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.
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