February 1960 Electronics World
Table of Contents
Wax nostalgic about and learn from the history of early electronics. See articles
Electronics World, published May 1959
- December 1971. All copyrights hereby acknowledged.
Home entertainment is
as big of a deal (or bigger) today as it was in the 1960's and 1970's when high
fidelity personal sound gear was coming into the mainstream. Capability and features
were going up while the price was coming down on really nice equipment. In order
to achieve theater quality sound from your stereo and/or large screen television,
thought and planning is essential or you will end up with a confusing mess of directed
and reflected sounds. This article, a continuation of "Room Acoustics for
Stereo" in the January 1960 issue of Electronics World, contains very valuable
information on room configuration and sound absorbing materials and strategies.
A fairly extensive table of common floor, wall, and ceiling sound absorption coefficients
is provided, as are charts of reverberation times of various venues and volumes.
See also "More About Wide-Stage Stereo" in the March 1960 issue of Electronics
Room Acoustics for Stereo - Part 2: Reverberation & Room Treatment
By Abraham B. Cohen / Advanced Acoustics
How the optimum room reverberation is obtained and room resonances are controlled
by the use of standard decorative and special acoustic materials.
One requirement for a good listening room is that it have the proper reverberation
time - or liveliness - to simulate good concert hall conditions. Control of reverberation
may be achieved by using standard home decorative materials and commercial sound
absorbent tiles. By means of absorption tables covering architectural materials
used in home construction, ordinary decorative materials, and commercially available
acoustic tiles, it is fairly simple to calculate and "correct" a room to provide
proper reverberation conditions.
Fig. 7 - When a sound source continues to operate, diffuse reflection
builds up until the sound level reaches steady state.
Fig. 8 - Reverberation time is the time, in seconds, taken for
steady sound to be reduced to a millionth of its original level
A room in which sound is to be produced or reproduced, whether live speech and
music direct from the piano sounding board or reproduced sound from loudspeakers
(monophonic or stereo), must meet certain acoustical conditions if the sound is
to be heard properly. A good "listening" room should have correct reverberation
properties and generate a minimum of multiple echoes.
Analytically, reverberation is a measure of how fast a room will cause sound
to be reduced to a certain level after the source of sound has ceased to emit its
signal, providing the sound has been fully built up in and has "saturated" the room
before the source was stopped. We naively state that reverberation is the quality
of "liveness" of a room (to sound) but what is the actual mechanism of room acoustics
that leads to reverberation?
In Fig, 7 A let the loudspeaker in the corner generate a constant-frequency and
constant-amplitude sound. The wavefront emerging from the loudspeaker builds out
in a spherical shell of constantly enlarging radius. As soon as that first wavefront
reaches a reflecting surface, as at point P1 on wall A-B, the wavefront will be
reflected, This reflection travels outward from the wall just as if there were an
additional actual sound source at wall A-B. Again the sound from this new source
travels out in ever-increasing spherical fronts until it hits another surface like
wall D-C from which it will again be reflected as if there were another sound source
at P2 on that wall. Thus these reflections will continue until there are a great
number of these reflected rays travelling through the room, as shown in Fig. 7B,
producing a generally diffused sound condition throughout the room - as long as
the source continues to operate.
When the source of sound stops, the diffuse reflections do not stop immediately
because the sound that has left the speaker has a finite speed of travel. The sound
will keep reflecting in a completely random manner, each reflection being successively
weaker than the preceding one. The more absorbent the walls, the less sound power
gets reflected and the quicker the sound dies out. When the sound dies out slowly,
there is little absorption within the room, and the room would be termed "highly
Reverberation, then, is correlated with time - how fast does the sound die out?
This is a measure of reverberation-"time." Consequently, "reverberation time" has
been defined as the time (in seconds) that a steady-state sound in a room decreases
to 1-millionth (1/1,000,000) of its original intensity (after the source has stopped).
See Fig. 8. In easier terms, when the intensity of the sound, after the source has
stopped, decreases by 60 db, the time it took to drop to this level is the reverberation
Now, just what sort of reverberation time period should one expect of a listening
room, especially one intended for stereo listening? In Part 1 of this series we
noted that perhaps a shorter reverberation time for stereo might be desirable since
the very essence of stereo itself "psycho-enlarges" the room. This question of optimum
reverberation time cannot be answered by complete specifications down to the last
decimal place. It is a variable, depending on room or auditorium size and the type
of program material being presented. Studies have shown that while an auditorium
sounds right - has an accepted reverberation time - for a full symphony orchestra,
it may not have the proper reverberation for chamber music or voice.
Fig. 9 - Optimum reverberation times.
Fig. 10 - Average absorption characteristics of representative
Many studies have been made of music rooms, auditoriums, concert halls, churches,
and similar structures to determine the acceptable range of reverberation times
and these figures have been generalized in Fig. 9. While it would be nice to have
an instrument at home which would enable us to check the reverberation time of our
listening room against this sort of chart, such equipment is quite expensive (for
home use) and requires an experienced professional hand at the controls. For such
non-critical conditions in the home it is possible to calculate the reverberation
time of any room by measuring its physical size and applying information from the
charts and tables to - be discussed shortly.
In very simple form, T = 0.05V/A, where T is the reverberation time of the room
(in seconds), V is the volume of the room (in cubic feet), and A is the total absorption
of the room and its contents. V, the volume of the room, is no trouble to calculate.
Total absorption may be judiciously estimated by reference to established tables
which give the absorption coefficients of various materials used in the construction
and decoration of the room. For purposes of this article, we must, of necessity,
limit ourselves in listing absorption coefficients. What we will do is list the
more common household materials which may affect the acoustic conditions of our
listening room and then work with these.
Table 1 itemizes such common household materials while Fig. 10 illustrates, in
chart form, the absorption characteristics, as a function of frequency, of some
representative commercial materials available for acoustical correction purposes.
It will be observed that, in general, the softer the material, the more it soaks
up the sound.
Another interesting item to be observed from Table 1 is that a given material
can be arranged so that its absorption characteristic may be varied. Velours, for
example, when hung flat, have much less absorption per unit area, as shown in Fig.
11 than, if we drape or "bunch" a length of velour into an area one-half its flat
length. As Table 1 indicates, we practically double the absorption. This, then,
is a basic principle by which highly absorbent surfaces are obtained ... "amplify"
a given absorptive area by providing a dimension in depth for more absorption.
Most of us are familiar with those off-white squares of "sound absorbent" material
widely used on the ceilings of public buildings. Such materials are known as "tiles"
in the trade. Fig. 13 shows some typical units of this type. The tiles are generally
a foot square and may be obtained in several thicknesses - 3/4," being standard
for home use. The tiles are made in a variety of materials and textures. Some are
simply rough surfaces with "cracks" or fissures apparent on the surface while others
are of the perforated variety, some with evenly spaced holes and some with randomly
Although an inspection of Fig. 10 indicates that the absorption characteristics
of these materials are roughly similar, the perforated variety has some advantages
in absolute absorption, especially in the mid-frequency range, although the "curve"
of absorption is not as smooth as for the unperforated type.
Fig. 11 - Here is a simple method of changing the amount of absorption
in a typical listening room, as required.
If such acoustic tile is to be applied for the purposes of reducing noise due
to the human voice alone, as in restaurants and offices, then the perforated material,
with its accentuated mid-frequency absorption (where most of the voice frequencies
are found) is perhaps most suitable since the ear is especially sensitive in this
If, however, we wish to absorb a fuller range of sound than that of the human
voice, then the fissured material is preferable because of the smoother absorptive
curve, although more of the material may be required. Moreover, as far as the home
is concerned, the fissured or sculptured material will fit in with the decor better
and look less "industrial" than the perforated variety.
It is interesting to note that the rise in the absorption curve of these materials
roughly coincides with the frequency range where the stereo differentiating effect
takes place. In the region above 300 cps, spatial differentiation becomes effective
and separate speakers reproducing these frequencies are necessary for proper spatial
perspective. Thus the effectiveness of these sound absorbing materials roughly matches
the frequency range where the stereo effect becomes discernible - making them eminently
suited for acoustic adjustments and compensation for stereo reproduction.
To illustrate the application of the reverberation formulas in conjunction with
the tables, let's choose a room with the typical dimensions shown in Fig. 12. In
its bare, untenanted state, the room consists of a pine floor with the walls and
ceiling of plaster, lime, and sand finish on metal lathing. The windows, of course,
are of standard glass. We know intuitively how a loudspeaker would sound in this
bare room, but just as a simple exercise to illustrate the use of our "mathematical
tools," let us put down some numbers that will provide the figure of (de) merit
for this room.
Fig. 12 - This completely bare hard room will be too "live",
while the room will be too "dead" if completely padded. Figures used are shown below.
We are going to apply the reverberation formula (T = 0.05V/A) previously discussed,
which relates room volume to total absorption. To get total absorption, we simply
add up all the absorption units of all the surfaces of the room by multiplying the
areas of the various exposed surfaces by their respective absorption coefficients.
In simple cases such as this, the absorption coefficient for 512 cps is used. Thus,
as indicated in Fig. 12, the floor has an area of 25' x 20' or 500 square feet.
It is made of pine flooring which has an absorption coefficient of 0.09 (per square
foot). Its total absorption is 500 x 0.09 or 45 units. The "units" for the walls,
ceiling, and windows are figured in the same way on the basis of their areas and
absorption coefficients. The absorption of the room totals 128 units and, as indicated
in Fig. 12, the reverberation time becomes 1.95 seconds. Checking back to Fig. 9
we find that this reverberation time is far too long for proper definition of sound
in a room this size (5000 cubic feet). The room will be too "live" which, of course,
we knew from the start.
Now let's go to the other extreme and drape all the walls (using a drape which
has an absorption coefficient of 0.55) and lay carpeting on the floor. The second
tabulation, Fig. 12, shows that the total absorption will now be 710 units. Again
applying our reverberation formula we get 0.35 second, which we can see from Fig.
9 is far too low - much too dead - which we also probably surmised from the excessive
padding added to the room.
Somewhere between the completely live room (128 units) and the over-subdued room
(710 units), there must be a happy medium. Without going through the calculations,
but following the same general method, we find that if we drape only one-fifth of
the area of each wall and carpet the floor we arrive at a total absorption of 357
units (per the third calculation of Fig. 12). This room, now re-adjusted to 357
units, yields a reverberation time of 0.70 second. Although this figure is a little
on the low side, it will insure a reasonable acoustic "liveness."
The room should actually be a bit more on the live side because if you put a
couple of upholstered settees in the room (at an absorption figure of 5 units each)
and sprinkle generously with seven or eight human beings (4 absorption units each),
the total absorption will be gradually increased to approximately 400 units which
will tend to pull down the reverberation properties of the room to near the lower
Fig. 13 - Commercial acoustic tiles. (A) Soft fibers pressed
into rigid, but acoustically soft mass with perforations to absorbent body; also
with surface absorption. (B) Soft fibers pressed into rigid, but acoustically soft
mass with surface cracked, or fissured, for added absorption. (C) Hard panel, such
as Masonite, perforated and backed by thickness of absorbent material; no absorption
by surface . (D) Same as (B) but with surface sculptured for decorative effect.
Altering the Room
As just demonstrated, much can be accomplished in the line of altering the reverberation
of a room by judicious application of various household materials, especially those
in the drapery category. Thus, for instance, if one were to hang draw drapes across
the four walls of a room (of the dimensions of the one given in Fig. 12) and kept
the floor linoleum-covered, as in a playroom, the room could measure from 2.5 seconds
with the drapes completely retracted to 0.46 second with the drapes fully extended
to cover the walls. With this type of arrangement, it would be fairly simple for
the host to adjust his room to match the number of guests: For a lot of people -
open the drapes and expose the walls; for an intimate soiree - draw the drapes.
On the basis that each adult is equivalent to 4 absorption units, one could "calibrate"
the drapery drawstring in units of number of guests.
We may, of course, use materials which have been developed specifically for sound
control, such as the acoustic tiles discussed previously. With these tiles we may
control the room acoustics more accurately and with smaller areas of material than
with conventional drapes, although combinations of tiles and drapes are practical.
Tiles are especially useful when the room decor will suffer from an overabundance
of drapery. Actually, the acoustic tiles may be quite decorative as well as functional
and lend a clean, uncluttered look to the room. With the modern trend toward "airy"
interiors, acoustic tile may actually prove a boon in maintaining this feeling while
controlling room acoustics. Many such tiles (especially the perforated ones) can
be painted to create interesting decorative effects.
One of the most important features of these tiles is that they enable us to treat
ceilings as well as walls. Since the ceiling of a room comprises approximately one-sixth
of the room's interior surface, it represents a good sized area which cannot be
treated other than with tiles. The ceiling cited in our example, 25 feet x 20 feet
with an absorption coefficient of 0.06 when bare has only 30 absorption units but
when covered with fissured acoustic tiles its absorption becomes 375 units - more
than enough to take care of the whole room without treating the other areas.
Fig. 14 - Absorbent material on one surface will inhibit a resonance
condition between that surface and the opposite one, but will not prevent resonance
from being set up between the other room surfaces.
Although this method of adjusting room acoustics in one grand stroke is fine
for general noise abatement, it is not the best approach to the problem of musical
acoustics control. The reason for this is that there may be room resonances set
up between the sets of untreated parallel walls which can produce quite disturbing
effects to musical sounds reproduced within the room. Room resonances may occur
when the dimensions of the room are close to the wavelength of the sound propagated
in the room and are most prevalent in the low-frequency region.
When the air within the listening room is energized by a sound source, the volume
of air enclosed by the walls, ceiling, and floor may be driven into resonance at
frequencies which coincide with the wavelength or half wavelength separation of
the walls. The very worst shape for a room, acoustically speaking, is a perfect
cube because all axes are of equal length and the room is highly excitable acoustically
and energetically resonant in all directions at the wavelength (and harmonics of
it) which is equivalent to the spacing of the walls. In general, no room dimension
should be close to a whole number ratio of another dimension. For normal living
rooms, you may consider that if the dimensions of your room are in the ratio of
1:1.25:1.58 then your listening room will be reasonably immune to resonances between
the three mutually perpendicular axes.
Although there is usually nothing we can do to the dimensions of our rooms, the
statement of the condition, as given, is important in that it allows us to allocate
our sound absorbent material intelligently. For instance. if there should be a strong
resonant condition between two highly reflecting walls, then even large amounts
of sound absorbing materials on the floor or ceiling will not affect that particular
wall-to-wall resonance to any great extent. See Fig. 14. Thus the technique of treating
the ceiling only is not the answer to adjusting the room for music purposes. We
must distribute the "treatment" in random fashion to include all surfaces in order
to minimize reflections along all three axes. If we thus distribute the treatment
throughout the room, not only will we minimize room resonance but we will improve
and equalize the general sound diffusion throughout the room - a condition of considerable
importance for stereo.
Speaker & Listener Locations
Table 1 - Absorption coefficients per square foot of some common
materials are shown.
One of the prime considerations in setting up a room for music reproduction -
live or recorded - is that the sound source be located in a relatively "live" area
so that the sound may be projected into the room. For monophonic applications, an
undraped corner of the room is perhaps the best "projector" area of the room. Many
articles have recommended locating the loudspeaker in a corner so that the corner
can act as a three-sided horn feeding the room. Too little mention is made, however,
concerning the acoustic condition of the walls and floors of these "desirable" corners.
If the walls and floors are heavily draped and carpeted, as shown in Fig. 15A, the
corner would be of very little use. The need for a live backing area behind the
sound (Fig. 15B) is dictated by the need for throwing the sound forward - just as
is done by an orchestral shell (Fig. 15C). Therefore, absorption techniques that
are to be applied should be in the general listening area, with a minimum in the
sound generating area.
Putting the absorbent material in the listening area serves to reduce unduly
strident echoes from live surfaces (removed from the original source) which by reflectively
interfering with the direct sound will cause confusion at the listener's ear. In
this connection we must also realize that the closer one sits to the source of the
sound, the less effective are the surroundings. This effect is especially noticeable
in very live rooms. If one were seated close to a hard plaster wall and the sound
source is 20 feet away, there would be considerable interference introduced by reflections
off the wall. On the other hand, if the walls were heavily draped then the ear would
receive very little reflected sound and its major acoustic stimulus would be direct
sound from the speaker. We must, therefore, strive for a balance between direct
and reflected sound so that while we hear the performance in front of us, we get
some general diffuse sound from around us to "liven" the performance.
Summary of Principles
Fig. 15 - Speaker placement should be such as to provide proper
Summarizing the various principles outlined for any good listening room - whether
for live music, reproduced monophonic or stereo, we have:
A. The room should have an absorption characteristic which will provide a reverberation
time of from 0.75 to 1.25 seconds.
B. The sound absorption material used to achieve this reverberation figure should
be distributed throughout the room rather than be concentrated in one area.
C. Large, hard parallel reflecting areas should be avoided as they lead to multiple
reflections and resonances.
D. The sound source should be located in a comparatively "live" area of the room.
E. The listener location should not be adjacent to either a hard reflective surface
or a totally absorptive surface since in neither case will the listener receive
the proper proportion of reflected-to-direct sound in keeping with accepted reverberation
(To be continued)
Posted July 2, 2018