david@agora.hf.intel.com (David Robinson) (08/27/90)
The following article is from POSITIVE FEEDBACK, the newsletter of the Oregon Triode Society, and is Copyright 1990, all rights reserved. This article may be reposted or reprinted, as long as it is not resold, and as long as proper attribution of the source is made in full. Please keep this header in all copies made of this article. David W. Robinson Editor, POSITIVE FEEDBACK david@agora.hf.intel.com THE ROOM PROJECT: Part One John Pearsall In Vol. 1, No. 2, I made a proposal that the members and friends of the Oregon Triode Society embark on a thorough exploration of the methods of taming some of our more troublesome listening room acoustics. Some of you have expressed an interest in developing these ideas as an alternative or supplement to the commercially available room treatment products. So...let's begin. In the first of three articles in Hi-Fi News & Record Review (August, September and October issues of 1981), Peter Mapp, an acoustics specialist working with Essex University in England, poses this question: What has a dynamic range of about 55 db. (or less), a frequency response from about 30 Hz upward within +/- 15 db. or so, adds colorations thoughout the audible range, significantly alters the harmonic structure and timbre of reproduced sound, reduces clarity and precision, distorts stereo imagery and perspective, alters perceived frequency balance and loudness, and has a transient response about as fast as a tortoise with wooden leg? Answer: The typical domestic listening room. Hear, hear! Well said! During the last several weeks I have been exploring the literature, both of British and North American authors. Two major philosophies in listening room design emerge. The American and Canadian designers seem to favor a thorough removal of all possible anomalies in the front half of the room (the speaker end), and a reflective seating and listening end. The British authors, on the other hand, are more likely to favor a randomization of room modes and faults. Different approaches and different results, but both having their own validity and range of application. The design of the "dead" speaker environment is based on absorption and trapping, whereas the "randomized" speaker environment is based on diffusion. Differing methods, but both effective. We need a starting point for dealing with the untreated room. Since the shape of the room came from the whim of fate or the mind of the architect, we might as well deal with speaker placement first. Assuming that your speaker wasn't designed for "boundary dependent" operation-- i.e., wall, corner, or shelf placement--we should look at a suggested system for good stereo imaging in an average room in an existing house. Mathematical considerations of the size distribution of your listening room is our next inquiry. Is there a "Golden Mean" for acoustical space? There seems to be. The two most commonly mentioned are 1.6:1.25:1 and 2.5:1.6:1, in order of length, width and height. My room misses the latter standard by a significant amount: Its length is 19', the width is 11', and its height is 8'4". This yields proportions of 2.28:1.32:1. Not bad, but not terrific either. [OK, OK, John. Now you've done it. I had to go out immediately and figure the proportions of my listening room, since I hadn't measured it in a while. Hmmm. 24' long by 14'1" wide by 8'6" high. That worked out to about 2.82:1.66:1. For what it's worth, I rather like the sound of my room... Ignorance is bliss, though. DWR] Rooms that have bad trouble in the frequency domain are those with any dimension being a multiple of another. Each dimension of a room has its own set of standing waves (resonances), and if the other dimensions reinforce the resonant modes, the room can be unbearable. On the other hand, if these resonant points can be scattered enough so that they are non-reinforcing, then the room has promise for really good results. This means that the obvious "worst case" would be the unlikely, but possible, cube shaped room. Leaving the flat surfaces and rectangles for a moment, let's look at some other shapes and their reflective modes. The concave shape in solid structures is deadly (e.g., curved alcoves or bay windows). It can "lens" or focus sound into a "hot spot." A convex shape, however, is usually not a problem, in that it diffuses sound energy. It has no real focus (divergent bounce), whereas the concave surface is convergent bounce. The flat surface can be bad or good, depending on the specific case. Enough basic theory for now. I'd like to mention a few more things that can be generating some nasties in your room: 1. Windows -- Old casement windows and double-hung sashes with thin glass and loose putty and no storm windows. A lot of spurious garbage and rattles can result at higher playback levels. 2. Glass doors over bookcases and storage shelving. I just removed the glass doors over the bookshelves that flank my fireplace. I could not get them to keep still, so I took them down and put them in the attic. This yields another advantage: The value of the uneven bookshelf surfaces in diffusing sound. 3. Glass screens on the fireplace rattle, too. Mine DO. The low frequencies set them in motion, so I leave them partly open when listening. 4. Flimsy room doors, cheap hollow core or thin plywood center doors do "talk back" to the room to a significant degree. 5. Shutters and venetian blinds can rattle. Check yours for resonances. In our next installment, we will begin to test the efficacy of acoustic materials from Johns-Manville (their acoustic fiberglass line), Owens- Corning Fiberglass products, U.S. Gypsum (rock-wool Thermafiber sound batts) and a new polyester felt-like fabric called "Quiet, Please" from Harold Davis Textile Corporation. Once we know the measured effects of the various materials on sound energy, then, and only then, can we come up with some cost-effective and attractive designs for the home listening room. It should be rewarding for all of us, and just in time for the indoor season when it rains here in the Northwest. And if any of you readers know of a new or existing acoustic material (outside of the standard tiles and traps), please pass on the suggestion in care of the Editor of POSITIVE FEEDBACK, DWR. A big [and not terribly organized! DWR] reading list this time: Stereophile, Vol. 9, #3, April 1986. "Equipment Report: ASC Tube Traps," by J. Gordon Holt, pp. 66-70. Stereophile, Vol. 11, #4, April 1988. "A Matter of Diffusion," by Keith Yates. pp. 59-77. Stereophile, Vol. 4, #2 & #3, c. 1977. "The Ultimate Component," by Roger Sanders, pp. 5-8 and pp. 7-9, 51, respectively. Stereophile, Vol. 13, #4, April 1990. "In Search of the Audio Abode," by J. Gordon Holt, pp. 84-103. The Vacuum Tube Logic Book, Second Edition, by David Manley. See "room acoustics" on page 22. Hi-Fi News and Record Review, August, September and October, 1981. "Interface I: Loudspeakers and Rooms," by Peter Mapp. Audio Magazine, December 1986 and January 1987. "Build a Live End/Dead End Listening Room," by William Hoffman. Audio Monitor: Journal of the N. California Audio Society, Issue IV, 1983. "Rooms Resounding," by Brian Cheney, pp. 8-11. Until the next time...get thee to a library! Peace through Mozart.... JP My opinions belong to me...and vice versa. They're not copyrighted; third party thinkers should feel free to clone them at will. david@agora.hf.intel.com tektronix!tessi!agora!david