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BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to
directional microphones, and more particularly to a structure having one or more microphone
BACKGROUND OF THE INVENTION Microphones with directional characteristics are useful in
many application areas. One well-known way to obtain directivity is the use of primary gradient
(FOG) microphone elements. It has a moveable diaphragm with front and back enclosed in a
capsule. The capsule has openings on each side to receive sound pressure which interacts with
the front and back surfaces of the diaphragm.
In response to this interaction, an electrical signal is generated, which is proportional to the
sound pressure difference on both sides (front and back) of the diaphragm. Sound coming from a
direction where the wave front of the sound wave arrives at the front and back of the diaphragm
simultaneously is ignored. In this case, the instantaneous sound pressure on each side of the
diaphragm is the same, so the sound pressure difference is zero.
Sounds coming from different directions arrive on one side of the diaphragm, but arrival on the
other side is delayed by an effective path distance "d" between the two sides (front and back).
Although this delay causes directivity, this delay also affects frequency response characteristics
for the following reasons. That is, it is because the path distance "d" corresponds to different
fractions of the wavelength at each of the different frequencies. The low frequency response is
low because sound waves having practically the same phase arrive simultaneously on both sides
(front and back) of the diaphragm.
US Pat. No. 3,715,500, issued Feb. 6, 1973 to Sessler et al., Is a technique referred to as a
unidirectional microphone. Cessler et al disclose that connecting the FOG microphone element
and the tube effectively increases the separation distance between the acoustic ports. While the
low frequency response of the microphone improves as the separation distance increases,
assembling the disclosed structure and attaching it to a modern acoustic input device is quite
US Pat. No. 4,742,548, issued on Mar. 3, 1988, to Cessler et al., Is a technique referred to as a
unidirectional second-order gradient microphone. In this patent, sensitivity is improved by
stuffing the FOG microphones in order to increase the effective acoustic path distance between
the acoustic ports of the FOG microphones. The baffle preferably has a square or circular plane.
Although this structure is an improvement over the prior art, it is physically large and protrudes
from the sound input device, so it can not address the problem of conveniently attaching a FOG
microphone to the sound input device .
A known microphone device called WM-46AAD 201, which is commercially available from
National / Panasonic (Matsushita Electric Industrial Co., Ltd.), has a cardioid polar sensitivity
characteristic. The FOG microphone element is enclosed within a rigid two-piece plastic housing.
The housing has openings for receiving sound waves on each side of the FOG microphone in the
The housing and the FOG microphones are joined together by an adhesive or other bonding
material, so that each side of the FOG microphone is affected only by the sound waves entering
the appropriate openings in the housing. Unfortunately, the construction of such a device is
laborious, with the use of adhesives and also extra steps requiring curing time. Furthermore, if
the application method is not good, a leak will occur, resulting in a change in the acoustic
One useful directional microphone assembly is shown in FIG. 2 which uses a tube to couple the
microphone element to the desired acoustic pick-up point. This figure is from Knowles Elecronics,
Inc. 2.) Technical Bulletin, a quote from "EB directional hearing aid microphone application
notes" in TB-21. Unfortunately, no structural means for supporting such an assembly within the
acoustic input device is provided, and it is believed that the tube can not be easily sealed to the
device surface.
Accordingly, it is desirable to provide a housing for a microphone element of relatively simple
construction that is easy to manufacture and install.
It is further desirable for the microphone assembly to be easily attachable to an acoustic input
device without inherently protruding while maintaining the functionality improvements provided
by Cessler et al.
SUMMARY OF THE INVENTION A directional microphone assembly is comprised of a
microphone element enclosed within a housing, which is made of a sound insulating and resilient
The microphone element comprises a diaphragm, which operates under the influence of the
sound pressure applied to its both sides, producing an electrical signal proportional to the sound
pressure difference.
The housing has a first channel of sound propagation, which channel transmits sound pressure
from the first opening in the housing to one side of the diaphragm.
The housing also has a second acoustic propagation channel, which is adapted to transmit sound
pressure from the second opening in the housing to the other side of the diaphragm.
In one embodiment of the present invention, the housing is formed of ethylene propylene diene
monomer (EPDM), which is a rubbery material having elasticity. As a result, a good seal is formed
at the periphery of the microphone element, so that the sound pressure of one channel does not
leak to the other. Furthermore, by making the housing in such a rubber-like material, a seal is
also formed on the surface of the acoustic input device to be stored.
In the same embodiment, the housing is formed of two identical parts, which are joined at the
microphone element area. In this embodiment, the use of adhesive material is not necessary for
the following reasons. That is, the same housing parts can be joined to the microphone element
with only enough friction to connect the entire structure.
A feature of the present invention is that the directional microphone assembly is advantageously
inserted into the outer surface of the acoustic input device, such that its channel opening is
located on one or more sides of the acoustic input device, or behind the outer surface. It can be
microphone can sense instantaneous sound pressure at its input sound port and produces a
voltage signal of an electrical output corresponding to the magnitude of the sound pressure.
Such a microphone is known as a "pressure microphone" and is usually configured as shown in
FIG. The acoustic port 101 introduces sound into the microphone 100 which interacts with one
side of the diaphragm 103 to generate a voltage.
The opposite side of the diaphragm 103 is a sealed area, the volume of which affects the
compliance of the diaphragm. Because the pressure microphone responds equally to sounds
coming from all directions, its sensitivity pattern is omnidirectional. FIG. 5 shows the
omnidirectional sensitivity pattern in the far field of the pressure microphone with several
relevant characteristics. The information in FIG. 5 is summarized using the following data. That
is, Knowles Electoronics, Inc. B) Technical Bulletin, "EB directional hearing aid microphone
application note" in TB-21.
Primary Gradient Microphone A gradient microphone measures differential pressure on both
sides of one or more diaphragms to obtain directional polar sensitivity characteristics. FIG. 2
shows a primary gradient (FOG) microphone 200 having input acoustic ports 201 and 202
provided on both sides of the diaphragm 203. These acoustic ports are separated by an effective
distance d. In order to propagate from one acoustic port 201 to another acoustic port 202, the
sound waves must propagate around the FOG by this distance.
The motion of diaphragm 203 is converted to a voltage as the output of the microphone. The
magnitude of the output voltage of the FOG microphone is a function of the instantaneous sound
pressure difference on either side of the diaphragm 203. As the distance d decreases, the output
voltage from the FOG microphone also decreases. On the other hand, the speed of sound at 21 °
C. in air is 344 m / sec, so the f = 2250 Hz audio signal has a wavelength of about 15.24 cm.
Therefore, even if the separation distance (effective distance) between the sound ports is small,
the phase difference between the sound ports 201 and 202 is sufficient, so that the FOG
microphone is bi-directional as shown in FIG. Shows a polar sensitivity pattern of In fact, this
polar sensitivity pattern is hardly affected by frequency as shown in Equation 2 described later.
Here, the polarity of the output voltage is determined by the particular side of the diaphragm
with which the wave front of the propagating sound wave first strikes. Also, FOG microphones do
not respond to sounds coming from certain directions known as nulls. The present invention
utilizes these properties. A suitable FOG microphone element for use in the present invention is
WM-55A103 manufactured by Panasonic Division of Matsushita Electric Industrial Co., Ltd.
The separation distance "d" between the acoustic ports on either side of the diaphragm 203 can
be varied. In the far field, the pressure gradient Δp has the relationship of d with the
following equation.
[Equation 1]
Θ = polar direction of the collision wavefront relative to the microphone main axis.
C = propagation velocity Equation 1 is simplified as follows for small values of kd.
[Equation 2]
FIG. 4 shows the sensitivity of the FOG microphone in the case of the direction θ = 0 °, that is,
the frequency response [Expression 1].
Frequency response and directivity patterns are known to change by changing the gradient
microphones themselves. For example, as shown in FIG. 3, an acoustic resistance Ra can be
introduced into one of the acoustic ports 302 leading to the diaphragm 303 of the FOG
microphone. Such resistance changes both the directivity pattern and the frequency response.
More generally, the directivity pattern D (θ) associated with the far-field-operated FOG
microphone is given by the following relation when kd <1.
[Equation 3]
In equation (3), 密度 is the density of air, V is the volume of the acoustic region on the back side
of the diaphragm, and Ca is the acoustic compliance (similar to capacitance) between the
diaphragm and the acoustic resistance Ra.
From Equation 3, cardioid sensitivity is obtained when B is set equal to one.
That is, it is obtained when the time constant RaCa is set to be equal to the time it takes the
sound wave to propagate the distance "d". FIG. 5 shows such a cardioid pattern and other
characteristics of this particular FOG microphone.
Another well known directional pattern is supercardioid. This is obtained by adjusting d, Ra and
V such that B is equal to the square root of three. When the value of B is further increased to 3,
hypercardioid directivity patterns are obtained. Each of the microphone directivity patterns
selected and shown in FIG. 5 has its own collection of characteristics. For example, (1) null
position (angle), (2) distance factor, that is, one directional microphone indicates how many times
the effective distance from the pressure microphone to the acoustic source can be obtained In
addition, a multiplier indicating that this directional microphone has the same signal-to-random
incident noise ratio, (3) front-to-rear sensitivity ratio, etc.
FIG. 6 is a diagram showing the prior art configuration described in detail in US Pat. No.
4,742,548. The baffle 205 surrounds the FOG microphone 200, which increases the distance
that the wavefront of the acoustic signal has to propagate in order to travel from one side of the
FOG microphone to the other. This distance "d" is a parameter that directly affects the sensitivity
and frequency of the associated microphone and sometimes affects polar sensitivity
characteristics. Unfortunately, this baffle has to be placed perpendicularly to the surface of the
acoustic input device, and in designing this acoustic input device the flexibility of the design has
to be stored, so that the flexibility of the design The ability is limited.
FIG. 7 shows a flat housing 110 for a FOG microphone element. This housing 110 effectively
stretches the distance 〓 "d" between the acoustic ports of the FOG microphone element located
therein. This rectangular block structure is molded from vulcanized (cured) rubber or other
suitable elastic material and used in place of the baffle 205 shown in FIG. One suitable
commercially available material is ethylene propylene diene monomer (EPDM).
The housing 110 of FIG. 7 is made of a sound insulating material that does not transmit sound
pressure as effectively as air. However, the housing 110 has openings 111 and 112 for
introducing sound pressure, and the sound pressure introduced from these openings passes
through the sound propagation channels 113 and 114, and the microphone element 200 (see
FIG. 8) is disposed. Is received into the cavity. The microphone element 200 has a set of wires
116 which are pulled out of the housing through the holes 115 of the self-sealing.
The housing 110 is sized to form a seal with the microphone element 200 so that the sound
pressure of one channel does not leak around the microphone element into the other channel.
Thus, by using an elastic material for the housing, it is not necessary to use a sealing adhesive,
which is advantageous. FIG. 8 is a cross-sectional view of the microphone / housing assembly
showing the mutual arrangement of the housing 110, the FOG microphone 200, the channels
113, 114, and the openings 111,112.
FIG. 9 shows one part of the housing 110, which is made of two such identical parts. The junction
117 of these two parts is shown in FIG. When the parts of the housing shown in FIG. 9 are joined
with another identical part, a cavity is formed in the area enclosed by the faces 118, 119. This
cavity is illustratively cylindrical and sized to form a seal with the particular microphone element
Since EPDM is an elastic material, it is not necessary to use an adhesive material. This is because
sufficient frictional forces allow each identical housing part to be joined with the microphone
element, as a result of which the resulting assembly can be held securely.
Applications The present invention can be attached and applied to any acoustic input device that
uses directional microphones. Typical examples of sound input devices include tape recorders or
telephone handsets. FIG. 10 shows an example application of the invention in a teleconference
device. The teleconferencing device 130 has a loudspeaker 131 and four directional microphone
assemblies (110-1, 110-2, 110-3, 110-4) arranged in four directions on the outer periphery of
the rectangular structure.
Each microphone element is arranged in a housing as shown in FIG. This microphone array
provides very useful full room coverage for conference call applications. Since only one person
usually speaks at a time, background noise and reverberation are minimized by activating only
one of the microphones at a time. Circuitry within teleconferencing device 130 compares the
output signals from each directional microphone assembly to determine which microphone
assembly is delivering the strongest signal.
FIG. 11 is a front view of the teleconference device 130, showing the position 110-1 as a
representative example of the microphone housing. As apparent from FIG. 11, this device is
excellent in that it is flat and compactly packaged. Four pockets are formed on the outer surface
of the teleconference device 130 for mounting the housings 110-1, 110-2, 110-3, 110-4. Each
pocket is designed to be slightly smaller than the corresponding housing, whereby the housing is
frictionally held in place.
While the retention of the housing as described above takes advantage of the resilient nature of
the housing 110, the resilient nature of such a housing is also utilized when sealing the
microphone element. The directional microphone is formed such that its channel opening is
inserted into or attached to the outside of the teleconference device 130 such that the channel
opening is located on the outside of the device. Thus, the housing is arranged inconspicuously.
Additionally, the rubbery housing 110 forms a seal with the face of the teleconference device.
FIG. 12 shows another application of the present invention. Here, the microphone housing 110 is
disposed along one side of the telephone 120. The telephone has a loudspeaker 121. This
loudspeaker allows hands-free operation, also known as speakerphone. The housing 110 has a
primary gradient microphone element as shown in FIG.
The distance between the openings of the housing 110 is selected to improve sensitivity and
narrow the microphone beam width, thereby forming a supercardioid polar sensitivity pattern. A
pocket is molded on the exterior of the phone to mount the housing 110. The pocket is slightly
smaller than the housing so that the housing is frictionally held in the pocket. Sound transmitting
material can also be used to conceal the opening of the housing 110.
While specific embodiments of the invention have been described above, it will be apparent that
further modifications are possible within the scope of the invention. Such modifications include,
for example, the use of elastic materials other than EPDM in the manufacture of the housing, the
use of non-moulded housings, non-circular openings in the housing, and non-coplanar openings
It can be mentioned. Of course, the present invention is not limited to the above embodiments
and their modifications. Furthermore, instead of using a single FOG microphone element, the use
of two electrically interconnected pressure microphones is also contemplated.
All of these are included in the spirit of the present invention. The reference numerals described
in the claims are for easy understanding of the invention and should not be construed as limiting
the technical scope.
As described above, the present invention provides an excellent housing for a microphone
element with a simple structure and easy to manufacture and attach.
Brief description of the drawings
1 shows a pressure microphone element with omnidirectional polar sensitivity characteristics.
2 is a diagram showing a primary gradient microphone element as used in the present invention.
FIG. 3 is a diagram showing a primary gradient microphone element having a limitation on one of
its acoustic ports.
4 is a diagram showing the frequency response of the microphone shown in FIG.
FIG. 6 is a table showing the characteristics associated with the microphone of FIG. 3 for different
values of FIG. 5B.
6 is a diagram showing a second-order gradient microphone whose sensitivity is improved by the
use of the baffle according to the prior art.
FIG. 7 is a perspective view of a preferred structure housing the primary gradient microphone
8 is a plan view of the housing of FIG.
9 is a perspective view of the structure of FIG.
10 is a plan view of a teleconference device using a primary gradient microphone according to
the present invention.
11 is a front view of the teleconference device of FIG.
FIG. 12 is a perspective view of a loudspeaker using a primary gradient microphone according to
the present invention.
Explanation of sign
Reference Signs List 100 microphone 101 acoustic port 103 diaphragm 110 (110-1 to 110-4)
microphone housing (microphone assembly) 111 opening 112 opening 113 channel 114
channel 115 channel 115 (self-sealed) hole 116 wire 117 joint (area) 118 surface 119 face 120
telephone 121 loudspeaker 130 teleconference device 131 loudspeaker 200 microphone
(element) 201 acoustic port 202 acoustic port 203 diaphragm 205 baffle 300 microphone 301
acoustic port 302 acoustic port 303 diaphragm
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