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FIELD OF THE INVENTION The present invention relates to directional microphones and acoustic
sensors. (Prior Art) Acoustic transducers having directional characteristics are used in many
fields. In particular, unidirectional microphones are used, which have a very large directivity
factor for their size. Many of these microphones are of the first-order gradient type, and have
directivity characteristics represented by (a 10 cos θ) depending on the detailed configuration.
Here, a is a constant (0 ≦ a ≦ 1) and θ is an angle with respect to a symmetrical rotation axis. A
directivity factor of up to 4 is obtained with this system. This directivity is improved by using a
second-order gradient microphone. This microphone has a ll directivity pattern representing (a +
c o sθ) (b + cO 3 θ) (where lal ≦, l b l ≦ 1), and a maximum directivity factor of 9 can be
obtained. The use of such second-order gradient microphones is hampered by more complex
designs, resulting in poorer signal-to-noise ratio as compared to first-order gradient designs. A
recent version of the secondary gradient microphone is disclosed in US Pat. No. 4,742,548.
Although the present disclosure is advanced as compared to the prior art, the arrangement and
sensitivity of the primary directional element employed therein are generally demanding,
especially such microphones. The requirements are severe if multiple secondary gradient
microphones are matched and used in an array of Therefore, a simple device for constructing a
secondary gradient microphone is desired. SUMMARY OF THE INVENTION According to the
present invention, a solution to the problem of a better unidirectional microphone and sensor is
to simulate the presence of a second (paired) directional sensor element, such as a directional
microphone or the like. Using a planar reflective element in the vicinity of the sensor element of
The apparatus of the present invention forms a second-order gradient microphone having
various patterns (e.g., unidirectionality characteristics, toroidal directional characteristics).
According to a first feature of the invention, the width of the reflective element and the position
of the sensor relative to its surface are such as to exclude destructive interference from other
reflective surfaces. According to a second feature of the invention, the first-order bi-directional
microphone and the other sensor elements are arranged apart from the acoustically reflecting
wall to improve the directional response of the device and to reflect the echo and noise in the
room It is something to suppress the influence. According to a third aspect of the invention, a
virtual-image-guided directional microphone is provided with a hands-free telephone problem
(eg, multiple path distortions (from room echoes), gain-switched voice multipaths, etc. Etc.) to be
The directivity characteristic of this array is the product of the gradient characteristic and the
linear array characteristic. A fourth aspect of the invention relates to the formation of a virtual
image induced directional acoustic sensor to form a unique directional pattern (e.g. a toroidal
pattern) and also to omnidirectionally modify the directional pattern. The present invention
relates to the combination with the acoustic sensor. DESCRIPTION OF THE PREFERRED
EMBODIMENTS General Description In the prior art, first-order gradient layer bi-directional
sensors (FOGs) are placed within a small distance from each other, with inherent phase and delay
added, second-order gradient (SOG) 1.) A unidirectional microphone is formed (see the abovementioned patent), which allows a frequency-independent directional response and a compact
and relatively simple design. These systems are of the free hanging type or of the type arranged
on a table. Also, they have toroidal pole characteristics or unilateral outer pole characteristics.
The pole characteristics of this microphone rely on tight matching of both amplitude and phase
between sensors over the frequency range of interest. Also by contrast, the device according to
the invention forms 5OGs of toroidal or other characteristics in the acoustically reflecting wall or
in the microphone or sensor unit which is arranged directly on the large acoustically reflecting
surface arranged at or near the wall It is easy to do. All the features of the conventional
secondary system are incorporated into the new system, and further, an improvement in the
signal to noise ratio (more than 3 dB for these new sensors) can be seen. The device of the
present invention requires only one sensor to achieve the second order gradient and other
directivity characteristics. And that image matches perfectly to the true sensor in both frequency
and phase. Although the limitations of the effects of omnidirectional or unidirectional sensors
placed near the reflective surface have been described in the literature (US Pat. No. 4,658,425),
the configuration and effects of the invention with respect to first order gradient sensors in
relation to reflectors Is not listed. EXAMPLE In FIG. 1, a directional microphone device 11 is
shown, which has a single commercially available linear gradient (FOG) sensor 13 (Vanasonic
WM-55 D103 type), which has a baffle It is fixed to a central opening 14 of 12 (3 cm diameter x
2, 5 mm thickness). The sensor 13 and the baffle 12 need to be sealed. The sensor 13 and the
baffle 12 are separated from the acoustic reflection surface 15 by a predetermined distance. The
plane defined by the sensor 13 and the baffle 12 is parallel to the reflecting plane 15. The bidirectional axis of the sensor 13 is orthogonal to the reflective surface 15.
The predetermined distance 2 [deg.] From the reflecting surface 15 is a function of the highest
frequency, and if it is zo-2, 5 cm, the upper limit of the frequency is 3.5 kHz. The effective
distance d2 between the two sides of the diaphragm including the baffle 12 was determined by
the size of the baffle 12 and was experimentally set to 2 cm. From geometrical considerations,
the output of a sensor is the sum of its output and its output from its virtual image. It shows that
this obtained sensor has a quadratic gradient characteristic. FIG. 2 shows a model of a two-pole
sensor P, P2, for example the two-pole elements 22, 23 of an electret FOG sensor arranged at a
general angle α on the reflecting surface 21. The following analysis shows that α equal to O is
optimal. For a plane wave of frequency ω, the field is decomposed into an incident source and a
reflected field. pi (') = POJ (-""' yy-4z) (1) pr (t) = Po ♂ 'x +, y + + where k 1, k 2, k is the
wavelength vector field X y With Z component, the total pressure at any place can be expressed
by the following equation. pr (t) = p + (+) + pr (+) = 2 PO cos (k, z) eJ (-"= + ky" (2) The second
equation shows that the obtained field has a standing wave in two directions, X direction , X
direction has a propagating plane wave field. In spherical coordinates, k 1, k 2 and k 3 can be
written as x y z as follows. Here, k is an acoustic wave number. Since the gradient sensor output
is proportional to the spatial derivative of the sound pressure in the direction of the dipole axis,
the output of the bipolar sensor can be written as: If it is assumed that k z << π, then 2 pd (α +
X + 3 '+ 4 t) 2 POke J ′ −
ky Y) a [j cos φ sin e sin α + k z cos 2 (θ) cos α], (5) α−0,
equation 6 Indicates that the directional response is CO 52 (θ) when the gradient axis is
disposed orthogonal to the reflective surface, which is the directivity of a linear four pole or
second order gradient transducer. If it is α-π / 2, this is a directional response of a linear
gradient. In general, assuming that k z <<<, the following is true. I p, ((1, Z) I = 2 Pok [cos 2 φ-θ
5 in 2 a + (k z) 2 cos' (θ) cos 2 α] T (8) Therefore, the bipolar sensor 13 of FIG. Will be placed. A
particular application of a directional microphone mounted on a wall is, for example, for use in a
conference room, a hands-free telephone in a car phone (FIG. 10).
In the car 101, the microphone device 102 of FIG. 1.2 is mounted on the inner surface of the
windshield 107. This device 102 has a primary gradient sensor element (FOG) 103 mounted in a
baffle, this baffle 104 being mounted with a plane parallel to the windshield 107 but with the
bidirectional axis of the sensor Then, the directivity pattern is orthogonal to the windshield 107,
and the distance of the sensor from there is ZO (FIG. 1). The spacing and orientation is
maintained by the body 105 and the glue 106, a mounting that does not transmit vibrations,
through which the microphone lead wire is connected to a car radio (not shown). Wall mounting
is toroidal system Wall mounting is toroidal microphones are designed to have two FOGs in a
baffle. FIG. 3 is a schematic representation of a transducer. From the above analysis, the output
of sensor 31.32 can be written as: pmoid "4Pok2e" a "'frcos2φ5in2esina + cos'e20CO5Ql (11) rs
in α 舅 Z O% CO5α-If pd, (α, r, z6) = 2 Po [jkx cos (k, zO) sin α + k, sin (kz 74 J) A cos Ql
umbrella [eJ (+, r +, y)]. Here, α, r and Zo are as shown in FIG. The toroidal is obtained by simply
adding the outputs of these two sensors. (For brevity, we have removed function attachments. ) L,
assuming that the distance between the two sensors and the wall is small compared to the
wavelength. , DI = 4 pok2 so that?-? / 2, so 'Ploroid l = 4 Pok2 K cos2 ?. As r-zo, C05 ((1) = sin a>
a = 45 ° or, generally, (13) (14) (15) phrase tan (α) =-(16) experimentally performed In the
inventive arrangement, the spacing between the transducers is equal to twice the height of the
transducers from the reflective surface. Therefore, the bipolar is rotated ± 45 degrees with
respect to the surface reference. In this system, two virtual images are generated which should
be summed together with the two sensors. A nice and intuitive way to look at the resulting
transducers is to view the toroid as the sum of two orthogonal arrays consisting of one sensor
and a virtual image of the opposing sensor. This decomposition results in two linear quadripolar
arrays that are orthogonal to one another.
Due to symmetry, the crossing points between the two linear quadrupolar arrays add in the
phase state, thereby forming a toroidal. This linear quadrupolar array has a directivity of cos 2θ
along its primary axis. Since the linear quadrupolar array is orthogonal to each other, a
coordinate system is taken on the primary axis of the linear quadrupolar array. By doing this, the
linear combination of the two microphones is 5 in 2θ + cos 2θ−1. Along the axis orthogonal to
the linear quadrupole arrangement, the response remains cos2θ. Therefore, the resulting
transducer response is second order toroidal. The frequency response of the sum of four sensors
(two real and two virtual sensors) is a function of the wave input angle. FIG. 4 shows a plot 41 of
the theoretical frequency response of the wave input at r = z o-2, 5 Cm in the Z-axis direction.
The dependence of ω2 can be read. Unlike conventional toroidal microphones, the microphone
array of the present invention requires fine matching of only two gradient transducers. A single
microphone consisting of one or two FOG sensors has been described to form the second order
unidirectionality and toroidal directionality characteristics. Those skilled in the art will appreciate
that the linear and planar arrays are also F, because of the quadratic gradient response of each
sensor and its virtual image. It is apparent that G sensors can be used, and that the array can also
be placed near the acoustical reflective surface, increasing the directivity factor of the array. A
similar description applies to toroidal or curved surfaces which are non-planar reflective
surfaces. An acoustically absorbing material or resonator at a particular frequency band is
included in the reflective surface so that the directivity factor of a single microphone can be
modified. For example, at low frequencies, the CO 52 θ response, at high frequencies, the COS θ
response is required. This requires a sound absorbing material that reflects at low frequencies
and absorbs at high frequencies. An arrangement of telephone lines in the conference room is
shown in FIG. 11 (right view is a side view, left view is a front view). Each primary gradient unit
111 is disposed in the baffle 112 to form a line array 113. This line array is arranged separately
from the acoustic reflection wall 114. The vertical alignment of the line array 113 provides a
very narrow pickup pattern in the vertical direction. And is appropriately combined with the
output of the omnidirectional sensor 52 (FIG. 5) and the effective secondary gradient sensor 51
(the axis of which is orthogonal to the table surface) of FIG.
This configuration is shown in FIG. Following the above deployment, the combined sensor output
can be written as: Two combinations-P total + P slope * H (ω) (17) Insert a filter function H (ω)
to compensate for the difference in frequency response between the second order gradient
sensor and the omnidirectional sensor. If H (ω) is set as follows, the toroidal system on tape: In
this system, the position for receiving directivity (sound) is as follows, located at the position of
the talker's head around the table. pc = 2Po eJ (w +, x +, yl gin 2 (θ). (19) From equation 19, it
can be seen that the combination of the filtered gradient and the omnidirectional gradient
becomes a toroidal sensitive to the plane parallel to the table surface. The measurements below
operation are directivity characteristics, frequency response, equivalent noise level, measured as
toroidal and unidirectional sensors on a reflective gradient microphone. Here, spherical
coordinates are used. φ is in the xy plane and θ is the angle from the Z axis. The directivity
characteristic of the device of the FOG and the acoustic reflection surface is given by Equation 6.
As can be seen from the analysis results, combining the FOG with its virtual image forms a
second order unidirectional microphone. Experimental results for various 2 ° points show a
system that corresponds well to the expected theoretical results. Figures 6 and 7 show the results
for 2 °-2 ° 5 cm for both the θ and φ planes. The beam width is about ± 35 degrees. The
accuracy of this system depends on the perfect match between the FOG and its virtual image. The
frequency response of this system depends on the predicted ω2. The corrected frequency
response is shown in FIG. The Alff1 mapped noise floor for the modified toroidal sensor is shown
in FIG. The A-weighted equivalent sound pressure of this sensor noise is 36 dB at 200 Hz or
higher. The above description relates to an embodiment of the present invention, and various
modifications of the present invention can be considered by those skilled in the art, all of which
fall within the technical scope of the present invention. Ru. For example, the line array of FIG. 11
can be replaced by a square array to narrow the pick-up pattern in the horizontal plane. It should
be noted that reference numerals of components in the claims should not be construed as
limiting the scope for easy understanding of the invention.
Brief description of the drawings
FIG. 1 is a diagram of a second-order gradient microphone with a baffle (interference prevention)
-order gradient microphone arranged on the reflection surface, FIG. 2 is a diagram of a first-order
gradient sensor arranged on the reflection surface, 3 is a diagram of the toroidal sensor placed
on the wall, FIG. 4 is a diagram showing the toroidal theoretical frequency response of the antiinterference gradient wall arrangement placed apart on the reflecting surface, and FIG. Figure 6
is a diagram of a toroidal sensor array on tape, Figure 6 shows the measured θ directivity at φ-
90 degrees aligned with the X axis, and the toroidal array of wall devices, Figure 7 is the X axis
Figure 8 shows the measured φ directivity of φ-90 degrees in a toroidal array of wall devices,
Figure 8 shows the toroidal measured (ω 2) corrected frequency response of the wall device ,
Figure 9 is an array of wall devices Fig. 10 shows the measured modified noise floor of the
present invention, Fig. 10 is an illustration of the inventive microphone of a mobile cell phone,
Fig. 11 is an illustration of the linear array of the invention, applicant: American Telephone and
FIG. .
1FIG、2FIG。 FIG、60FIG。 FIG。 FIG。 FIo, 8FIG, 9 drawings of clean
(without change in contents) FIG, 10
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