In Search Of The Audio Abode---The Hi-fi House Letters

Letters in response appeared in Vol.13 No.7, the July 1990 issue of Stereophile:

The quest for the perfect abode
It was with great interest that I read J. Gordon Holt writing on the horrors of the quest for the perfect listening abode in Vol.13 No.4.

I have recently been upgrading my audio system and it has proven to be a most satisfying experience. But with improvement comes the endless search for optimal performance from one's equipment to further the experience of musical enjoyment.

I'm indebted to Stereophile for helping to steer the right course in properly mating my components. Keep up the fine work! I look forward to JGH's next item on the ideal listening room and dream of that day when I can build a room from scratch instead of being prisoner to my rented house and its unbecoming and sometimes unmusical environment.
---Ken Calascione Santa Fe, NM

Walls for the perfect abode
Mr. Holt's article on listening rooms in Vol.13 No.4 was informative.

When building a partition wall with offset studs, it is beneficial to use different thicknesses of sheetrock for the two sides---resonances will be less well transmitted. A wall with 5/8" gypsum board on one side and 1/2" on the other is "quieter" than one using the thicker board on both sides. (Belts, Lumleys, and lair-bound paranoids will want to investigate the lead-shielded sheetrock used in hospital X-ray rooms.)

Also, if the homeowner or builder is skilled, a coffered ceiling whose sections enclose shallow pyramids is superb.

I look forward to more articles on listening room acoustics, including one on Mr. Harley's new room.
---Lev Botstein Shutesbury, MA

Ducts for the perfect abode
As a designer of residential HVAC systems, I found Mr. Holt's April article, "In Search of the Audio Abode" (regarding climate control), to be pointed in the right direction, but slightly misleading.

Mr. Holt's suggestion of expanding duct openings will help quiet the air, but it will also diminish the throw pattern needed for proper circulation. A less expensive, more effective solution is to replace the standard registers with models featuring a 1/2"-spaced fin.

The baffle farm suggested in fig.7 will do much more harm than good. True, it will lower air noise, but it will also reduce air flow to a disabling level. By restricting the air flow as shown, the possibility of freezing the evaporative coil becomes a likelihood. After the coil ices up, the life of the compressor then becomes jeopardized. An inexpensive alternative is to lower the fan speed at the furnace. Most new blowers have multiple speeds. Before doing this, consult your furnace manual or contractor. Too low a setting could overheat the furnace or freeze the evaporative coil.

Another inexpensive method to attenuate air noise is to replace the last few feet of each individual supply with insulated, flexible ductwork. Supply and return trunks can be treated with duct-liner insulation to absorb any blower-motor or air noise.

Living with warm-air heating and central air-conditioning need not be a noisy experience. If you are having a home built for you or if you are replacing your existing HVAC system, contact a contractor who will be sympathetic to your requirements. A properly designed duct system can produce amazing results. If you can afford it, many manufacturers now produce high-efficiency, variable speed units. These systems vary the fan speed to deliver only what is needed to maintain the desired temperature.
---Gerald Andy Andeskie Total Energy Service, Inc. Bound Brook, NJ

The inverse-square law
I particularly enjoy reading well-written pieces about audio and room acoustics, so it was with much anticipation that I began to read J. Gordon Holt's feature, "In Search of the Audio Abode," in the April issue. I was stopped stone cold, however, when I read, and reread, how sound expanding balloon-like (as a point source) diminishes by 12dB per doubling of distance.

This just ain't so. The sound level from a point source decreases by 6dB per doubling of distance. The one-quarter attrition rate, stated in the article, is in fact correct from a power soundpoint. That is to say, from a point source the sound power per unit area does become one quarter as great when the distance is doubled. However, the sound pressure is reduced by only one half with each doubling of distance.

Taking this one step further, the correct formulation for sound power is 10log(power1/power2). Substituting 1/4 for the power ratio gives 10log(1/4), or -6dB. For sound pressure, the correct formulation is 20log(pressure1/pressure2). Substituting 1/2 for the pressure ratio gives 20log(1/2), or again, -6dB. I can only presume JGH substituted 1/4 into the 20log(pressure1/pressure2) formula, thus getting the incorrect -12dB figure.

So what does this mean? It means that for a point source, JGH's fig.1 is incorrect, and that the projected size of the lot required for acoustical isolation would have to be much bigger.

But in JGH's defense, he does wisely go on to present some limitations in the model. Given that these limitations also apply to the 6dB lapse rate, then why am I bothering with all this anyway?

The answer is simply that articles can be valuable as tutorial explanations. But, and here is the catch, the tutorials must first be valid and accurate. Otherwise, the ten paragraphs so carefully developed in the Privacy section of the article could be more appropriately summarized by one sentence saying "Close the windows."

What sayest ye, JGH?
---Gary J. Loikith California, MD

Mr. Loikith is, of course, correct when he states that the well-known inverse-square law equates to a 6dB reduction in sound-pressure level with each doubling of the distance between the listener and a point source in free space. The incorrect 12dB rate had unfortunately slipped by me when I was preparing the article for publication. Mea culpa. When I asked JGH about his derivation, he had indeed confused power with pressure. Gordon's corrected fig.1 has been used in the web version of the article.---JA