roll-off starting around 8 kHz is a natural consequence of absorption, diffusion, and progressive high-frequency attenuation with distance). Needless to say, these acoustic curves are considerably influenced by the character of the room ... whether it has an average mix of reflective and absorptive elements, carpeting, furniture, etc. or is highly absorptive with much drapery, carpeting, and overstuffed furniture ... or is very reflective with a lot of glass area, wood paneling, vinyl or tile floor, and sparse modern furniture upholstered with leather or vinyl. However, assuming an average room, analysis will reveal standing waves which, below 2 kHz, gives rise to audible ringing modes. At higher frequencies the standing waves become so complex and "cluttered" that they have a decreasing effect on sound reproduction. You may not consciously be aware of it, but these modes usually add a reverberant quality to the listening room. Obviously you would like to flatten the acoustic curve in your listening room, and while observing some technical constraints, there are a number of ways this can be accomplished.

Acoustic Analysis

Before remedial measures can be undertaken, the room must be acoustically analyzed, and fortunately for those who do not have access to the aforementioned instruments, there is an inexpensive but excellent alternative. This is the Crown Equalization Test Record, available through Crown dealers as part of the Crown EQ-2 Synergistic Equalizer system, and the dealer is also supposed to have high quality SPL (sound pressure level) meters for rent. By buying the record, renting the meter, and buying a pad of special graph paper, ruled with a range from 20 Hz to 20 kHz, with standard ISO 1/3-octave centers in between, you're in business. The test record has 1-kHz reference levels for right and left channels. In the position in which you normally listen to your system, the SPL meter is referenced to 80 dB. The record then gives a voice announcement, followed by 10 seconds of pink noise of each of the 1/3-octave bands between 20 Hz to 20 kHz, and the graph is marked with the readings obtained from the SPL meter. Done with care, this gives quite an accurate acoustic response curve for your room.

There is an extra added advantage in using this method, inasmuch as it incorporates the phono cartridge, RIAA circuitry, etc. in the overall response. There are, of course, a number of excellent pink-noise generators available from the likes of 'vie, UREI, and one is built into the Shure analyzer. These must be used with calibrated mikes and do not reflect the phono data.

Needless to say, the Crown people, Soundcraftsmen, Shure, etc., all make equalizers, and all the aforementioned analysis is to convince you of the need to buy one. Gadzooks! You can hear the purists screaming a mile away! None of this kind of circuitry for them, say they, mumbling about distortions and phase anomalies, etc. All right .. . leaving that subject for the moment, what can be done to correct the acoustic deficiencies in the room?

Damping with Fiberglas

The answer is that a new type of Fiberglas comes to the rescue ... Fiberglas 703 is a semi-rigid material, supplied in 2 by 5-foot sheets in 1-, 2-, and 3-inch thicknesses. It is characterized by an almost 91 percent absorption coefficient from 125 Hz to beyond 4 kHz. The 703 can successfully cope with most of the ringing modes in the room and deaden the room considerably. Of course, for the home listening room, we don't want it to be "anechoically" dead, so ideally, the 703 should be applied in certain measure, and then the room is reanalyzed, and the final amount determined by results. How do you apply it? Here is where "saying what goes" comes into play. The 703 is the usual unattractive yellowish color, and, of course, you have the abrading of the glass fibers to contend with, which can cause physical discomfort such as itching. The problem can be solved by using a light Monks cloth cemented to the rear of the 703 batt without any loss of absorption. insulation contractors will have special "spikes" ... which have a square self-adhesive base and metal spikes of varying length; the spikes are set at intervals on the wall, and the batts of 703 are impaled on the spikes. Be warned ... if you remove the spikes from painted or papered walls, they will usually remove some of the wall covering. Three-quarter-inch furring strips can also be used to mount the 703, with the plus here of some dead air space behind the batt, which helps absorption. The 703 batts can be fairly easily cut to various sizes and shapes for appropriate placing on the walls.

Where do you put the 703? Every room will be different, of course, but generally on a long wall in a rectangular room, with the opposite wall uncovered, unless the modes and reverberation in the room are quite pronounced. It is usually placed on the wall opposite your speakers, and if you are using a permanent quadraphonic speaker setup, then place it on the facing wall as well. The two-inch thickness is most practical to work with, and as to cost, 10 of the 4 by 5 foot batts are about $36.00. The 703 does a good job at a reasonable price.

There is another material, much fancier than the 703, and this is called Soundfoam. It is a polyurethane material which comes in 2 by 4-foot panels with self-adhesive backing, and thicknesses up to 1 inch. It is also available in an embossed pattern and in various colors. A special version comes with a layer of lead sandwiched between the foam, which affords a barrier against sound transmission, as well as its absorptive properties. Just the thing to cut down noise between apartments, although bass frequencies will still be transmitted through the floor and the adjacent studs. The Sound foam is roughly 75 percent as efficient as the 703, and it costs quite a bit more, but it is an attractive looking material and that may weigh heavily with many people.

Owens-Corning Fiberglas 703 can be obtained from most insulation contractors. The Soundfoam is obtainable from Soundcoat Co., 175 Pearl St., Brooklyn, NY 11201, who also make foam wedges for use in the construction of anechoic chambers.

Okay, so we have conquered a good part of the room acoustic problems, but what about the rising bass end and the treble roll-off? The answer has to be an equalizer ... there is just no other way, except elaborate poly-cylindrical and free-form plywood diffusers and even these are not always the answer. Equalizers are like any other piece of audio equipment ... you usually get what you pay for. In the graphic types there are the Crown, Klark-Technik, and Soundcraftsmen, and in the parametric type there are units from Technics, Orban, and SAE. All of them and others will correct the bass end. As for the treble, it really should be left in its naturally attenuated state, but if you insist in trying to restore some of the top end, do it quite moderately, remembering that a totally flat top will sound very hard and overbright.

(Source: Audio magazine, Jun. 1978; Bert Whyte)

= = = =