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Claims  |
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We claim:
1. A loudspeaking telephone station comprising: microphone apparatus for
converting audible sounds into electrical signals, a loudspeaker for
converting electrical signals into audible sounds, and a speech network
that electrically connects the microphone apparatus and the loudspeaker to
a telephone line, the microphone apparatus having a directional polar
response characteristic such that the microphone apparatus is more
sensitive to sounds emanating from one direction than from other
directions,
characterized in that:
the polar response characteristic of the directional microphone includes a
major lobe, one or more side lobes, and nulls between pairs of lobes, said
major lobe having substantially greater amplitude than any of the side
lobes;
the loudspeaker is positioned in the null of said polar response
characteristic that resides between the major lobe and an adjacent side
lobe;
the loudspeaker is aimed in a first direction and the microphone apparatus
is positioned so that its major lobe is aimed in a second direction, said
first and second directions being approximately orthogonal; and
the microphone apparatus comprises a first-order-gradient (FOG) microphone
having sound ports on opposite sides of a common diaphragm that are
separated by distance "d," the FOG microphone having a free field
directivity pattern, D(.theta.), given by:
##EQU2##
where: B>1.
2. The loudspeaking telephone station of claim 1 wherein the speech network
includes a hybrid for connecting the microphone apparatus to a telephone
line via a transmit circuit and for connecting the telephone line to the
loudspeaker via a receive circuit, the speech network further including a
first echo canceler that responds to signals in the receive circuit and
generates an estimate of an acoustic echo that couples between the
loudspeaker and the microphone, the transmit circuit including subtracting
means for combining the estimate of the acoustic echo with the output
signals from the microphone apparatus; whereby the acoustic echo between
the loudspeaker and the microphone apparatus is reduced.
3. The loudspeaking telephone station of claim 2 wherein the speech network
further includes a second echo canceler that responds to signals in the
transmit circuit and generates an estimate of an electrical echo that
couples between the transmit and receive circuits via the hybrid, the
receive circuit including subtracting means for combining the estimate of
the electrical echo with signals received from the hybrid; whereby
acoustic and electrical echos are reduced.
4. The loudspeaking telephone station of claim 1 wherein the FOG microphone
resides within a housing that includes openings at opposite ends thereof,
the structure including a pair of channels, each channel extending between
one sound port of the FOG microphone and one of the openings; whereby the
housing effectively increases the distance between the sound ports for
improved sensitivity.
5. The loudspeaking telephone station of claim 1 wherein the FOG microphone
is embedded in a baffle which is coplanar with the microphone's diaphragm
and extends around its perimeter; whereby the baffle effectively increases
the distance between the sound ports for improved sensitivity.
6. A loudspeaking telephone station comprising: microphone apparatus for
converting audible sounds into electrical signals, a loudspeaker for
converting electrical signals into audible sounds, and a speech network
that electrically connects the microphone apparatus and the loudspeaker to
a telephone line, the microphone apparatus having a directional polar
response characteristic such that the microphone apparatus is more
sensitive to sounds emanating from one direction than from other
directions,
characterized in that:
the polar response characteristic of the directional microphone includes a
major lobe, one or more side lobes, and nulls between pairs of lobes, said
major lobe having substantially greater amplitude than any of the side
lobes;
the loudspeaker is located at the center of the telephone station and
positioned in the null of said polar response characteristic that resides
between the major lobe and an adjacent side lobe;
the loudspeaker is aimed in a first direction and the microphone apparatus
is positioned so that its major lobe is aimed in a second direction, said
first and second directions being approximately orthogonal;
the microphone apparatus comprises four first-order-gradient (FOG)
microphones that are positioned around an outside surface of the telephone
station, each FOG microphone including sound ports on opposite sides of a
common diaphragm that are separated by distance "d," and each FOG
microphone is characterized by a free field directivity pattern,
D(.theta.), that is given by:
##EQU3##
where: B>1.
7. A loudspeaking telephone station comprising: microphone apparatus for
converting audible sounds into electrical signals, a loudspeaker for
converting electrical signals into audible sounds, and a speech network
that electrically connects the microphone apparatus and the loudspeaker to
a telephone line, the microphone apparatus having a directional polar
response characteristic such that the microphone apparatus is more
sensitive to sounds emanating from one direction than from other
directions,
characterized in that:
the polar response characteristic of the directional microphone includes a
major lobe, one or more side lobes, and nulls between pairs of lobes, said
major lobe having substantially greater amplitude than any of the side
lobes;
the loudspeaker is positioned in the null of said polar response
characteristic that resides between the major lobe and an adjacent side
lobe;
the loudspeaker is aimed in a first direction and the microphone apparatus
is positioned so that its major lobe is aimed in a second direction, said
first and second directions being approximately orthogonal; and
the microphone apparatus comprises three second-order-gradient (SOG)
microphones that are positioned around an outside surface of the telephone
station and, each SOG microphone comprising a pair of collinear
first-order-gradient (FOG) microphones.
8. A loudspeaking telephone station comprising: microphone apparatus for
converting audible sounds into electrical signals, a loudspeaker for
converting electrical signals into audible sounds, and a speech network
that electrically connects the microphone apparatus and the loudspeaker to
a telephone line, the microphone apparatus having a directional polar
response characteristic such that the microphone apparatus is more
sensitive to sounds emanating from one direction than from other
directions,
characterized in that:
the polar response characteristic of the directional microphone includes a
major lobe, one or more side lobes, and nulls between pairs of lobes, said
major lobe having substantially greater amplitude than any of the side
lobes;
the loudspeaker is positioned in the null of said polar response
characteristic that resides between the major lobe and an adjacent side
lobe;
the loudspeaker is aimed in a first direction and the microphone apparatus
is positioned so that its major lobe is aimed in a second direction, said
first and second directions being approximately orthogonal; and
the microphone apparatus comprises a pair of collinear first-order-gradient
(FOG) microphones separated by distance d.sub.1, each FOG microphone
generating an electrical signal that is connected to a different input of
a subtracting circuit, one of the electrical signals being delayed by a
time interval .tau.; whereby a second-order-gradient polar response
characteristic is achieved.
9. The loudspeaking telephone station of claim 8 wherein the time interval
.tau.=d.sub.1 /c, where c is the speed of sound in air.
10. The loudspeaking telephone station of claim 8 further including means
for reversing the inputs to the subtracting circuit; whereby the polar
response characteristic is reversible upon activation of the reversing
means. |
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Claims |
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Description |
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TECHNICAL FIELD
This invention relates to a loudspeaking telephone station, and more
particularly to the use of one or more directional microphones therein.
BACKGROUND OF THE INVENTION
Loudspeaking telephones (also known as speakerphones or hands-free
telephones) are ones that locate a microphone and a loudspeaker outside of
a conventional telephone handset in somewhat close proximity to each
other, thereby creating the opportunity for sustained oscillation to
occur. This situation, known as singing, is often encountered in public
address systems when signals from a loudspeaker are coupled to an
associated microphone. Loudspeaking telephones generally include
amplifiers in both the transmit and receive channels of a telephone set as
well as a hybrid circuit that interconnects the transmit and receive
channels to a telephone line. Although the hybrid circuit couples most of
the transmitted signal energy to the telephone line, a portion, known as
hybrid echo, finds its way back into the receive channel. In a similar
manner a portion of the acoustic energy emanating from the loudspeaker is
picked up by the transmitting microphone and is known as acoustic echo.
Thus a loop is created that includes the transmit channel and the receive
channel. They are coupled by hybrid echo at one end and by acoustic echo
at the other. Oscillation occurs when the net gain around the loop exceeds
unity (0 dB).
Perhaps the earliest technique used to circumvent the oscillation problem
was the so-called "push-to-talk" system. In its normal state the transmit
channel is disabled and the receive channel is enabled. When a user wants
to talk he depresses a manual switch to enable the transmit channel and
simultaneously disable the receive channel. Oscillation can never occur
because the transmit and receive channels are never on at the same time.
An improvement in the push-to-talk system came when the manual switch was
replaced by circuitry that detected speech energy at the transmitter which
thereafter enabled the transmit channel and disabled the receive
channel--a technique known as voice switching. A refinement of the
voice-switched system came with the inclusion of circuitry to compare the
magnitude of the transmit and receive signals and enable the loudest
talker to dominate. Apart from the dubious wisdom of rewarding such
behavior, there is the problem of losing a syllable or two during the time
that the direction of transmission is being reversed. Recognizing the
desirability of full-duplex operation in a loudspeaking telephone (i.e.,
simultaneous conversation in two directions), other techniques are sought
that reduce coupling between the loudspeaker and the microphone.
U.S. Pat. No. 4,658,425 discloses a microphone actuation control system
such as used in the Shure ST 3000 Teleconferencing System. In this system,
three first-order-gradient (FOG) microphones, each having a heart-shaped
(cardioid) polar response pattern, share a common housing with a
loudspeaker. Each of the microphones faces outward so that the direction
of maximum sensitivity emanates radially from the center of the housing.
The overall pattern provided by the three microphones allows full room
(360.degree.) coverage, although normally only one microphone is on.
Unfortunately, manufacturing variations among cardioid microphones, as
well as the telephone housings that hold them, lead to the creation of
side lobes in the polar response pattern. (Although other lobes may be
defined, for the purposes of the present invention, all lobes other than
the major lobe are designated "side" lobes.) Such unintended side lobes
indicate an increased responsiveness to sounds coming from the directions
toward which the side lobes point. Frequently this direction is where the
loudspeaker is located, and thus the likelihood of sustained oscillation
is increased.
Loudspeaking telephones also suffer from reverberation (barrel effect) in
which the microphone picks up non-direct speech coming from reflections of
direct speech from a wall or ceiling. Sounds emanating from the receiving
loudspeaker are similarly reflected and picked up by the microphone and
can create a reverberant echo from the far-end talker. U.S. Pat. No.
4,629,829 discloses a full-duplex speakerphone that uses an adaptive
filter (echo canceler) to reduce acoustic coupling. Echo cancelers
mitigate echos by generating an estimate of the echo and then substracting
the estimate from the signal corrupted by the echo. However, echo
cancelers are only useful over a limited signal range and provide the
greatest benefit when the acoustic coupling between microphone and
loudspeaker is minimized.
It is therefore desirable to configure a loudspeaking telephone station in
a manner that provides a stable polar response pattern of its associated
microphone(s) with respect to manufacturing variations.
Additionally, it is desirable to provide a loudspeaking telephone station
capable of full-duplex operation in locations where reverberation and room
noise exist.
SUMMARY OF THE INVENTION
A loudspeaking telephone station comprises at least one directional
microphone and a loudspeaker mounted in a common housing. The polar
response pattern of the directional microphone includes a major lobe, one
or more side lobes, and nulls between pairs of lobes; where the nulls
represent substantial decreases in microphone sensitivity when compared
with adjacent side lobes. Further, the loudspeaker is positioned in the
null of the polar response pattern between the major lobe and an adjacent
side lobe to substantially reduce acoustic coupling between the
loudspeaker and the microphone.
In an illustrative embodiment of the invention, a first echo canceler is
used to further reduce acoustic coupling between the loudspeaker and the
microphone, and a second echo canceler used to reduce electrical coupling
across the hybrid circuit. Positioning the loudspeaker and microphones in
accordance with the invention yields full-duplex operation in a
loudspeaking telephone station.
The features and advantages of the present invention will be more fully
understood where reference is made to the detailed description and the
drawing.
BRIEF DESCRIPTION OF THE DRAWING
FIG. 1 discloses a pressure microphone element having an omnidirectional
polar response characteristic;
FIG. 2 discloses a first-order-gradient microphone element such as used in
the present invention;
FIG. 3 discloses a first-order-gradient microphone element having a
restriction in one of its sound ports;
FIG. 4 illustrates the frequency response of the microphone shown in FIG.
2;
FIG. 5 illustrates, in table form, characteristics associated with the
microphone of FIG. 3 for different values of B;
FIG. 6 discloses a second-order-gradient microphone comprising a pair of
first-order-gradient microphones;
FIG. 7 illustrates the frequency response of the second-order-gradient
microphone shown in FIG. 6;
FIG. 8 illustrates the polar response characteristic of the
second-order-gradient microphone shown in FIG. 6;
FIG. 9 discloses a second-order-gradient microphone having improved
sensitivity through the use of baffles;
FIG. 10 discloses a perspective view of a preferred structure for housing a
first-order-gradient microphone element;
FIG. 11 shows a top view of the housing of FIG. 10;
FIG. 12 shows a cross-section of the housing of FIG. 10;
FIG. 13 discloses a top view of a teleconferencing unit using
first-order-gradient microphones in accordance with the invention;
FIG. 14 discloses of front view of the teleconferencing unit shown in FIG.
13;
FIG. 15 discloses a top view of a teleconferencing unit using
second-order-gradient microphones in accordance with the invention;
FIG. 16 discloses of front view of the teleconferencing unit shown in FIG.
15;
FIG. 17 discloses a perspective view of a speakerphone adjunct device using
a second-order-gradient microphone in accordance with the invention;
FIG. 18 is a top view of the speakerphone adjunct device of FIG. 17
illustrating its associated directivity pattern;
FIG. 19 discloses a perspective view of a loudspeaking telephone using a
first-order-gradient microphone in accordance with the invention;
FIG. 20 is a top view of the loudspeaking telephone of FIG. 19 illustrating
its associated directivity pattern; and
FIG. 21 discloses, in block diagram form, the significant functional
components of a speech network for a loudspeaking telephone station that
includes echo cancelers.
DETAILED DESCRIPTION
GENERAL
Pressure Microphones
Single port microphones are capable of sensing instantaneous sound pressure
at their input sound port and producing an electrical output voltage
signal corresponding to the magnitude of the sound pressure. Such
microphones are known as "pressure microphones" and are generally
constructed as shown in FIG. 1. Sound port 101 admits sound into
microphone 100 which interacts with one side of diaphragm 103 to produce
an electrical voltage. The other side of diaphragm 103 resides in a closed
cavity whose volume affects the compliance of the diaphragm. Pressure
microphones are equally responsive to sounds coming from any direction
and, therefore, their response patterns are omnidirectional. FIG. 5
discloses the omnidirectional response pattern of the pressure microphone
along with other selected characteristics associated with it. (The
information in FIG. 5 was compiled using data in the Knowles Electronics,
Inc. Technical Bulletin, TB-21: "EB Directional Hearing Aid Microphone
Application Notes.")
First-Order-Gradient Microphones
Gradient microphones are those which achieve a directional polar response
characteristic by measuring the differential pressure on opposite sides of
one or more diaphragms. FIG. 2 discloses a first-order-gradient (FOG)
microphone 200 having input sound ports 201, 202 positioned on opposite
sides of diaphragm 203. The sound ports are separated by distance "d"
which represents the distance that a sound wave must travel around the FOG
in going from one sound port 201 to the other 202. Movements of diaphragm
203 are converted into voltages at the output of the microphone. The
magnitude of the voltage output of the FOG microphone is a function of the
difference in sound pressure on the opposite sides of diaphragm 203. As
distance "d" becomes smaller and smaller, so too does the output voltage
from the FOG. Recall that the velocity of sound in air at 70 degrees
Fahrenheit is 1128 feet per second, so that a f=2250 Hz audible signal has
a wavelength of about six inches. Thus, even small separation distances
provide sufficient phase difference between the sound ports 201, 202 so
that the FOG microphone has a bidirectional polar response pattern such as
shown in FIG. 5. Note that the polarity of the output voltage is
determined by the particular side of the diaphragm that is first impinged
upon by the moving wavefront. Note also that the FOG microphone is
unresponsive to sounds coming from certain directions that are known as
nulls. This property is of use in the present invention. A FOG microphone
element, suitable for use in connection with the present invention, is the
WM-55A103 manufactured by the Panasonic division of Matsushita Electric
Corp.
The spatial separation "d" between the sound ports leading to opposite
sides of the diaphragm 203 may be varied. The pressure gradient .DELTA.p,
in the far-field, has the following relationship to "d".
.DELTA.p.alpha. sin (1/2kd cos .theta.) (1)
where:
k=(2.pi.f)/c;
.theta.=polar orientation of the impinging wavefront with respect to the
major axis of the microphone; and
c=wave velocity.
Equation (1) may be simplified for small values of kd to become:
.DELTA.p.alpha.1/2kd cos .theta. (2)
The sensitivity or frequency response of a FOG microphone [equation (1)],
for the direction .theta.=0.degree., is shown in FIG. 4. It is known that
the frequency response and the directivity pattern may be changed by
altering the gradient microphone itself. For example, acoustic resistance
R.sub.a may be introduced into one of the sound ports 302 (see FIG. 3)
leading to diaphragm 303 of the FOG microphone. Such resistance alters
both the directivity pattern and the frequency response.
More generally, the directivity pattern D(.theta.) associated with FOG
microphones operating in the far field, and where kd<1 is given by the
following relationship:
##EQU1##
In equation (3),.eta. is the density of air, V is the volume of the
acoustic cavity behind the diaphragm, and C.sub.a is the acoustic
compliance (similar to capacitance) between diaphragm and R.sub.a. From
equation (3), a cardioid response is achieved when B is set equal to 1,
which is to say that the time constant R.sub.a C.sub.a is set equal to the
time it takes for a sound wave to travel distance "d." FIG. 5 illustrates
such a cardioid pattern as well as other characteristics of this
particular FOG microphone. Another popular shape is known as a
supercardioid. It is obtained when d, R.sub.a and V are adjusted such that
B is set equal to the square root of 3. Further, by increasing the value
of B to 3, a hypercardioid directivity pattern is created. Each of the
selected microphone configurations, shown in FIG. 5, has its own set of
characteristics such as: (i) the location (in degrees) of its null, (ii)
distance factor-a multiplier indicating how many times more than the
distance from a pressure microphone that a directional microphone has the
same signal-to-random incident noise ratio. (iii) front-to-back response
ratio etc.
Microphone elements having cardioid directivity patterns are sometimes used
in hands-free telephony and are commercially available. One drawback to
the use of cardioid microphones is their reduced signal-to-electrical
noise performance at low frequencies when compared to a pressure
microphone. However, the directivity of the cardioid microphone provides
better signal-to-acoustic noise performance than a pressure microphone
since it is less sensitive to sounds emanating from sources other than the
desired direction. Indeed, FIG. 5 indicates that it is 4.8 dB less
sensitive to random incident energy than the pressure (omnidirectional)
microphone. Another drawback to the use of cardioid microphones is
illustrated in FIG. 5. The cardioid microphone has a null located at
180.degree. which exists only as long as B=1. Since the magnitude of B is
influenced by a number of factors (see equations immediately following
equation 3), and since changes in B cause a lobe to form exactly at the
180.degree. point, reliance on this null is undesirable. Referring once
again to the polar response patterns shown in FIG. 5, it can be observed
that when a side lobe already exists, the direction of the null residing
between the major lobe and the adjacent side lobe does not vary
appreciably as the magnitude of B changes (note the variation in the
position of the null from the supercardioid pattern to the bidirectional
pattern). Accordingly, this particular null is most useful in reliably
reducing the acoustic coupling between the loudspeaker and the
microphone(s) when faced with manufacturing and other variations. It must
be noted that patterns in FIG. 5 hold for far-field behavior where wave
amplitudes are constant. While this isn't exactly the case for sound
coming from a near-field loudspeaker, the same qualitative results are
applied. As noted earlier, although various polar response patterns
include a lobe which might be referred to as a "back" lobe, all lobes
other than the major lobe are designated "side" lobes.
Second-Order-Gradient Microphones
Second-order-gradient (SOG) microphones provide greater directivity than
FOGs, and are used in applications that require substantial ambient noise
rejection (i.e., noisy rooms, automobiles, etc.). SOGs, because of their
gradient nature, work in the same general manner as FOGs (i.e., they
achieve directivity by responding to differential sound pressure).
Second-order systems are typically formed by electrically subtracting the
signals from two spatially separated FOGs. The subtraction is shown more
clearly in the diagram of FIG. 6 which discloses FOG microphones 200-1 and
200-2 separated by spatial distance d.sub.1, each including a separation
distance d between its sound ports. Time delay circuit 220 provides .tau.
seconds of delay for electrical signals passing through it, but does not
otherwise change the signals. Differential amplifier 230 subtracts the
delayed electrical signal of microphone 200-2 from the electrical signal
of microphone 200-1 to produce an output signal. When the time delay .tau.
is equal to d.sub.1 /c, the directivity pattern shown in FIG. 8 is
produced. It is noted that the direction of the FIG. 8 response pattern
can be reversed by removing delay element 220 from the output of
microphone 200-2 and inserting it into the output of microphone 200-1.
Also, the polarities shown in differential amplifier 230 of FIG. 6 must be
reversed. This is achieved by actuating contacts K1-K4, or an equivalent
operation. Suitable networks for supplying time delay .tau. are well
known; one such network is shown in FIG. 6 of U.S. Pat. No. 4,742,548
which is hereby incorporated by reference.
To better understand the relationship between sensitivity and bandwidth it
is useful to introduce the mathematical definition of the far-field
sensitivity of a SOG microphone, which is proportional to:
D(.theta.)=sin (1/2kd.sub.2 +1/2kd.sub.1 cos .theta.) sin (1/2kd cos
.theta.) (4)
where:
d.sub.2 =c.tau..
The sensitivity of a SOG microphone is seen to increase with d, d.sub.1 and
.tau.. Unfortunately, as d and d.sub.1 are increased to allow greater
sensitivity, the bandwidth of the SOG microphone is decreased. Owing to
the reciprocal relationship between sensitivity and bandwidth, parameters
d and d.sub.1 must be carefully selected in accordance with design
requirements of the SOG microphone unit. For example, referring to FIG. 7,
if a bandwidth from 0.3-3.3 kHz is desired, it is advantageous to position
the maximum of the unequalized frequency response at 3.3 kHz (i.e., for
.tau.=d.sub.1 /c, set f.sub.0 .perspectiveto.c/4d.sub.1 =3.3 kHz). The
following values for d, d.sub.1, and .tau. are employed in the preferred
embodiment of the present invention: d=0.029 meter, d.sub.1 =0.043 meter,
and .tau.=d.sub.1 /c=0.000125 second. With these values, the frequency
response characteristic of FIG. 7 and the polar response characteristic of
FIG. 8 are achieved. However, different values for d, d.sub.1, and .tau.
modify these characteristics in such a way that when sensitivity is
increased, bandwidth is decreased.
Although sensitivity and bandwidth are variables in SOG microphone designs
(i.e., they may be changed to meet design needs), one characteristic
remains unchanged, namely the location of its nulls at .+-.90.degree. from
the principal axis of the microphone. This remains strictly true in the
near-field over only part of the frequency band. In accordance with the
invention, these are the nulls adjacent to the major lobe of its polar
response characteristic, and which may be verified by setting
.theta.=.+-.90.degree. in equation (4). Since these nulls are
perpendicular to the axis of the major lobe, it is possible to reverse the
direction of the response pattern while maintaining the loudspeaker in the
null between its major lobe and an adjacent side lobe. This feature is
exploited in the teleconferencing system of FIG. 15 which provides
360.degree. coverage using only three SOG microphones, each with a
reversible major lobe.
Referring now to FIG. 9, there is disclosed a configuration that
illustrates the positioning of FOG microphones 200-1, 200-2 used in
constructing a SOG microphone. Baffles 205-1, 205-5 surround the FOG
microphones and provide an increase in the distance that the wavefront of
an acoustic signal must travel, around the baffle, in going from one side
of the FOG microphone to the other. This distance "d" (shown in FIG. 6) is
a parameter that affects the sensitivity, the frequency, and the polar
response characteristic of the associated microphone. FOG microphones
200-1, 200-2 are maintained in a particular orientation by support device
215. The FOG microphones are separated by spatial distance d.sub.1 which
is carefully selected to achieve a particular set of design requirements
as discussed above. A more complete discussion of this particular
unidirectional second-order-gradient microphone is presented in U.S. Pat.
No. 4,742,548.
FIG. 10 discloses a low profile housing 110 for a FOG microphone element
that effectively extends the distance "d" between sound ports of the FOG
microphone element contained therein. This rectangular block structure
(housing 110) is molded from rubber or other suitable compliant material
and replaces each baffle 205-1, 205-2 shown in FIG. 9. Housing 110 is a
cost effective way to capture and seal the microphone unit upon insertion
without adhesives. It includes openings 111, 112 at opposite ends
connected by a pair of channels 113, 114 (see FIG. 12), each channel
extending between one sound port of the inserted microphone and one of the
openings. FIG. 11 is a top view of housing 110 that illustrates its
general shape, while FIG. 12 provides a cross-section view of the
microphone/housing assembly, illustrating the interrelationship of housing
110, FOG microphone 200, channels 113, 114 and openings 111, 112. In
various applications, SOG microphones are constructed using a pair of
collinear microphone/housing assemblies such as shown in FIG. 10.
TELECONFERENCING APPLICATION
FIG. 13 discloses a top plan view of teleconferencing unit 130 including
upwardly aimed loudspeaker 131 and four microphone housings
110-1,-2,-3,-4. The array of microphones provides full room coverage which
is most useful in a conference telephone application. Since, normally,
only one talker is speaking at a time, background noise and reverberation
are minimized by only activating one microphone at a time. Circuits within
the teleconferencing unit 130 compare the output signals from each of the
microphone elements in housings 110-1,-2,-3,-4 to determine which one is
the strongest. One such system is disclosed in U.S. Pat. No. 3,755,625. In
response, only the signals from that microphone are transmitted to the
distant end. In accordance with the invention, loudspeaker 131 is located
in the null of the polar response pattern of each of the
microphone/housing assemblies, and that null resides between the main lobe
and an adjacent side lobe. This particular null is located at
125.degree.-which accounts for the particular positioning of the
microphones around the teleconferencing unit 130. This performance is
achieved by placing a microphone element, as disclosed in FIG. 3, into the
housing, thus forming a supercardioid polar response pattern (see FIG | | |
