October 1959 Popular Electronics
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While acoustical tiles are not exactly the stuff of RF engineering,
their properties and their effects on sound waves are analogous
to RF absorbers and their effects on electromagnetic waves.
Reflections that cause multipath reception of signals that contain
the same information but are out of phase and unequal in amplitude
to the primary (direct) path seldom combine to enhance the overall
signal-to-noise ratio, so placing absorbent material in the
surrounding environment is necessary to improve signal quality.
This article from Popular Electronics goes through the process
of outfitting an area with acoustical tiles and give some empirical
test data from before and after.
Acoustical Tile - A New Hi-Fi Component
By Andrew Lorant
You can improve the listening quality of your hi-fi system,
especially stereo, with acoustical tile. How this new component
helps make hi-fi sound better, and how you can install it in
your house, will be describes in this, and future articles.
The manufacturers of hi-fi equipment have been able to provide
many methods of controlling sound - bass and treble tone controls,
filters, loudness compensating controls, and so on. But all
of these exert their effects on the sound before it is produced.
To provide complete control of sound, we need some way of regulating
the sound after it is produced. This we can do only by altering
the acoustical properties of the listening room.
Enjoyment of hi-fi - especially stereo -
can usually be improved by installing acoustical tile in the
One way to achieve acoustical control is by putting sound-absorbent
furnishings - chairs, rugs, drapes, etc. - in the room. Obviously,
this method is limited in flexibility by practical factors;
after all, only just so many overstuffed chairs can be fitted
into any given room.
Measurements have been made to determine
the practical effects of installing acoustical tile. Above is
the test room before it received acoustical treatment. The many
hard surfaces in this room made the enjoyment of music very
A simpler method involves the use of acoustical
tile. This material is particularly good because, in addition
to its sound-controlling properties, acoustical tile is easy
to install, flexible in installation, and is available in a
variety of decorator patterns. Cost is surprisingly low and
the results, from an acoustical standpoint, are well worth while.
Treatment of test room with acoustical tile
results in better room acoustics and better looks, too.
How does a sound-absorbent material like acoustical tile
work? Well, imagine that you are in a room with four hard walls
and a volley of ping-pong balls, rather than sound waves, is
being propelled from your speaker system. The ping-pong balls
will bounce merrily back and forth from wall to wall. Now, while
the original balls are still bouncing around, if a second volley
of balls were to be "shot" out of the speaker, all the balls
would mix together and it would be impossible to distinguish
one group from the other.
Professional sound-measuring instruments
were necessary to obtain data on the results of installing various
amounts of acoustical tile.
This, in effect, is what happens to sound when it is reflected
from hard-surfaced walls, ceilings and floors. The direct wave
of sound from the speakers mixes together with the reflected
sound. By the time the music reaches your ears, it has been
so colored by room acoustics that no amount of knob-twisting
can make it sound realistic.
Now consider what happens when sound-absorbent materials
are added to the room. When sound hits them, it stops bouncing
around like so many ping-pong balls, But you can go overboard
using these materials; unless you are careful, too much sound
absorption will create a "dull" or "hollow" effect.
It's plain to see, therefore, that acoustical problems cannot
be solved by merely adding sound-absorbent materials indiscriminately.
The acoustic surroundings must be controlled. And this is where
acoustical tile comes to the rescue.
By installing acoustical tile on the ceiling and then applying
varying amounts of tile on the walls, any degree of sound absorption
can be attained. Because of its extreme flexibility of installation,
acoustical tile offers infinite opportunities for the design
of a listening room which will suit any listening taste.
In addition to its importance as a sound-controlling factor,
acoustical tile is useful in cutting down unwanted sounds. If,
for example, you wish to listen to hi-fi in the basement while
the rest of the family is watching TV upstairs, acoustical tile
on the ceiling will help prevent sound transmission from one
room to another.
Since the introduction of stereo, the acoustical treatment
of the listening room has become even more important than with
monophonic hi-fi systems. Ideally speaking, each ear should
receive sound from only one of the stereo speakers. The left
speaker, for example, should make its strongest impression on
the left ear, and sound from the right speaker should be concentrated
on the right ear.
For acoustical treatment in a stereo room, therefore, you
may find it advisable to put acoustical tile part way down the
walls along each speaker enclosure. This will help prevent sounds
from the speakers from getting bounced over to the wrong side
of the room.
Obviously, then, the treatment of a room with acoustical
tile involves not only the amount of tile used, but also where
it is placed. By carefully studying the many scholarly books
that have been written about acoustics, you can find out how
much sound-absorbent material is optimum for a room with a given
volume of air; but it's more difficult to get some practical
guidance that tells where and how to install it.
Accordingly, a test room has been set up to make the necessary
measurements and evaluations Since measurements under as wide
a variation of conditions as possible were desired, a basement
room with concrete walls, rubber tile flooring, and open beam
ceiling was chosen. By erecting "dummy" walls, false ceilings,
etc., a number of different conditions could be simulated without
too much difficulty.
The preliminary results of these tests (which are presented
in the section below) indicate that the application of acoustical
tile has undoubtedly improved the acoustics of the test room,
but since the study is necessarily detailed, the conclusive
report on the measurements will be presented next month rather
than in this issue. In the meantime, you might start checking
into the advisability of installing acoustical tile in your
Does Acoustical Tile Really Make a Difference?
What does acoustical tile add to the enjoyment of a stereo
system? To find the answer, a series of tests was undertaken
during the installation of acoustical tile in a typical listening
Stereo Acoustics. To obtain the greatest stereo effect, the
left ear should receive the sound from the left speaker, the
right ear from the right speaker. This situation, however, can
be only partially realized because each ear also hears the "other"
Fig. 1. The solid lines indicate direct sound
paths; the dashed lines show some possible paths for reflected
Our hearing mechanism is able to pinpoint a sound source
because of a difference in time and level. Since the path from
the left speaker to the right, ear is longer than from the left
speaker to the left ear, as indicated by the solid lines in
Fig. 1, a sound emanating from the left speaker reaches the
right ear an instant later than the left ear. Also, the sound
level at the right ear will be less than at the left ear.
Fig. 2. Untreated room shows negligible differences
in sound levels at left and right "ears" beyond 6 feet from
In a "hard" room, sound also reaches the ears through reflection
from walls, ceiling, and floor. One possible reflected path
is shown by the dashed lines in Fig. 1. These reflected sounds
not only upset the ear's "timing" mechanism, but they also tend
to equalize the level on both ears. As a result, the ability
to pinpoint the sound source is reduced.
Fig. 3. After treatment with acoustical tile,
test room shows improved stereo separation up to 15 feet from
In order to be able to locate the sound source easily, the
reflected sound must be minimized. This is accomplished by the
application of acoustical tile.
Measurements. To measure the effects of acoustical tile,
the left and right ears were simulated by a sound level meter.
With the left speaker providing a sound source, it was possible
to measure the sound levels at the d left and right "ears."
The results are represented in Fig. 2 and Fig. 3.
Without acoustical tile, the difference between the sound
levels at the left and right ears is negligible beyond six feet
from the speaker (see Fig. 2). In the treated room, however,
the difference between sound levels at the left and right "ears"
is significant up to 15 feet from the speaker (see Fig. 3).
More results will be given next month.
Posted May 21, 2013