ELECTRONIC SOUND ABSORBER

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• Collection: General CIA Records
• Document Number (FOIA) /ESDN (CREST): CIA-RDP78-03300A001900110032-3
• Release Decision: RIPPUB
• Original Classification: K
• Document Page Count: 7
• Document Creation Date: December 22, 2016
• Document Release Date: May 3, 2011
• Sequence Number: 32
• Publication Date: July 23, 1956
• Content Type: MISC

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OUSTICAL SOCIETY OF AMERICA, Vol. "a,,, No. 6, 1130-1136, November, 1953
Pub, 1933, by the Acoustical Society of America
Erlectronic Sound Absorber
RRY F. OLSON AND EVERETT G. MAY
CA Laboratories, Princeton, New Jersey
(Received July 23, 1953)
an electronic transducing system, as contrasted to conventional sound absorption by direct conversion from
acoustical to heat energy. The electronic sound absorber consists of a microphone, amplifier, and loudspeaker
connected so that, for an incident sound, wave and sound pressure at the microphone is reduced. Thus it will
be seen that the electronic sound absorber is a feedback system which operates to reduce the sound pressure
in the vicinity of the microphone. The sound pressure in the neighborhood of the microphone can be reduced
10 to 25 decibels over a frequency range of three octaves in the low-frequency portion of the audio-range.
The electronic sound absorber may be used to reduce the noise over a small volume, that is, spot type noise
reducer or it may be used with an acoustical resistance to obtain a high order of sound absorption in the
low-frequency range.
W HEN sound energy impinges upon a surface, it
may be tacitly assumed that the energy is
divided into three portions, namely, the incident,
reflected, and absorbed energy. Furthermore, it may
also be assumed that the fraction of absorbed incident
energy is a property of the physical characteristics of the
surface exposed to the sound. It is upon these assump-
tions that the classical theories of sound absorption are
based. From these theories evolved a quantity termed
"the sound absorption coefficient of a material," which
is the ratio of the absorbed sound energy to the incident
sound energy. In general, the object is to obtain a large
absorption coefficient over a wide frequency range with
a practical material.
All conventional sound absorbing materials are made
of some sort of porous material. The absorption of
sound is due to dissipation of energy incurred by
viscosity as the sound passes through the narrow
tortuous passages in the porous material. When this
material is used as a surface sound absorber, the
volume current is inversely proportional to the acousti-
cal impedance of the material. The sound absorption is
P,
the product of the square of the volume current and
the acoustical resistance. Since the acoustical impedance
of all practical sound absorbing materials in conven-
tional mounting arrangements is very high in the
low-frequency range, the resultant volume current is
small, and as a consequence, the sound absorbing
efficiency is poor.
There are many applications for a sound absorber
which exhibits high absorption efficiency in the low-
frequency range. Some of the applications are as
follows: the reduction of noise in spot locations in the
vicinity of machines, in airplanes, in automobiles, and
in trains, a direct reduction in sound output of a ma-
FIG. 2. Sound-pressure reduction frequency characteristic
of a Helmholtz resonator having a volume of two cubic feet.
chine by application of the absorber at the source of
the noise and the absorption of sound by the use of the
absorber similar to that of conventional materials.
Conventional sound absorbing systems with high
efficiency in the low-frequency range are extremely
bulky. Therefore, conventional sound absorbing systems
are unsuitable because the above-mentioned applica-
tions require a compact high-efficiency ?,,und absorbing
system.
The efficiency of sound absorption can be increased
by using a resonator with the absorbing material. In
effect, this improves the coupling between the medium
and the absorbing material. However, the frequency
range of high sound absorption obtained by the use of
a resonator is confined to a fraction of an octave.
SECTIONAL VIEW
rat M1 rAt MY CAI
P,
FIG. 1. Sectional view and acoustical network of a Helmholtz
resonator sound absorber. pi-sound pressure in free space.
MI-inertance of the air load. rAI-acoustical resistance of the
air load. M, - inertance of the aperture. CAI-acoustical capaci-
tance of the volume. ru:-acoustical resistance of the cloth over
the opening of the resonator. p:-sound pressure at the mouth.
STAT
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ELECTRONIC SOUND ABSORBER
Furthermore, the resonator must be of considerable size
to obtain high absorption. These two factors make the
range of usefulness of the resonator somewhat limited
because several resonators are needed to obtain absorp-
tion over one or two octaves. Such a system is too
bulky for most applications.
There is another sound absorbing system which
can be developed to exhibit high absorption over many
octaves in the low-frequency range, namely, an elec-
tronic sound absorbing system. It is the object of this
paper to describe a sound absorbing system employing
all electronic components. However, the first considera-
tion will be a resonator absorber which will illustrate the
problems of high-order sound absorption in the low-
frequency range.
RESONATOR SOUND ABSORBER
The resonator sound absorber' consists of a simple
resonant acoustical system. One type consists of a
Helmholtz resonator with some absorbing material
located in the cavity of the resonator, or with a cloth
Fte. 3. Sound-absorption coefficient frequency
characteristic of a Helmholtz resonator.
over the mouth to provide additional acoustical
resistance. The acoustical performance is practically
the same for all types of simple resonators. Therefore,
a consideration of the performance of the Helmholtz
type will indicate the characteristics of resonant sound
absorbers.
A sectional view and acoustical network of a Helm-
holtz resonator are shown in Fig. 1. The performance of
the system can be determined from the acoustical
network and the constants of the system. In one
application, the problem is to reduce the sound pressure
over a small volume. Under these conditions, the
acoustical resistance rns should be made as small as
possible. A typical measured sound-pressure frequency
characteristic at the mouth of the resonator is shown in
Fig. 2. The characteristic shows that there is a reduction
in sound pressure in the frequency range above 72
cycles and an increase in sound pressure in the fre-
quency range below 72 cycles. Since there is an increase
of pressure over a certain portion of the frequency range,
I C. M. Harris, and C. T. Malloy, J. Acoust. Soc. Am. 24, 1
(1952). This paper contains references to the publications of other
investigators on the subject of resonator sound absorbers,
CABINET' ABSORBING
MATERIAL
FIG. 4. The elements of an electronic sound absorber. A sectional
view of the loudspeaker and cabinet and a schematic view of the
microphone and amplifier.
it is obvious that the simple resonator is not suitable for
a free-field, spot-type sound reducer.
When the resonator is used as a sound absorber, the
action is somewhat different. In order to obtain the
maximum sound absorption, the acoustical resistance
rAl should be equal to the acoustical resistance rA2.
This can be accomplished by selecting the proper value
of the acoustical resistance rA2 provided by the cloth
over the opening of the resonator. If a bank of resona-
tors is used, so that the ultimate acoustical resistance
rAi is obtained, the maximum efficiency of sound
absorption will be obtained. A typical sound-absorption
frequency characteristic of a Helmholtz resonator is
shown in Fig. 3. An examination shows that a high
value of sound absorption is obtained over only a very
narrow frequency range. At least twenty resonators
would be required to. cover the frequency range from
30 to .200 cycles with tolerably good absorption. Since
the average cubical content of each resonator is two
cubic feet, the total cubical content would be 40 cubic
feet. Since a bank of resonators is required for each
frequency, the entire assembly becomes an arrangement
of tremendous bulk.
ELECTRONIC SOUND ABSORBER
The electronic sound absorber absorbs or reduces
sound by means of an electronic transducing system, as
contrasted to conventional absorption by direct con-
version from acoustical energy to heat energy. Specifi-
cally, the electronic sound absorber consists of a micro-
Ftc, 5, A perspective view of the elements of the
electronic sound absorber.
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H. F. OLSON AND E. G. MAY 1132
phone, amplifier, and loudspeaker, shown schematically
in Fig. 4. A perspective view of the microphone, ampli-
fier, loudspeaker, and battery supply is shown in Fig. 5.
A photograph of the system is shown in Fig. 6. The
system is connected so that for an incident sound wave,
the sound pressure at the microphone is reduced. Thus
it will be seen that the electronic sound absorber is a
feedback system which operates to reduce the sound
pressure in the vicinity of the microphone. In order to
achieve this type of operation, special precautions in
the choice and design of the elements used in the
electronic sound absorber are necessary. The sections
which follow will describe the elements and the per-
formance characteristics of these elements.
Microphone
The microphone which seems to be particularly
suitable for this application is the electronic microphone2
in which the impinging sound vibrations directly control
the electron stream in a vacuum tube. The advantages
of the electronic microphone for this application are
as follows: the response in the low-frequency range is
independent of the frequency, the output impedance is a
Fin. 7. Sectional view of the electronic microphone used
in the electronic sound absorber.
constant electrical resistance, the phase relation between
the sound pressure and the voltage output is a constant
in the low-frequency range, and the sensitivity is
relatively high. A sectional view of the electronic
microphone is shown in Fig. 7. The microphone consists
of a diaphragm connected to a mechano-electronic
transducer. The voltage output of the mechano-
electronic transducer is proportional to the amplitude
of the anode rod. In the frequency region in which the
vibrating system is stiffness-controlled, the ratio of the
electrical output to the, impinging sound pressure is
Fic. 8. Response frequency characteristic of the electronic
microphone shown in Fig. 7.
independent of the frequency. The response frequency
characteristic of the microphone is shown in Fig. 8.
The resonant frequency of the electronic microphone is
about 5000 cycles. The response is very smooth below
1000 cycles. Furthermore, . uniform response extends
down to zero cycles or dc sound pressure. Under these
conditions, the variation in phase angle between the
Fic. 9. Sectional view of the loudspeaker mechanism used
in the electronic sound absorber.
actuating sound pressure and the voltage output is less
than two degrees from 20 to 400 cycles. Since the
electrical impedance of the microphone is an electrical
resistance of about 10 000 ohms, the problems of
maintaining uniform phase and response in coupling an
amplifier to the microphone are simplified.
Loudspeaker
Coupling transformers between a vacuum tube and a
loudspeaker introduce considerable phase shift in the
low-frequency range. Therefore, in this system it
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1133
appeared that the logical solution was the elimination
of the transformer by the use of a high-impedance voice
coil which could be coupled directly to a vacuum tube.
Accordingly, the voice coil was wound with wire which
gave an electrical resistance of 200 ohms.
In this feedback system, the loudspeaker should be a
zero-order radiator, which means that the back of the
loudspeaker mechanism must be enclosed. In order to
obtain an enclosure of reasonable size, the loudspeaker
diaphragm must be relatively small. For a cone diameter
of three inches, the required cabinet volume is about
one-half cubic foot. This is a cabinet of tolerable size.
The resonant frequency of the loudspeaker mecha-
nism alone should be about 30 cycles in order to keep
the resonant frequency in the cabinet to around 45
cycles. A limp suspension is required to obtain a
resonant frequency of 30 cycles with the small vibrating
mass of the cone and voice coil. The suspension arrange-
ment developed for this loudspeaker is shown in Fig. 9-.
Placement of the outer suspension near the plane of the
voice coil makes a center suspension unnecessary. The
suspension was made of sheet rubber in order to obtain
a sufficiently low value of stiffness.
FIG. 10. Response frequency characteristic of the loudspeaker
mechanism of Fig. 9 operating in a cabinet of one-half cubic foot.
The response of the loudspeaker in the cabinet
indicated that some damping was required. The damp-
ing is provided by an acoustical resistance in the form
of silk cloth covering the holes in the top plate of the
magnetic structure which completely encloses the back
of the diaphragm.
The response frequency characteristic of the loud-
speaker is shown in Fig. 10. It will be seen that the
performance is remarkably good for a loudspeaker of
such a small size.
Amplifier
The schematic diagram for the amplifier of the
electronic sound absorber is shown in Fig. 11. A battery
powered amplifier was selected so the sound absorber
could be tested in locations remote from power lines.
Separate filament batteries were used on each stage,
and separate plate supplies were used on the voltage and
power stages to reduce amplifier regeneration difficulties.
Ten 3S4 vacuum tubes in parallel were connected
directly to the 200-ohm voice coil. The plate resistor,
combines with the relatively low electrical impedance
of the loudspeaker, to reduce the amount of power that
can be obtained from this combination as contrasted to
FIG. 11. Circuit diagram of the amplifier used in the
electronic sound absorber.
that which could be obtained under optimum operating
conditions. Nevertheless, the maximum undistorted
power output to the loudspeaker is 0.5 watt. This is
more than adequate for this application.
The response frequency characteristic of the amplifier
from the input to the grid of the first tube to the point
0 of Fig. 11 is shown in Fig. 12. The reduction in output
with frequency range is introduced to reduce noise and
positive feedback in the high-frequency range. Further
reduction in high-frequency response is obtained from
the inductance-capacitance network between the ampli-
fier and loudspeaker.
Operation
A sectional view, schematic electrical diagram, and
acoustical circuit of the electronic sound absorber are
shown in Fig. 13. The system is connected and equalized
for frequency response and phase so that the sound
pressure is reduced at the microphone. The driving
pressure p2 is given by
where B=flux density in the air gap of the loudspeaker,
I= length of the conductor of the voice coil, i= current
in the voice coil, and S= area of the cone.
The amplitude and phase relations between the sound
pressures pr and P2 are such as to make the sound
pressure p3 as small as possible over a wide frequency
range. Under these conditions of operation the system
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FIG. 12. Response frequency characteristic of the
amplifier of Fig. 11.
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FIG. 13. Sectional view, schematic electrical diagram, and
acoustical network of an electronic sound absorber. p,=sound
pressure in free space. M,=inertance of the air load. r41=acous-
tical resistance of the air load. Ms=inertance of the cone and
voice coil of the loudspeaker. CAI=acoustical capacitance of the
suspension system of the cone. rAS=acoustical resistance of the
cloth over the apertures in the back plate. CAS=acoustical
capacitance of the volume of the cabinet. rAS=acoustical resist-
ance of the sound absorbing material in the cabinet. P2=driving
sound pressure in the loudspeaker. ps= sound pressure at the
microphone.
is a sound pressure reducer. The system can also be
designed so it will absorb sound. This can be done by
designing the system so the proper phase relations are
ACOUSTICAL NETWORK
FIG. 14. Sectional view, schematic electrical diagram and
acoustical network of an electronic sound absorber. pl=sound
pressure in free space. M, = inertance of the air load. TAI = acousti-
cal resistance of the air load. Ms=inertance of the cone and voice
coil of the loudspeaker. fAR=acoustical resistance of the screen
covering the microphone and cone. Ms=inertance of the screen.
CAI=acoustical capacitance of the suspension system of the cone.
rAS=acoustical resistance of the cloth over the apertures in the
back plate. CAS=acoustical capacitance of the volume of the
cabinet. rAS=acoustical resistance of the sound absorbing material
in the cabinet. P2=driving sound pressure in the loudspeaker.
ps=sound pressure at the microphone.
INCIDENT
SOUND
Fic. 15. Test arrangement for the electronic sound absorber.
A, B, C, and D are test locations. A. At the face of the microphone.
B. Four inches from the face of the microphone. C. Ten inches
from the face of the microphone. D. Twenty-four inches from the
face of the microphone.
obtained. However, for this application, the system
shown in Fig. 14 is somewhat simpler in operation.
The problem in low-frequency sound absorption is to
provide an acoustical impedance of a relatively small
value so that the volume current which introduces the
sound absorption will not be limited by a high acoustical
impedance. The electronic sound absorber provides a
low acoustical impedance for terminating a dissipative
acoustical impedance. The dissipative acoustical im-
pedance is provided by the screen rAS, Ms in Fig. 14.
If a bank of electronic sound absorbers is used so that
the ultimate acoustical resistance of rAl is obtained,
then the inertance M1 becomes practically zero. Under
these conditions, the acoustical impedance of the screen
should be an acoustical resistance equal to r. i. Then,
if the sound pressure ps is zero, 100 percent absorption
is obtained.
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Fic. 16. Sound pressure reduction frequency characteristic
of the electronic sound absorber at location A of Fig. 15.
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The electronic sound absorber was placed in a plane-
wave sound field as shown in Fig. 15. The pressure
response frequency characteristics at point A at the
face of the microphone were obtained with the amplifier
turned on and off. The difference between these two
characteristics gives the reduction in sound pressure due
to the electronic sound absorber. The sound pressure
reduction frequency characteristic at point A is shown
in. Fig. 16. It will be seen that a very high order of sound
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FiG. 17. Sound pressure reduction frequency characteristics of
the electronic sound absorber at locations B, C, and D of Fig. 15.
reduction is obtained over a frequency range of more
than three octaves. It would be practically impossible
to obtain this order of sound reduction with a resonator
over even a very small fraction of an octave. The
pressure reduction frequency characteristics at various
distances from the electronic sound absorber are shown
in Fig. 17. It will be seen that a high value of sound
reduction is obtained at a considerable distance.
Applications
There are many applications for a sound absorber
which exhibits high sound reduction or high sound
absorption efficiency in the low-frequency range because
conventional sound absorbing materials exhibit very
low absorption in this range. The sound absorption
coefficients of a typical acoustical material as a function
of the frequency are shown in Table 1. It will be seen
TABLE I. Absorption coefficient of a typical sound absorbing
material, 1} inch in thickness, cemented to a wall.
Frequency 64 128 256 512 1024 2048 4096
Coefficient 0.08 0.14 0.42 0.99 0.74 0.60 0.50
that the absorption coefficient is very small in the low-
frequency range. Therefore, the sound reduction which
can be obtained in the low-frequency range is only
one or two decibels. It is in this range that the electronic
sound absorber is very efficient. It is conceivable that
the cost could be brought to a point where the electronic
sound absorber could be used as a wall treatment for a
room to provide absorption in the low-frequency range.
The other application for the system is that of a spot-
type sound reducer, that is, for reducing the sound level
over a limited space:
FIG. 18. Electronic sound absorber used in an airplane or
automobile to reduce the noise in the vicinity of the occupant's
head.
One application for the electronic sound absorber is in
the form of a sound reducer in airplanes and automobiles
where the noise level is very high in the low-frequency
region. As pointed out above, the conventional sound
absorbing material will not reduce the noise level in
this frequency region to any appreciable degree. The
electronic sound absorber can be installed on the back
of a seat as shown in Fig. 18. In this way a sound
reduction of 10 to 20 decibels can be obtained in the
low-frequency range at the ears of the passenger. There
are many other applications for a spot-type low-
frequency noise reducer in factories, shops, and offices
where the frequency noise level is high and the position
of the person is fixed. For example, the application of
LOUDSPEAKEI
oo..~MICROPHONE
Ftc. 19. Electronic sound absorber used to reduce the low-
frequency machinery noise in the vicinity of the head of an
operator.
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SOUND
wr.
FIG. 20. Applications of electronic sound absorber for reducing
the low-frequency sound output from different sound sources
and the acoustical circuit depicting the performance of these
systems. A. Sectional views of an air conditioner output. B. End
view of a generalized machine. C. Sectional views of a muffler for
an internal combustion engine. D. Acoustical circuit of systems
in A, B, and C. pi- driving sound pressure of the sound or noise
source. sAl-acoustical impedance of the source. res=acoustical
radiation resistance of the source. Ms=inertance of the source.
pi-radiation sound pressure.
an electronic sound absorber in a machine shop is
shown in Fig. 19. Here the electronic sound reducer is
located directly above the head of the operator.
The electronic sound absorber provides a system
which reduces the sound pressure over a considerable
space. This suggests applications for reducing the sound
output of sound producing systems by acoustically
short circuiting the acoustical generator. Three examples
of this application of the acoustical sound absorber are
shown in Fig. 20A, B, and C. The acoustical circuit
which applies to all three systems is shown in Fig. 20D.
The arrangement in Fig. 20A depicts the outlet
portion of an air conditioning duct. It is well known
that the sound in mid- and high-frequency ranges can
be absorbed with high efficiency by lining the duct with
acoustical sound absorbing material. However, the
sound in the low-frequency range is not absorbed with
good efficiency. As a result, the sound output in the
low-frequency range is above the tolerable point. The
sound output in the low-frequency range can be
reduced by the use of an electronic sound absorber as
shown in Fig. 20A. The sound pressure P2 at the micro-
phone and in the vicinity of the microphone is reduced
to a very low value. Under these conditions the sound
output is reduced, as can be deduced from a considera-
tion of the acoustical circuit of Fig. 20D. Measurements
upon an air duct indicated a reduction in sound output
from the duct approximately the same as the charac-
teristic of Fig. 16.
The low-frequency sound output of internal com-
bustion engines is very difficult to reduce by the means
of mufflers. Here again the electronic sound absorber
can be used to reduce the sound output in the low-
frequency range as shown in Fig. 20B. As in the preced-
ing example, the acoustical circuit of Fig. 20D illustrates
the action.
The low-frequency output of many machines is very
high and cannot be reduced to a tolerable value by the
use of sound absorbing materials. The electronic sound
absorber can be used to reduce the sound output as
shown in Fig. 20C. As in the preceding example, the
idea is to reduce the sound pressure at the sound
generators. Since the wavelength is relatively long in
the low-frequency range, the sound absorber exerts its
influence to reduce the sound pressure over a large space.
Experimental tests have indicated that a reduction
similar to the characteristics of Figs. 16 and 17 can be
Fic. 21. Three electronic sound absorbers located
in the corner of a room.
obtained. The amount of sound reduction by a single
unit depends upon the dimensions of the noise source.
In the case of large sources, several electronic noise re-
ducers may be used and thereby obtain the maximum
reduction in noise.
The electronic sound absorber can be used in the
same manner as conventional wall materials for the
absorption of sound in the low-frequency range by the
use of the system shown in Fig. 14. In order to obtain
a high acoustical resistance load the absorbers should
be mounted in the corner at the intersection of three
surfaces as shown in Fig. 21. As contrasted to the
resonant absorber with a narrow frequency range of
sound absorption, the electronic sound absorber exhibits
high sound absorption over a wide frequency range.
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