January 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.
In the beginning, man
created monophonic (mono) radios and phonographs that had sound with no spatial
separation (left and right) in the source(s) and featured a single speaker. As
such, except for being sure to not locate your radio or phono behind the sofa,
sound perception at any point the room was fairly consistent - except maybe for
volume level. Still, there was ample opportunity for the time of arrival
due to multipath effects to distort the sound. Up until the 1950s or so, most
homes had hardwood floors (with a few rugs) and rock-hard plaster walls to
reflect sound waves, and rooms were relatively sparsely populated with furniture
and wall hangings (look at photos in vintage magazines for proof), all of which
provided means for distorted sound at a distance. And man said, "Let there be
stereophonic (stereo) sound," which, while more faithfully reproducing sound to
simulate a live setting, it also more than doubled the opportunity for multipath
to distort the perceived sound. In 1960 when this article appeared in
Electronics World magazine, a growing awareness of how a closed environment
affected sound distribution helped to mitigate deleterious effects of
multi-multipath (a term I just created) due to two or more speakers pumping out
sound. More ardent audiophiles installed carpeting, ceiling-to-floor drapes over
windows, acoustic ceiling tiles, tall leafy plants, and padded-fabric-covered
furniture in order to minimize reflections. Stereo was huge in 1960s and 70s as
documented by the plethora of articles about it in nearly every type of
Room Acoustics for Stereo - Part 1: Basic Principles
Fig. 1 - An acoustically unbalanced room may cause the speaker
system to sound unbalanced to the listener. Stereo speakers are at A and B, listener
is at X.
By Abraham B. Cohen / Advanced Acoustics Corp.
There is an intimate relation between the stereo speaker system and the listening
room. Here are the general principles to be followed for best results.
Good sound reproduction requires controlled acoustics in the listening room.
For stereo listening, the room acoustic problem becomes even more critical because
the reverberant room conditions and reflective wall surfaces may either greatly
enhance the stereo effect or completely destroy it.
In achieving optimum stereo effect, there is an intimate relationship between
the type of stereo speaker system used and the acoustic conditions of the room.
Because of the many different types of speaker systems and the innumerable types
of listening rooms, a simplified approach to the speaker-room combination will be
covered. This approach may then be extended to diverse types of loudspeaker systems.
Proper speaker placement for high-fidelity stereo reproduction is considerably
more complex than for high-fidelity monophonic program material. In single-channel
reproduction we have but one problem as far as the loudspeaker is concerned, i.e.,
at which spot in the room will the loudspeaker perform best? With stereo we have
a compound problem which involves: (a) how far apart shall the speakers be? and
(b) how shall they be angled toward the listening area? This leads to the next point
(c) which involves the best place to sit, stand, or recline to get the best stereo
effect. Unfortunately, many articles in the stereo literature treat only lightly
one additional factor that influences all of these considerations. This factor is
the effect of the acoustical condition of the room in which the "stereorama" is
Fig. 2 - An arrangement, whereby improved acoustic symmetry among
listener, speakers, and room is obtained by a rearrangement of room furniture of
The conditions which characterize a good listening room, whether for monophonic
or stereo service, involve two simple factors. The first is that the room be reverberant
to the proper degree to give the reproduced sound "liveness." The room should not
be too reverberant for then the reproduced sound would appear too cavernous, hollow,
and indistinct. Nor should the room be overstuffed for then it would sound dead
and somber. To a considerable degree, stereo systems themselves produce a psycho-acoustical
revision of the absolute acoustic properties of the listening room. This, in effect,
may make a room seem larger than it really is simply because of the apparent discrete
localization of different sections of a large orchestra to separated areas within
the room. The very essence of stereo sound reproduction thus "pushes" the walls
of the room apart. Our minds automatically tend to convert this "spread out" reproduced
orchestra into a "stage" for the orchestra.
Such psycho-acoustic enlargement, when listening to monophonic, or single-channel,
program material radiated from one central speaker opening, would be virtually impossible.
Some efforts are made to produce artificial enlargement of monochannel recordings
at the recording studio by the judicious injection of controlled amounts of reverberation.
There have been many "pop" selections recorded with this artificial reverberation
added - but in many instances so poorly done that the music and singer are all but
lost in the muddled "barrel of sound."
Thus, in monochannel reproduction, size of the room in "undirectional," that
is, controlled in a "reverberation direction" but only by the reverberation inserted
at the source. This is in contrast to stereo recording where spaciousness is not
only injected at the source but is, moreover, under the control of the listener
with respect to placement of the speakers and their acoustical environment.
The fact that there has been a fad for injecting reverberation into monophonic
recordings, even though overdone, is indicative of the reeling that "spaciousness"
does add psychological "depth" to our listening area. There are even electrical
reverberation devices, intended for home use, which are designed to "liven up" (psycho-acoustically)
the monochannel reproduction. In the case of monophonic reproduction, however, where
the original source of the reproduced sound is a single cabinet, it- is more difficult
to get a wide-stage psycho-acoustic effect without resorting to these highly accentuated
effects incorporated in the original selection. It would be considerably better
to make the recording under natural reverberation conditions and let the acoustic
enlargement be obtained by means of the listening room environment. This, however,
is a rather utopian ideal. Perhaps someday the standard living room will come with
adjustable sound-conditioning facilities similar to the air-conditioning systems
now in widespread use.
In the early days of stereophony, it was frequently noted that, when a switch
was made from monophonic to stereophonic playback, to get the same "loudness" effect,
the total gain of the amplifiers had to be reduced. Perhaps these were the first
straws in the (acoustic) wind that tended to show that with monophonic reproduction
the ear needed more reinforcement from its surroundings to get the same loudness
effect as for the stereo reproduction. From this we may possibly conclude that a
"live-r" reverberation condition is desirable for monophonic reproduction than for
stereo reproduction. This leads us to the thought that it would be desirable not
only to be able to correct our listening rooms in general so as to present the proper
acoustic environment but to make this acoustic adjustment variable to accommodate
changing conditions of playback methods, types of programs, or number of people
in the room.
Stereo has the latent power to really put us in the concert hall, spatially,
without monophonically overdone artificial effects if the speaker system is properly
balanced in performance and is judiciously installed in a manner consistent with
good concert hall practice. This means that we must look to our living room for
the last link in the "realism" chain, with the aim of bringing it as close to good
musical acoustic practice as possible.
In bringing the concert hall into the stereo room there is nothing we can do
about the actual physical dimensions - in terms of feet and inches - of the listening
room to approximate the concert hall. However, there is much we can do to change
the psycho-acoustic dimensions of the room through adjusting its reverberation characteristics
to a satisfactory "concert hall" level. Once this has been done, we can place the
speakers so that they function best with the adjusted room conditions to heighten
the illusion of expanded acoustic space.
The general principles of room acoustics are easily grasped. We sound very stentorian
and robust when we sing in the bathroom because the hard, smooth surfaces bounce
the sound around and around with little absorption so that the sound seems to persist
loud (but not necessarily clear) as we continue to bellow to our heart's content.
On the other hand, if we were enmeshed deep in a well-stocked clothes closet, we
could shout until we turned blue with exertion and would still sound weak and feeble.
The hard reflective surfaces of the bathroom make the room "live." The soft, absorbent
"stuffing" of the clothes closet makes the closet "dead." In between these acoustical
extremes lies an acceptable mean for good concert hall - or for that matter - listening
room liveness. Our acoustic goal is to achieve a room condition which is live enough
to give fullness and body to all the musical resonances yet not so live that recurrent
echoes will blur the original rich sound. There have been many determinations as
to the optimum degree of liveness for the concert hall, and these same principles
may be readily applied to your own stereo listening area. However, before we illustrate
the application of "acoustic room conditioning," with emphasis on stereo speaker
placement, it would be helpful to examine the reverberation units with which we
will be concerned.
Fig. 3 - Room acoustics are controlled by absorption properties
of material surfaces.
Room Interior Surfaces
If a room is "live," its boundary surfaces are very reflective. If it is "dead,"
they are highly absorptive. The common factor is, then, absorption. The great body
of research that has laid the foundation for dealing with this factor of absorption
(and reverberation) was done by Sabine. He proposed that the very logical, and very
understandable, unit of absorption be, literally, an open window. Obviously, if
sound is generated in a room and some of that sound reaches an open window, that
portion of the sound that falls on the open window will leave the room entirely,
as shown in Fig. 3. As far as the interior of the room is concerned, the open window
completely "absorbed" the sound that hit it - for none of that sound found its way
back into the room. A square foot of open space in a wall thus becomes an "absorption
Nothing, but nothing, can quite equal the absorption of a hole in a wall looking
out into the world. Whatever the material may be, if it is physical, if it can be
weighed, calipered, touched - it will absorb some sound, reflect some sound, and
transmit some sound. Substantive materials thus have individualistic absorption
characteristics defined by their "absorption coefficient" in relation to "open space."
Whatever the material being considered, its absorption is always less than "open
space," a square foot of which is considered to have "unit absorption." Thus, the
absorption coefficient of any material is always less than 1.
Referring to Fig. 3, the sound from the speaker system that finds its way out
of the window will be completely lost as far as further usefulness to the room is
concerned. It has, in effect, all been absorbed by the open window. On the other
hand, the sound from the loudspeaker which hits the carpet is partly absorbed and
partly reflected back into the room. Likewise, the sound hitting the plastered walls
and ceiling is absorbed to a lesser degree and is reflected back into the room with
more intensity than is the case with sound striking the carpet. As far as the listener
is concerned, the aggregate sound that reaches him is composed of the direct ray
A, the highly reflected ray B, and the somewhat reflected ray C. It is obvious,
then, that the degree of liveness, or "concert-hallism," presented to the listener
by his room is controlled by the absorptive properties of his surroundings.
Special Stereo Needs
Fig. 4 - There are many stereo speaker systems in use today as
is shown above.
This problem of the room condition is important for any type of sound reproduction
but for stereo it is doubly important (no pun intended). For the sake of preliminary
illustration of the effect of the acoustics of a room on the resultant sound field
of a stereo speaker system, we will use as the stereo sound source two speakers,
6 to 8 feet apart, oriented toward the central listening area by approximately 15
degrees each and we will assume that the speakers are as nearly balanced in their
performance as is possible.
Now, even though we accept this "paired speaker" condition as being most suitable
for our illustration, it should not be inferred that it is the only acceptable stereo
speaker configuration. Much definitive work still needs to be done on speaker pairs
(or trios), balanced or unbalanced, and we shall have considerable comment to offer
concerning this problem after we have explored the preliminary example of the effect
of room acoustics on the reproduced sound from the balanced and separated speaker
Suppose, now, that we place our perfectly balanced speakers, at the previously
specified distance and angular relationships, in a room typical of many new home
constructions where the living room and the dining area form a sort of an "L," as
shown in Fig. 1. The speakers are neatly balanced on either side of the equipment
cabinet. At the opposite wall is a foam rubber settee backed up against a hard plasterboard
wall (decoratively papered, of course) with probably an arrangement of picture miniatures
hung above the couch (a large, heavy picture would probably rip its supporting hooks
right out of the plasterboard) . In any event, small pictures or large, the backing
wall will be rather "hard" acoustically - it will reflect quite a bit of the sound
that hits it.
We turn on our stereo system, lower ourselves onto the comfortable settee and
listen. But although our reproducing speakers are balanced, phased, separated, and
angled properly, the listening system is no longer balanced. Not even the channel-balancing
control on the amplifiers or "stereo control center" will correct this acoustically
Speaker "A" sends a direct sound to the listener via ray 1 and considerable wall-reflected
sound to the listener via rays 2 and 3. On the other hand, Speaker "B," while it
may transmit direct sound to the listener, sends very little reflected sound to
his ears for there is the "open space" of the dining ell in front of it. Thus the
sound from Speaker "A" will be louder to the listener. A second effect is that there
will be considerable high-frequency loss from Speaker "B" as heard by the listener.
The high frequencies from Speaker "B" that continue on straight ahead are almost
completely lost to the listener while the highs from Speaker "A" come to him directly
and by reflection. Consequently, although the speakers may have been perfectly balanced
acoustically, their reproduction at the ear's location is unbalanced, with the unbalance
being preponderantly in the high-frequency range, due to the differential reflective
conditions bridging two speakers and the listener. This is unfortunate because much
of the stereo effect is contributed by the higher frequencies. If we lose the highs,
we lose the directional sense of the system.
Fig. 5 - A stereo reproducing system should be able to "place"
the instruments in a manner duplicating their original location.
We may overcome these deleterious effects of the distortion of the direction
of the sound by one of two methods - or preferably - a combination of both. The
first obvious method is to rearrange the furniture; the second method is to alter
the reflective properties of the room. Either of these problems may prove difficult
to overcome unless one finds some way of convincing the lady of the house that it
is actually her idea to rearrange the furniture, or to hang a drape here or there,
or to install acoustic tiles hither and yon. Having thus easily (!) surmounted this
first "managerial" problem, let us see what might be done with the furniture arrangement
shown in the room of Fig. 1 to provide a better acoustic balance among the speakers,
room, and the listener.
Fig. 2 shows one alternate arrangement of the speakers where almost complete
acoustic symmetry among the speakers, room, and listener is available. The listener
is in the direct way of each speaker. Each speaker also faces almost equivalent
open-ended spaces, that is, Speaker "B" radiates into the dining ell and Speaker
"A" radiates into the living room. To the listener, then, there is presented a condition
wherein there is a balance between the direct rays from each speaker and the reflected
There is also a second-order effect, but a desirable one. If the rooms are moderately
"live" and the program is played at good volume (who ever heard of low volume?)
- then an additional acoustic enlargement of the room takes place. The sound components
from the speakers that travel into the dining ell area and into the living-room
area find themselves bouncing around within these somewhat live end spaces. A "liveness"
effect - or reverberance - is thus produced which adds to the direct rays received
at the listening area. This partly gives rise to a psycho-acoustical enlargement
of the listening room in a way that somewhat simulates the larger chamber music
room - provided, of course, that the sound is reproduced loud enough to energize
these reflective areas. This arrangement of Fig. 2, though quite severely limiting
the effective stereo listening area, does provide a solution (for illustrative purpose)
of balanced stereo listening.
Some Different Approaches Before we opened the discussion of the above problem
of the acoustics of an unbalanced room upon a balanced speaker system, we deferred
our comments about other types of speaker systems and configurations for stereo
application. It would now be well to give this matter some consideration before
going into the specific problem of room acoustical analysis and adjustment where
these different types of speaker systems are used.
There are in use today speaker systems of many types which, as shown in Fig.
4, although greatly different, yield discernible and psycho-acoustically acceptable
stereo. In addition to the two separate speaker systems (Fig. 4A), there are systems
which consist of two full-range end systems with a "phantom fill" as a third channel
in the middle (Fig. 4B). There is a system where the major bass information is all
blended in one central system and the higher frequency stereo-determining components
are displayed by end-placed outboard speaker components (Fig. 4C). There is also
a system which is enclosed in one cabinet where the stereo effect is produced by
reflecting into the room - from each of its back hinged end doors - the full program
content of each channel respectively (Fig. 4D). In addition, there is a system where
the lows are blended in one woofer and dispersed by the rear wall, and the high-frequency
components of each channel are guided by the front hinged end doors to be randomly
reflected by the room walls (Fig. 4E). Finally (at this writing, at least), there
is the system where the blended lows and highs from one channel radiate from one
system and only the highs from the other channel have a separate speaker (Fig. 4F).
Obviously, not all these systems are 'best" systems. They each have their individual
merits and drawbacks. And obviously. they cannot all produce exact replicas of the
original sound coming out of the instruments. But one thing they all do. They give
some impression of the stereo effect, whether it be a correct or an incorrect one,
to which we then add our own personally devised psycho-acoustic image of what we
think the stereo reproduction should sound like - and we then have "stereo."
The question then arises how do we treat all these systems when integrating them
to the acoustical condition of a particular room. Unfortunately, there is no definite
and rigorously circumscribed "best" relation between the speakers of these various
systems and any room at random. The number of solutions would obviously approach
infinity, depending upon room size, shape, acoustic condition, speaker system, the
listening habits of the auditor, and the type of music being played.
Fig. 6 - (A) Higher frequency radiation should overlap for best
results. The overlap area is determined by speaker separation and angular orientation.
(B) Graph of maximum stereo listening area as a function of speaker separation and
orientation, based on radiation from 12" loudspeakers.
To briefly illustrate the complexity of the problem, take the last item just
noted-the type of music being reproduced in relation to speaker system and room
acoustics. If the stereo program material were that of a string quartet, we would
expect, for stereo realism, that the stereo effect encompass a rather small area
governed by the usual intimate geometrical relation among the two violins; viola,
and cello, as shown in Fig. 5. If the speaker system were an adjustable one in the
matter of speaker separation and speaker orientation - as would be possible with
the units of Figs. 4A, 4B, 4C, and 4F - then after auditing a few programs the listener
might be able to strike an arrangement where the stereo effect produced would be
fairly close to the original sound distribution. The arrangement in Fig. 4D could
also be acceptably adopted so that the doors, hung from the back edges, and properly
angled, would project the quartet image into a centrally confined area with a minimum
of deteriorating acoustic splash from the side walls which would otherwise tend
to swell the "size" of the quartet. Fig. 4E would probably not be as adaptable to
this type of music for, at best, even with the front-hung doors completely closed
(onto the front) the stereo-determining middle - and high-frequency components would
still be strongly projected towards the side walls of the room and would reach the
listener with strong reflected level from these comparatively distant areas with
the result that, again, the quartet's geometrical reproduction would be "swelled."
On the other hand, if the reproduced program were a full-bodied and large symphony
orchestra, then this last sys-tem with its doors wide open, would direct the sound
toward the outer reflecting wall to give a resultant enlarged acoustical image more
in keeping with the original effect. The same effect could, of course, be obtained
with the other systems through greater separation of the individual speakers, con-trolled
center fill, and system angling. Perhaps this brief dissertation on the correlation
of the reflecting properties of the room, the speaker system philosophy, and the
program material will bring home the difficulty of achieving one perfect solution-obviously
a single solution to suit all conditions is out of the question. But, despite the
complexity of the problem, or rather be-cause of its complexity, we must make some
simplifying assumptions in an effort to approach analytical results which may then
be applied to more complex systems.
Balanced Speaker System
Even if we limit ourselves to analysis of the stereo effect of two identical
systems, we may run into an imponderably large number of physical geometrical relationships
between the two loudspeakers that do not necessarily bear any definite relation
to the room characteristic. In order to resolve this matter of the placement of
two identical systems in order to get the maximum stereo effect, the writer undertook
to examine the matter in an analytical fashion in a paper which was presented before
the Audio Engineering Society Convention in September 1958. The analysis endeavored
to determine how far apart two identical speakers should be placed and how they
should be angled toward each other in order to obtain the maximum area in which
the stereo effect would be perceived when the individual speaker system consisted
of a simple 12" speaker. One of the results of this analysis is shown in Fig. 6
from which it is seen that for such a system the maximum stereo listening area will
exist when the speakers are placed about 7 feet apart and are both toed-in by about
15 degrees from the wall against which they are mounted. Now, obviously, this result
cannot be accepted as gospel truth for all of the other types of speaker systems
previously illustrated but it does at least give us a toe-hold on a premise that
will permit us to discuss the acoustics of a room and its adjustment in terms of
a frequency space pattern of which we can be reasonably sure.
After we have gone more intensively into the study of the room itself and its
acoustics, we may then be able to convert these findings for the balanced system
for application to those situations where the composite type of system is to be
(To be continued)
Posted May 3, 2019