U.S. patent number 3,573,400 [Application Number 04/752,560] was granted by the patent office on 1971-04-06 for directional microphone.
This patent grant is currently assigned to Bell Telephone Laboratories, Inc.. Invention is credited to Gerhard M. Sessler, James E. West.
United States Patent |
3,573,400 |
Sessler , et al. |
April 6, 1971 |
DIRECTIONAL MICROPHONE
Abstract
A directional microphone with toroidal sensitivity
characteristics which can be selectively distorted is constructed
with an arrangement of acoustic tubes which sample an acoustic
field at a number of separated points on an inner circle and at
separated points on a concentric outer circle. The acoustic signals
from the inner points are summed in a first acoustic cavity and the
signals from the outer points are summed in a second cavity. The
first and second cavities are separated by a foil electret or other
electroacoustic transducer which produces a signal proportional to
the difference in sound pressure between them. Several of the
acoustic tubes may be adjusted to alter the shape of the
microphone's sensitivity pattern in the plane of maximum
sensitivity and the entire acoustic system is selectively
dimensioned to equalize the system's inherent frequency
response.
Inventors: |
Sessler; Gerhard M. (Summit,
NJ), West; James E. (Plainfield, NJ) |
Assignee: |
Bell Telephone Laboratories,
Inc. (Murray Hill, NJ)
|
Family
ID: |
25026806 |
Appl.
No.: |
04/752,560 |
Filed: |
August 14, 1968 |
Current U.S.
Class: |
381/357; 381/338;
381/353 |
Current CPC
Class: |
H04R
1/38 (20130101) |
Current International
Class: |
H04R
1/38 (20060101); H04R 1/32 (20060101); H04r
001/32 () |
Field of
Search: |
;179/121,121 (DIR)/
;179/1 (DIR)/ ;179/111 |
References Cited
[Referenced By]
U.S. Patent Documents
Foreign Patent Documents
|
|
|
|
|
|
|
1,003,835 |
|
Jul 1962 |
|
GB |
|
884,516 |
|
Jul 1953 |
|
DT |
|
Primary Examiner: Cooper; William C.
Assistant Examiner: Kundert; Thomas L.
Claims
We claim:
1. A directional transducer comprising, first and second
selectively dimensioned acoustic chambers, electroacoustic
transducer means for converting the acoustic signals in said
acoustic chambers into electrical signals, a first plurality of
exposed acoustic tubes opening into said first acoustic chamber and
extending radially outward therefrom to sample a sound field at a
plurality of points on a first circle exterior to said chambers,
and a second plurality of exposed acoustic tubes opening into said
second acoustic chamber and extending radially outward therefrom to
sample a sound field at a plurality of points on a second circle
exterior to said chambers, said first and said second circles being
concentric and having different radii.
2. A directional electroacoustic transducer which comprises, a
first selectively dimensioned acoustic chamber, a second
selectively dimensioned acoustic chamber, at least one
electroacoustic transducing element associated with said first and
second acoustic chambers for converting the acoustic energy in said
chambers into electrical signals, a first group of acoustically
conductive tubes with exposed ends opening into said first acoustic
chamber and extending outward from said first acoustic chamber a
first selected distance to reception points on a first circle of
radius r, and a second group of acoustically conductive tubes with
exposed ends opening into said second acoustic chamber and
extending outward from said second acoustic chamber a second
selected distance to reception points on a second circle of radius
R, where R is greater than r and where said first and second
circles are concentric and lie in the same plane.
3. A directional electroacoustic transducer as defined in claim 2
wherein a selected number of said acoustic tubes are of variable
length so as to permit adjustment of the shape of said directional
sensitivity pattern.
4. A directional electroacoustic transducer as defined in claim 2
wherein said first and second selectively dimensioned acoustic
chambers and said first and second groups of tubes are dimensioned
such that the chamber-tube systems from a Helmholtz resonator with
a resonance frequency positioned at the lower end of the frequency
range of interest.
5. A directional electroacoustic transducer as defined in claim 4
wherein said first group of acoustically conductive tubes comprise
four tubes orthogonally positioned with respect to a central axis
and wherein said second group of acoustically conductive tubes
comprise four tubes orthogonally positioned with respect to a
central axis.
6. A directional transducer comprising, an enclosed cylindrical
casing containing an electroacoustic transducer positioned to
divide said casing into first and second selectively dimensioned
acoustic cavities, means sampling the sound field exterior to said
casing at a plurality of selected points in the same plane for
supplying acoustic signals to said cavities, said means including a
first plurality of acoustic tubes opening into said first acoustic
cavity and extending radially outward from said casing, and a
second plurality of acoustic tubes opening into said second
acoustic cavity and extending radially outward from said
casing.
7. A directional microphone as defined in claim 6 wherein said
first and second plurality of acoustic tubes each comprise four
tubes orthogonally positioned about the central axis of said
cylindrical casing.
8. A directional microphone comprising, a cylindrical casing
enclosed at both ends, a circular electroacoustic transducer
element coextensive with the internal cross section of said
cylindrical casing and located so as to separate said cylindrical
casing into two selectively dimensioned acoustic chambers, a first
group of four open acoustic tubes of equal length extending
radially outward from the sides of said cylindrical casing, the
tubes of said first group being mated to said casing so as to open
into a first of said selectively dimensioned acoustic chambers and
extending outward to a plurality of selected reception points
located on a first circle, a second group of four open acoustic
tubes of equal length extending radially outward from the sides of
said cylindrical casing, the tubes of said second group being mated
to said casing so as to open into a second of said selectively
dimensioned acoustic chambers and extending outward to a plurality
of selected reception points located on a second circle, said
second circle being concentric with said first circle and having a
radius greater than the radius of said first circle.
9. A directional microphone comprising first and second selectively
dimensioned acoustic chambers spaced apart by a selected distance,
first and second electroacoustic transducers, said first transducer
being associated with said first cavity and said second transducer
being associated with said second cavity, a first plurality of
equal length acoustically conductive tubes opening into said first
acoustic chamber and extending outward to a plurality of reception
points located on a first circle in a reception plane intermediate
said first and said second chambers, a second plurality of equal
length acoustically conductive tubes opening into said second
acoustic chamber and extending outward to a plurality of reception
points located on a second circle in said reception plane, said
first and second circles being concentric, and subtractive circuit
means for electrically interconnecting said first and second
electroacoustic transducers.
10. A directional transducer as defined in claim 6 wherein the
acoustic tubes of said first plurality of tubes and the acoustic
tubes of said second plurality of tubes are of equal length and
equal length and e-ual cross section, and wherein said first and
second acoustic cavities are dimensioned to be of equal volume,
thus to achieve for said directional transducer a substantially
toroidal sensitivity pattern.
Description
This relates to electroacoustic transducers and more particularly
to a directional electroacoustic microphone with a toroidal
sensitivity pattern which can be selectively distorted.
BACKGROUND OF THE INVENTION
1. Field of the Invention
Directional microphones are commonly employed in a great variety of
audio communications systems; frequently to emphasize weak signals
in a noisy environment and to reduce the effects of reverberation.
Microphones for these purposes are currently available with
numerous diverse sensitivity patterns ranging from the
omnidirectional pattern to the long range unidirectional beam type
pattern. One such microphone which is becoming more and more
important in a great variety of recurrent communication situations
is the microphone with toroidal sensitivity characteristics; that
is, a microphone which is highly sensitive to acoustic signals
impinging approximately from within a single plane and which
rejects acoustic signals impinging from a direction normal to that
plane. Such toroidal microphones are valuable, for example, in
sound studio recording situations or in conference room telephone
arrangements where many talkers located around a single conference
table use a single telephone channel for two-way communication to a
distant point.
2. Prior Art
Prior toroidal microphones have been constructed with a pair of
mutually perpendicular pressure gradient receivers, the electrical
outputs of which are combined after a 90.degree. phase shift. Such
a toroidal microphone is disclosed in Olson, Pat. No. 2,539,671
issued Jan. 30, 1951. Olson shows two perpendicular ribbon
microphones with their outputs combined in a network including a
pair of broadband phase shifters. This arrangement has the
following properties which may be significant shortcomings in
certain situations: Firstly, as the angle of incidence of the
impinging sound wave increases away from the plane of maximum
sensitivity, the sensitivity decreases in proportion to the cosine
of the angle of incidence. Secondly, in the Olson arrangement the
phase response in the plane of maximum sensitivity is a function of
the angle of incidence of the sound wave in that plane. And
finally, the broadband phase shifter which is required by the Olson
arrangement is cumbersome, and an unnecessary source of potential
malfunction of the system.
A different arrangement which is claimed to provide a toroidal
sensitivity pattern is shown in Weingartner Pat. No. 3,201,516
issued Aug. 17, 1965. In this arrangement, an encased transducer is
exposed to an acoustic field through a number of selected acoustic
channels. However, this system does not achieve full toroidal
sensitivity and has substantial sensitivity for signals impinging
from its underside. Additionally, it provides only one
nonadjustable sensitivity pattern and is highly frequency
sensitive. That is, its sensitivity decreases in proportion to the
square of the frequency of the impinging acoustic signal.
Thus, it is an object of the present invention to overcome the
faults of prior microphone arrangements in a simple, compact
electroacoustic transducer with toroidal sensitivity
characteristics.
SUMMARY OF THE INVENTION
In accomplishing this and other objects and in accordance with the
invention, a directional second order microphone with toroidal
sensitivity characteristics is constructed with an arrangement of
exposed acoustic tubes positioned to sample a sound field at a
plurality of points on an inner circle and at a plurality of points
on a concentric outer circle. The acoustic signals gathered from
the inner points are summed in a first acoustic chamber and the
signals from the outer points are summed in a second acoustic
chamber, with the signal in the first chamber being subtracted from
the signal in the second either mechanically by exposing the two
sides of a microphone membrane to the two signals or electrically
in a differential amplifier. In accordance with two prominent
features of the invention, the length of several of the acoustic
tubes may be varied to adjust the sensitivity pattern of the
microphone to specific conference situations, and the dimensions of
the acoustic chambers are established to emphasize preselected
frequency components of the received signal and thus reduce the
frequency dependence of the sensitivity of the system. In addition,
higher even order systems may be arranged by adding additional
rings of sampling points.
BRIEF DESCRIPTION OF THE DRAWING
The invention will be more fully understood from the following
theoretical considerations and detailed description of a preferred
embodiment thereof taken in conjunction with the appended drawings
wherein:
FIG. 1 graphically depicts an array of eight pressure microphone
point sensors arranged in accordance with the invention;
Fig. 2 is a perspective view of a directional transducer utilizing
a single transducer element constructed in accordance with the
invention;
FIG. 3 shows a partially sectioned view of the transducer depicted
in FIG. 2;
FIG. 4 is a microphone constructed in accordance with an
alternative embodiment of the invention employing two separated
cavities and a pair of transducer elements with the electrical
outputs of the two transducer elements being electrically combined
in accordance with the invention; and
FIG. 5 shows the shape of the sensitivity pattern of a microphone
constructed in accordance with the invention.
THEORETICAL CONSIDERATIONS
Fig. 1 graphically portrays an arrangement of eight pressure
responsive acoustic point sensing elements located in accordance
with one embodiment of the invention so as to be highly sensitive
to sounds impinging from the x, y plane and to reject sounds
impinging normal to this plane. Such sampling elements, represented
in FIG. 1 by circles 10 through 17, may be open ends of
acoustically conductive tubes arranged in a manner to be described
more fully in conjunction with the detailed description of an
illustrative embodiment of the invention that follows. The
arrangement shown in FIG. 1 samples a sound field at eight points
in a plane, four inner points 14, 15, 16 and 17 on a circle of
radius r and four outer points 10, 11, 12 and 13 on a circle of
radius R: the circles r and R are concentric. It is to be
understood that more or less than eight sensors may be arranged in
accordance with the description that follows, for example 16
sensors may be employed, eight located on the outer circle and
eight on the inner circle. The analysis which follows is directed
to the embodiment shown in FIG. 1 and is intended as an example
only. It is also to be understood that higher even order toroidal
microphones may be constructed by sampling the sound field on
additional concentric rings of radius greater than R. In processing
the acoustic signals received at each point in accordance with the
invention, the sound pressures at the outer points are added
together by circuitry or acoustic apparatus, now shown, and their
sum is subtracted from the sum of the sound pressure at the inner
points. The resulting received total sound pressure may be
designated p(.alpha., .phi.) where .alpha. is the angle between an
arbitrarily selected line in the plane of the eight sensing
elements, in this case the line through circles 12 and 15
corresponding to the x axis, and the direction defined by the
projection into this plane of the propagation vector of an acoustic
wave shown as path A in FIG. 1. .phi. represents the angle of the
elevation of the impinging sound wave above the plane defined by
the x and y axis.
For sound waves impinging in the plane defined by the x and y axis,
that is the plane in which .phi. = 0, the sound pressure at the
outer sensors adds up to, ##SPC1##
where p.sub.0 represents the phase and amplitude of the pressure of
the undisturbed sound field at the center of the system and k is
the wave number. Using a similar relation for the sum of the sound
pressures at the inner points, there is obtained for the received
sound pressure p(.alpha., 0)= p.sub.in (.alpha., 0)- p.sub.out
(.alpha., 0), ##SPC2## where the common factor exp(-i.omega.t) has
been suppressed. For sound waves impinging at an angle .phi. above
the plane of the sampling points, k must be replaced by k' =k cos
.phi.. Thus, equation (2) yields, ##SPC3##
This equation shows that the phase of the received sound pressure
is either equal or opposite to that of the undisturbed sound
pressure at the center of the system.
Using a series expansion for the terms in Equation (3) there is
obtained for kR cos .phi. less than or approximately equal to 1,
##SPC4##
If kR cos .phi.<< p(.alpha., .phi.) is approximately given
by
At low frequencies, the received sound pressure is therefore
proportional to,
k.sup.2 =(.omega./c).sup.2, (6)
and to (cos .phi.) .sup.2 as expected for a second order gradient
microphone. In this range, the amplitude of the sound pressure is
independent of .alpha., and the phase is independent of .alpha. and
.phi. and equal to the phase of the undisturbed sound pressure.
Thus, point sensors arranged as shown in FIG. 1 provide an
electroacoustic transducer with toroidal sensitivity
characteristics in which the sensitivity decreases in proportion to
the square of the cosine of the angle between the direction of
incidence of a sound wave and the plane of the sensors. The
microphone is uniformly sensitive for sound waves impinging in the
plane of the sensors and the phase is constant in all directions.
Note, however, that the microphone described above is very
frequency dependent; that is, the sensitivity increases in
proportion to the square of the frequency of the signal being
received (Equation 6). While this response is to be expected in a
second order pressure gradient microphone, it would be preferable
if this effect were eliminated.
In accordance with a feature of the present invention, this
frequency dependence can be substantially reduced by selectively
dimensioning the various elements of the acoustic system designed
to implement the principles discussed above. Thus, in one
alternative implementation, the sound field is sampled at eight
points, as indicated above, by eight acoustic tubes, such as tubes
20 through 27 in FIG. 2. In this arrangement, the acoustic signals
received at the four outer tubes are directed to a first acoustic
cavity and the signals received at the four inner tubes are
directed to a second acoustic cavity. This entire acoustic system
preferably is selectively dimensioned to be acoustically inversely
frequency dependent thus to counteract the normal frequency
dependency of the microphone.
In order to achieve this acoustic inverse frequency dependence,
each acoustic cavity and its associated tubes are made to form a
Helmholtz resonator or an acoustic low pass filter. An analysis of
such systems based on well known electrical analogies will be found
in Morse, Vibration and Sound, 2nd edition, McGraw Hill Book
Company, 1948 at pages 233 to 237. As indicated by Morse, the
pressure in the cavity of a tube-cavity system is maximum at the
Helmholtz resonance frequency given by .omega..sub.0 =
c(.sup.S/.lambda. V).sup.1/2 , where .lambda. and S are the length
and cross section of the tube, V is the volume of the cavity, and c
is the velocity of sound. The pressure drops off in proportion to
.omega..sup..sup.-2 at higher frequencies. By adjusting the
dimensions of the system so as to place the resonance frequency at
the lower end of the frequency range of interest, the normal
frequency dependency of the microphone is counteracted. At the same
time, the overall signal to noise ratio of the system is improved
since the signal is increased at low frequencies, in the vicinity
of the resonance frequency, where electronic noise is highest.
In order to achieve the required equal resonance frequencies, in a
two cavity system, such as that shown in FIGS. 2 and 3, the two
cavities with their tubes must form acoustically equivalent systems
over the entire frequency range of interest. To ensure this
condition, the tubes feeding both cavities are preferably of equal
length and the cavities are preferably of the same volume. A
preferred manner of accomplishing these conditions will be
described in conjunction with the detailed description of a
preferred embodiment below.
In accordance with a second feature of the invention, a system
having a distorted toroidal sensitivity pattern, for example a
pattern with greater sensitivity for .alpha. = 0 and .alpha. =.pi.
than for
may be obtained by changing the position of the two outer sensors
10 and 12. If the distance between the sensors 10 and 12 is
extended from 2R to 2R, where R is greater than R, then Equation
(3) assumes the form; ##SPC5##
Series expansion yields for kR cos .phi.<< ##SPC6##
This equation is equivalent to Equation (5)
For all other azimuth angles, p(.alpha., .phi.)/p.sub.0 from
Equation (8) is greater than the values obtained from Equation (5)
and reaches maximum for .alpha. = 0 and .alpha. = .pi.. For each
angle .alpha., the dependence on (cos .phi.).sup. 2 is, however,
preserved.
DETAILED DESCRIPTION
Referring to the drawing, the microphone shown in FIG. 2 embodies
the principles of the invention in a compact, durable device which
is suitable for use in a conference room telephone arrangement. The
microphone samples the surrounding sound field at eight selected
points in the same plane, four on an inner circle and four on a
concentric outer circle in accordance with the theoretical
discussion above.
The sampling is accomplished with eight radially extending acoustic
tubes, 20 through 27, which feed acoustic signals into a
cylindrical body member 28. The tubes 20 through 27 are all of
equal length. However, an upper set of tubes, including tubes 20,
21, 22 and 23 are secured through apertures in the upper half of
the lateral portion of the cylindrical casing 28 so that only a
selected relatively short segment of the tube protrudes from the
cylindrical casing. These upper tubes are orthogonally positioned
with regard to the central axis of the casing 28 are are secured to
the casing by an acoustically tight connection. The tubes of the
upper set may be angled downward slightly so that their ends fall
in a single plane. As will be described in more detail in
conjunction with FIG. 3, tubes 20, 21, 22 and 23 feed acoustic
signals to a first selectively dimensioned acoustic cavity within
the casing 28.
The lower set of tubes 24, 25 26 and 27 are secured through
apertures in the lower portion of the cylindrical casing 28 so that
a relatively long segment of each tube is exposed beyond the wall
of casing 28. These tubes are also orthogonally disposed about the
central axis of the cylindrical casing. In the case shown in FIG.
2, the longer tubes are positioned in holes directly above the
shorter tubes. Thus, tube 20 is positioned above tube 25, and so
forth. It is to be understood, however, that either set of tubes
could be rotated, say by 45.degree. , about the central axis of the
cylinder and still produce a toroidal sensitivity pattern in
accordance with the invention. Tubes 24, 25, 26 and 27 are angled
slightly upward so that their free ends fall in the plane defined
by the ends of tubes 20, 21, 22 and 23.
In accordance with a feature of the invention, a number of tubes
may be adjusted to alter the points at which they sample the sound
field. Thus, for example, tubes 24 and 26 may include a threaded
portion 29 which is mated with threading on the interior of the
appropriate aperture. The length of threaded portion 29 is selected
according to the degree of variation in the sensitivity pattern
desired. To adjust the pattern, tubes 26 and 24 are rotated so that
they are advanced into or withdrawn from the cylindrical body 28,
thus altering the distance between the exposed ends of tubes 26 and
24. The effective alteration of the toroidal sensitivity pattern
caused by this variation is identical to the result achieved by
altering the position of receivers 10 and 12 in FIG. 1, which has
already been described in conjunction with the theoretical
discussion above.
FIG. 3 shows the microphone assembly of FIG. 2 in partially
sectional view exposing the interior of casing 28. The casing
includes two acoustic cavities 31 and 32 separated by an
electroacoustic transducer element 33. The casing 28 comprises a
cylindrical upper section 34 mated to a cylindrical lower section
35. The upper cylindrical section is enclosed by a contiguous
circular top element 36 which may be formed with the upper
cylindrical section 34 from a single homogeneous piece of material.
The lower cylindrical section 35 is enclosed by a threaded circular
bottom plate 38 which mates with a threaded portion 40 of the
cylindrical lower section. These threads are preferably very fine
so that as the bottom plate is rotated it advances slowly into the
lower cylindrical section 35 reducing the size of the lower
acoustic cavity 32. Plate 38 is equipped with a slotted disc 45 to
facilitate rotation. This adjustment permits the volume of the
upper and lower acoustic cavities to be equalized so as to provide
a low pass filtering effect and thus reduce the frequency
dependence of the system in accordance with the theoretical
considerations discussed above.
Tubes 20, 21, 22 and 23, which open into the upper cavity 31, are
secured to the upper cylindrical section 34 with only a relatively
short length extending out beyond the wall of the casing. Thus,
these tubes sample the sound field at four selected points on an
inner circle as shown in FIG. 2. Since, as indicated in the
theoretical discussion above, it is desirable that all tubes
entering the cylindrical casing be of equal length, a substantial
portion of each of the tubes 20, 21, 22 and 23 remain in the
interior of cavity 31. These tubes tubes may preferably be arranged
as shown, with tubes 21 and 23 lying adjacent to plate 36, tube 23
being forward of tube 21, and tubes 20 and 22 being located below
tubes 21 and 23.
Tubes 24, 25, 26 and 27 open into the lower cavity 32 with their
interior ends approximately abutting the interior wall of
cylindrical section 35. As indicated above, tubes 24 and 26 include
a threaded portion 29 which permits them to be advanced into cavity
32 thus altering the position at which the sound field is sampled
and hence the sensitivity characteristic of the microphone.
The circular electroacoustic transducer element 33 may be any one
of many such transducers which are well known in the microphone
arts. However, it has been found that an electrostatic transducer
of the foil electret type described in Pat. No. 3,118,022 issued
Jan. 14, 1964 to G. M. Sessler and J. E. West is particularly
desirable. A transducer of this type comprises a thin foil layer 37
superimposed upon a solid sheet of dielectric material 39 which is
prepolarized in an electrostatic field at an elevated temperature.
The foil and dielectric layer is stretched across a perforated
backplate 41 which is insulated from the casing by an annular
section of electrical insulating material 45.
Casing 28 may be specifically designed to accommodate such a foil
electret transducer. Thus, the lower extremity of the cylindrical
upper section 34 of the casing is found to include an annular
channel 42 and the upper extremity of the cylindrical lower section
35 includes an arced annular nub 43. The two sections are held
tightly together, e.g., by adhesive bonding or by mechanical means.
Additionally, the cylindrical upper section includes an annular
ledge 44 positioned slightly away from the annular channel so that
the circular backplate 41 can be fitted below the ledge 44 with the
foil and dielectric 37 and 39 stretched over the backplate to be
retained between the annular channel 42 and the annular nub 43. The
microphone's electrical output is then taken between the
cylindrical casing 28 and the perforated backplate 41. This
arrangement permits simple assembly of the entire microphone
unit.
A toroidal microphone, constructed in accordance with a second
embodiment of the invention is shown in FIG. 4. The principle of
this microphone is theoretically identical to that of the
microphone described above. The physical embodiment differs in that
two separated acoustic chambers 46 and 47 are employed in
conjunction with a pair of electroacoustic transducer elements 61
and 62, one associated with each cavity. The electrical output of
the first transducer 61 is subtractively combined with the output
of the second transducer 62 in an appropriate differential
amplifier, now shown, of a type well-known in the electronic art.
Alternatively, oppositely polarized air gap condensers or electret
transducers may be employed in which case their outputs will be
added. Like the prior embodiment, the present system comprises
eight acoustic tubes 48 through 55 arranged in sets of four. Again
in this arrangement, all tubes are of equal length. The first set
of tubes, comprising tubes 48, 49, 50 and 51, are angled
sufficiently away from the vertical to sample the sound field at
four points on an inner circle, while the second set including
tubes 52, 53, 54 and 55, are angled to sample the field at four
points on an outer circle.
Unlike the prior arrangement, however, the acoustic tubes 48
through 55 are closed off at their ends by small circular plates 56
and sound energy is permitted to enter the tubes only through a
plurality of ports 60 cut in the lateral portion of each tube at
its outer extremity. This arrangement is required since the first
set of tubes are directed generally upward and the second set of
tubes are directed generally downward so that, with simple open
tubes, one or the other microphone is always more directly exposed
to the transmitting sound source. The present arrangement
eliminates this difficulty by requiring sound pressure from all
directions to enter the tubes approximately uniformly.
FIG. 5 shows the toroidal form of the sensitivity pattern of a
microphone constructed in accordance with the invention. As
indicated in the course of the discussion above, this pattern may
be altered by rearranging the points at which the sound field is
sampled.
It is to be understood that the above described arrangements are
merely illustrative of application of the principles of the
invention. Other arrangements may be devised by those skilled in
the art without departing from the spirit and scope of the
invention. For example, the arrangement shown in FIG. 2 may be
altered so that the two acoustic cavities are contained within an
annular ring shaped casing. The first set of acoustic tubes then
protrude outward from the ring's exterior and the second set extend
into the open space in the ring's center.
* * * * *