United States Patent (19)
`Ono et al.
`
`(54) PRESSURE GRADIENT TYPE
`MCROPHONE APPARATUS WITH
`ACOUSTIC TERMINALS PROVIDED BY
`ACOUSTIC PASSAGES
`
`75 Inventors: Kiminori Ono, Katano; Satoru
`Ibaraki, Higashioosaka, Yuji
`Yamashina, Takatsuki, all of Japan
`73 Assignee: Matsushita Electric Industrial Co.,
`Ltd., Osaka, Japan
`
`Appl. No.: 283,912
`21
`22 Filed:
`Aug. 3, 1994
`(51
`int. Cl. ......................................... H04R5/00
`52 U.S. Cl. ................................................. 381/26; 381/91
`58 Field of Search ................................. 381/26,91, 169
`
`56
`
`References Cited
`U.S. PATENT DOCUMENTS
`
`IIIHIIIHIIII
`US005526430A
`11) Patent Number:
`5,526,430
`(45. Date of Patent:
`Jun. 11, 1996
`
`4,633,498 12/1986 Warnke et al. ........................... 381126
`4,819,270 4/i989 Lombardo ................................. 381A26
`4,836,326
`6/1989 Wehner et al. ........................... 381A91
`Primary Examiner-Stephen Brinich
`Attorney, Agent, or Firm-Wenderoth, Lind & Ponack
`
`ABSTRACT
`57)
`In a small-sized pressure gradient type microphone appara
`tus, a plurality of omni-directional microphone units are
`encased within a microphone holder. A plurality of acoustic
`passages having first and second ends are provided within
`the microphone holder for coupling the sound inlets of the
`plurality of omni-directional microphone units respectively
`to an outer space of the microphone holder. The second ends
`of the acoustic passages opened to the outer space of the
`microphone holder are arranged to be apart from each other
`at distances larger than distances between the sound inlets of
`the corresponding microphone units coupled at the first ends
`of the acoustic passages.
`
`4,070,547
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`1/1978 Dellar ........................................ 381/26
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`3 Claims, 6 Drawing Sheets
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`1
`PRESSURE GRADIENT TYPE
`MICROPHONE APPARATUS WITH
`ACOUSTIC TERMINALS PROVIDED BY
`ACOUSTIC PASSAGES
`
`2
`acoustic passages, or pipes, provided within the microphone
`holder and having first ends which are respectively coupled
`to the sound inlets of the plurality of omni-directional
`microphone units and having second ends which are opened
`to an outer space of the microphone holder for coupling the
`sound inlets of the plurality of omni-directional microphone
`units to the outer space of the microphone holder respec
`tively by the plurality of acoustic passages. The second ends
`of the acoustic passages are arranged to be apart from each
`other at distances larger than distances between the sound
`inlets of the corresponding microphone units coupled at the
`first ends of the acoustic passages.
`Distances between acoustic terminals of this pressure
`gradient type microphone apparatus are determined by the
`distances between the open ends of the acoustic passages
`provided in the microphone holder. That is, the distances
`between the acoustic terminals, or the sensitivity to sound
`pressure, can be maintained while reducing the distances
`between the microphone units, or reducing the size of the
`microphone apparatus.
`
`BRIEF DESCRIPTION OF THE DRAWINGS
`FIG. 1a is a schematic perspective view of a microphone
`apparatus according to an embodiment of the present inven
`tion;
`FIG. 1b is a cross sectional view of the microphone
`apparatus shown in FIG. 1a;
`FIG. 2 is a block diagram showing an example of a signal
`processing circuit used for the microphone apparatus shown
`in FIGS. 1a and 1b,
`FIG. 3 is a schematic diagram showing an arrangement of
`two omni-directional microphone units in the conventional
`pressure gradient type microphone apparatus;
`FIG. 4 is a frequency response diagram showing a sen
`sitivity to sound pressure in the front direction of the
`conventional first-order pressure gradient type microphone
`apparatus;
`FIG. 5 is a schematic perspective view of a microphone
`apparatus according to another embodiment of the present
`invention;
`FIG. 6 is a block diagram showing an example of a signal
`processing circuit used for the microphone apparatus shown
`in FIG. 5;
`FIG. 7 is an equivalent circuit diagram of an acoustic
`system consisting of an omni-directional microphone unit
`and an acoustic passage coupled to the microphone unit,
`FIGS. 8 a diagram showing a change of a frequency
`characteristic of the output of the microphone unit depen
`dent on the length of the acoustic passage in the system
`shown in FIG. 7; and
`FIG. 9 is a diagram showing a change of the frequency
`characteristic of the output of the microphone unit depen
`dent on the diameter of the acoustic passage in the system
`shown in FIG. 7.
`
`DESCRIPTION OF THE PREFERRED
`EMBODEMENTS
`FIG. 1a is a schematic perspective view of a microphone
`apparatus according to an embodiment of the present inven
`tion, and FIG. 1b is a cross sectional view of the microphone
`apparatus shown in FIG. 1a. A microphone holder 11a
`comprises a pair of unit holders 11b and 11c holding therein
`two omni-directional microphone units 12 and 14 respec
`tively. The omni-directional microphone 12 comprises a
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`BACKGROUND OF THE INVENTION
`1. Field of the Invention
`The present invention relates to a microphone apparatus
`for use in a small-size recording apparatus having an audio
`recording function, and more particularly to a pressure
`gradient type microphone apparatus having a plurality of
`omni-directional microphone units.
`2. Description of the Prior Art
`Video cameras are widely known as small-size recording
`apparatus having an audio recording function. Particularly,
`consumer-use video cameras have been remarkably reduced
`in size. Installation of the microphone apparatus in such
`small-sized consumer-use video cameras has changed from
`the type in which the microphone apparatus is mounted
`outside of the camera body to the type in which the micro
`phone apparatus is encased in an inner space within a part of
`the camera body. The so-called pressure-gradient type
`microphone apparatus having a plurality of omni-directional
`microphone units has been widely used as such an encased
`microphone apparatus. The pressure-gradient type micro
`phone apparatus comprises a plurality of omni-directional
`microphone units arranged on an outer horizontal surface of
`the camera body, and a directivity forming circuit for
`30
`processing output signals of the plurality of microphone
`units. The pressure-gradient type microphone apparatus gen
`erally has the following advantages:
`1)The microphone units are less affected by reflection and
`diffraction from the camera body, so that good sound
`pickup characteristics can be obtained.
`2) The directivity can be changed easily.
`However, the sensitivity to sound pressure of the pressure
`gradient type microphone apparatus is proportional to the
`distance between the microphone units (the distance
`between the centers of the sound inlets of the microphone
`units), i.e., the distance between acoustic terminals of the
`microphone units. That is, the reduction in overall size of the
`microphone apparatus inherently sacrifices the sensitivity to
`sound pressure. Accordingly, it has been difficult to largely
`reduce the overall size of the conventional pressure gradient
`type microphone apparatus while maintaining a practically
`required sensitivity to sound pressure.
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`SUMMARY OF THE INVENTION
`Accordingly, it is an object of the present invention to
`provide a pressure gradient type microphone apparatus
`which can be remarkably reduced in size while maintaining
`a practically required sensitivity to sound pressure and thus
`can be mounted in a reduced installation space in a recording
`apparatus.
`To achieve this object, a pressure gradient type micro
`phone apparatus according to the present invention com
`prises: a plurality of omni-directional microphone units,
`60
`each of the plurality of microphone units having a dia
`phragm provided perpendicularly to an axial direction of the
`unit and a sound inlet for exposing therethrough the dia
`phragm; a microphone holder for encasing therein the plu
`rality of omni-directional microphone units which are
`arranged in parallel in the axial direction so that the dia
`phragms direct in a same direction; and a plurality of
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`diaphragm 12a and a back plate 12b which are mounted
`parallel to each other in an inner casing 12c to constitute a
`parallel plane capacitor, and an outer casing 12d encasing
`therein the inner casing 12c. The outer casing 12d has a
`sound inlet 13 provided at a center of a front surface thereof
`opposing the diaphragm 12a to expose therethrough the
`diaphragm 12a. Similarly, the other omni-directional micro
`phone 14 comprises a diaphragm 14a, a back plate 14b, an
`inner casing 14c, and an outer casing 14d. The two micro
`phone units 12 and 14 are inserted into the unit holders 11b
`and 11c from the front ends of the outer casings 12d and 14d
`at which the sound inlets 13 and 15 are provided. The outer
`casing 14d has a sound inlet 15 provided at a center of a front
`surface thereof to expose therethrough the diaphragm 14a.
`Two passages, or pipes, 16 and 17 are provided in the holder
`11a for acoustically coupling the sound inlets 13 and 15
`respectively to the open space (front open space) outside the
`holder 11a. The acoustic passage 16 has opposite ends, one
`end being connected to the sound inlet 13 of the microphone
`unit 12 and the other, open end being opened to the front
`outer space of the holder 11a. Similarly, the acoustic passage
`17 has opposite ends, one end being connected to the sound
`inlet 15 of the microphone unit 14 and the other, open end
`being opened to the front outer space of the microphone
`holder 11a. The two passages (pipes) 16 and 17 are arranged
`such that the distanced between centers of the open ends
`of the acoustic passages 16 and 17 is larger than the distance
`d between centers of the sound inlets 13 and 15 of the
`microphone units 12 and 14.
`The acoustic passage connected to each omni-directional
`microphone unit adds an acoustic mass to the microphone
`unit. The addition of the acoustic mass provides the effects
`of reducing the resonance frequency of the acoustic system,
`which is the upper frequency limit of the sensitivity to sound
`pressure, and increasing the resonance Q value. An equiva
`lent circuit of an acoustic system consisting of an omni
`directional microphone unit and an acoustic passage coupled
`to the microphone unit is shown in FIG. 7. In FIG. 7, S
`denotes the sound source, and Zp denotes the acoustic
`impedance of the acoustic passage. The part enclosed by a
`40
`broken line represents the microphone unit, in which MO, CO
`and R0 are respectively the acoustic mass, acoustic compli
`ance and acoustic resistance of the diaphragm, and C1 is the
`acoustic compliance of the rear space in the microphone
`unit. The frequency characteristic of the output signal of the
`microphone unit is determined by the mutual relationship
`between the impedance of the acoustic passage and the
`impedance of the microphone unit. FIGS. 8 shows a change
`of the frequency characteristic of the output signal of the
`microphone unit dependent on the length of the acoustic
`passage in the system shown in FIG. 7 in a case that a
`cylindrical passage having a diameter of 2 mm is connected
`to a cylindrical omni-directional microphone having a diam
`eter of 6 mm, and M0, C0, R0 and C1 are properly set. In
`FIG. 8, 8a show a frequency characteristic when the acoustic
`passage is not connected to the microphone unit, and 8b, 8c,
`8d, 3e and 8fare frequency characteristics when the length
`of the acoustic passage connected to the microphone unit is
`changed to 2 mm, 4 mm, 6 mm, 8 mm and 10 mm,
`respectively. As seen from FIG. 8, when the length of the
`acoustic passage is increased, the resonance frequency of the
`acoustic system decreases and the resonance Q value
`increases, so that the frequency characteristic is disturbed
`more largely. FIG. 9 is a diagram showing a change of the
`frequency characteristic of the output of the microphone unit
`in the system shown in FIG. 7 in a case that a cylindrical
`passage having a length of 2 mm is connected to the
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`cylindrical omni-directional microphone having the diam
`eter of 6 mm. In FIG. 9, 9a, 9b, 9c, 9dand 9e are frequency
`characteristics when the diameter of the acoustic passage
`connected to the microphone unit is changed to 2 mm, 1.6
`mm, 1.2 mm, 0.8 mm and 0.4 mm, respectively. As seen
`from FIG. 9, the frequency characteristic is disturbed more
`as the diameter of the acoustic passage is decreased. In the
`cases shown in FIGS. 8 and 9, the acoustic passage may be
`designed to have a length of about 2 mm and a diameter of
`about 2 mm to produce a practically usable microphone. As
`described above, the acoustic passage connected to the
`microphone may be designed so as not to cause a large
`disturbance of the frequency characteristic of the output
`signal of the microphone unit.
`A design example of the microphone apparatus shown in
`FIGS. 1a and 1b may be such that each of the omni
`directional microphone units 12 and 14 has a diameter of 6
`mm, each of the acoustic passages 16 and 17 has a length of
`2 mm and a diameter of 2 mm, the distanced between the
`centers of the sound inlets 13 and 15 of the microphone units
`12 and 14 is 6.1 mm, and the distanced between the centers
`of the open ends of the acoustic passages 16 and 17 is 10
`
`FIG. 2 is a block diagram showing an example of a signal
`processing circuit used for the microphone apparatus shown
`in FIGS. 1a and 1b. Two signals S1 and S2 are respectively
`the output signals of the omni-directional microphone units
`12 and 14 mounted in the unit holders 11b and 11c if the
`holder 11a shown in FIGS. 1a and 1b. The signals S1 and S2
`are fed to a directivity forming circuit 21 which comprises
`a phase shifter 22 for phase-shifting the signal S2, and an
`adder for receiving the signal S1 at its non-inverting (+)
`terminal and an output signal of the phase shifter 22 at its
`inverting terminal (-) for adding an inverted form of the
`output signal of the phase shifter 22 to the signal S1 to
`produce a sum signal S3.
`As the result, the microphone apparatus of this embodi
`ment operates as a first-order pressure gradient type micro
`phone apparatus. The operation of a conventional first-order
`pressure gradient type microphone apparatus will be
`described for comparison. FIG. 3 is a schematic diagram
`showing an arrangement of two omni-directional micro
`phone units in the conventional first-order pressure gradient
`type microphone apparatus. Two omni-directional micro
`phone units 32 and 33 are mounted on apart of the outer wall
`31 of the video camera body to be spaced from each other
`by a center-to-center distanced. The two directions denoted
`by 0° and 180° respectively represents the front end and rear
`end directions of the microphone apparatus. Each of the two
`microphone units 32 and 33 has a diametera. The length of
`the area on the outer wall of the video camera body
`necessary for installing the two microphone units is
`expressed by d+a at maximum. The output signals of the two
`microphone units 32 and 33 are processed by the signal
`processing circuit as shown in FIG. 2. FIG. 4 shows a
`frequency response of the sensitivity to sound pressure of
`the conventional first-order pressure gradient type micro
`phone apparatus in the front direction (direction of 0). The
`sensitivity to sound pressure becomes maximum at a fre
`quency Fp expressed by Fp=d/2C, where C is the velocity of
`sound. Usually, sounds are picked up in the frequency range
`below Fp. The sensitivity to sound pressure is proportional
`to frequency in the frequency range below Fp. When the
`distance between the two microphone units 32 and 33 is
`reduced to be shorter than d, the frequency response curve
`shifts to the high frequency side as represented by a doted
`line in FIG. 4. Accordingly, the sensitivity to sound pressure
`will decrease in the frequency range below Fp.
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`On the other hand, according to the microphone apparatus
`shown in FIGS. 1a and 1b, in which the omni-directional
`microphone units 12 and 14 are encased within the holder
`11a, the length on the outer surface of the body of the
`recording apparatus such as the video camera necessary for
`installing the microphone apparatus may be d+t, where t is
`the diameter of the opening end of each of the acoustic
`passages 16b and 17 coupled to the microphone units, and
`d is equal to d. Accordingly, the microphone apparatus can
`be reduced in size while substantially maintaining the dis
`tance d between the acoustic terminals.
`FIG. 5 is a schematic perspective view of a microphone
`apparatus according to another embodiment of the present
`invention. A microphone holder 51a for holding therein
`omni-directional microphone units has three unit holders
`51b, 51c and 51d for holding three omni-directional micro
`phone units, respectively. The structure of each omni-direc
`tional microphone unit and the internal structure of the
`holder are basically the same as those in the embodiment
`shown in FIG. 1b although the number of the microphone
`units and the number of unit holders are increased from two
`to three. That is, each of the three omni-directional micro
`phone units encased within the microphone holder 51a is
`coupled through an acoustic passage to the outer space of the
`microphone holder 51a. The three omni-directional micro
`phone units are mounted in the microphone holder 51a such
`that the distance between the centers of the sound inlets of
`each two microphone units is shorter than d. Three acoustic
`passages are provided in the microphone holder 51a such
`that the distance between the centers of the open ends of
`each two acoustic passages is d. Accordingly, the micro
`phone apparatus can be reduced in size while maintaining
`practically required distances between acoustic terminals
`and thus maintaining a practically required sensitivity to
`Sound pressure.
`FIG. 6 is a block diagram showing an example of a signal
`processing circuit used for the microphone apparatus shown
`in FIG. 5. Signals Sb, Sc and Sd are respectively output
`signals of the three omni-directional microphone units
`encased and held within the three unit holders 51b, 51c and
`51d. A directivity forming circuit 61 for processing the
`signals Sb, Sc and Sd comprises a phase shifter 62 for
`phase-shifting the signal Sd, an adder 63 for receiving the
`signal Sb at its non-inverting terminal (+) and an output
`signal of the phase shifter 62 at its inverting terminal (-) for
`adding an inverted form of the output signal of the phase
`shifter 62 to the signal Sb to obtain a left channel signal S,
`and an adder 64 for receiving the signal Sc at its non
`inverting terminal (+) and the output signal of the phase
`shifter 62 at its inverting terminal (-) for adding the inverted
`form of the output signal of the phase shifter 62 to the signal
`Sc to obtain a right channel signal S. Accordingly, the
`microphone apparatus of this embodiment operates as a
`stereo microphone apparatus.
`What is claimed is:
`1. A microphone apparatus comprising:
`a plurality of omni-directional microphone units, each of
`the microphone units having a diaphragm provided
`perpendicularly to an axial direction of the unit and a
`sound inlet for exposing therethrough the diaphragm;
`a microphone holder for holding therein the plurality of
`omni-directional microphone units to be arranged in
`parallel in the axial direction; and
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`a plurality of acoustic passages provided within the
`microphone holder and having first ends which are
`respectively coupled to the sound inlets of the plurality
`of omni-directional microphone units and having sec
`ond ends which are opened to an outer space of the
`microphone holder for coupling the sound inlets of the
`plurality of omni-directional microphone units to the
`outer space of the microphone holder respectively by
`the plurality of acoustic passages, the second ends of
`the acoustic passages being arranged to be apart from
`each other at distances larger than distances between
`the sound inlets of the corresponding microphone units
`coupled at the first ends of the acoustic passages.
`2. A microphone apparatus comprising:
`first and second omni-directional microphone units, each
`of the first and second microphone units having a
`diaphragm provided perpendicularly to an axial direc
`tion of the unit and a sound inlet for exposing there
`through the diaphragm;
`microphone holder for holding therein the first and second
`omni-directional microphone units to be arranged in
`parallel in the axial direction; and
`first and second acoustic passages provided within the
`microphone holder and having first ends which are
`respectively coupled to the sound inlets of the first and
`second omni-directional microphone units and having
`second ends which are opened to an outer space of the
`microphone holder for coupling the sound inlets of the
`first and second omni-directional microphone units to
`the outer space of the microphone holder respectively
`by the first and second acoustic passages, the second
`ends of the acoustic passages being arranged to be apart
`from each other at distances larger than distances
`between the sound inlets of the first and second micro
`phone units coupled at the first ends of the acoustic
`passages.
`3. A microphone apparatus comprising:
`first, second and third omni-directional microphone units,
`each of the first, second and third microphone units
`having a diaphragm provided perpendicularly to an
`axial direction of the unit and a sound inlet for exposing
`therethrough the diaphragm;
`microphone holder for holding therein the first, second
`and third omni-directional and third omni-directional
`microphone units to be arranged in parallel in the axial
`direction; and
`first, second and third acoustic passages provided within
`the microphone holder and having first ends which are
`respectively coupled to the sound inlets of the first,
`second and third omni-directional microphone units
`and having second ends which are opened to an outer
`space of the microphone holder for coupling the sound
`inlets of the first, second and third omni-directional
`microphone units to the outer space of the microphone
`holder respectively by the first, second, and third acous
`tic passages, the second ends of the acoustic passages
`being arranged to be apart from each other at distances
`larger than distances between the sound inlets of the
`first, second and third microphone units coupled at the
`first ends of the acoustic passages.
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