first , a fundamental principle of an optical microphone device in this invention is explained . a diaphragm of the optical microphone device is actuated in accordance with the principle of the microphone called velocity type microphone . now , a microphone that causes an output voltage in proportion to a difference in sound pressure between two adjacent points is presumed . as shown in fig4 an object a may move along the axis y which crosses by an included angle θ with the direction x of the sound . the difference in force to function in both end faces , namely , the driving force f to the object a in the direction of axis y is shown by an expression : in the above , s is the area of the end face vertical to the axis y of this physical object a , velocity v in the axis direction is shown by an expression : v . - f . / z .  m = jωρ 0  sd   cos   θ z .  m · u . ( 2 ) in the above , zm is the mechanical impedance of this object a . therefore , the velocity v in the axis direction of the velocity type microphone is in proportion to the particle velocity , the frequency and the area of the diaphragm . further , it is inversely proportional to the mechanical impedance of the diaphragm . an optical microphone is structured to make the light emitted from the light source put on the diaphragm and to detect the reflection light . therefore , the output voltage of the microphone is in proportion to the amplitude of the diaphragm ( displacement ) x . x . = v . i   ω = ρ 0  sd   cos   θ z .  m  u . ( 3 ) an amplitude of the diaphragm of the optical microphone becomes biggest when the direction of the sound is the same as the direction of the moving axis of the diaphragm ( θ = 0 , 180 [ deg ]), and the amplitude becomes smallest when the both directions are right - angled ( θ = 90 , 270 [ deg ]). because the amplitude of the diaphragm is in proportion to the sensitivity , the directional characteristics to show the sensitivity is shown in fig5 . x . = ρ 0  sd   cos   θ z .  m  p . ρ 0  c = sd   cos   θ c  z .  m  p . ( 4 ) in the above , p is the sound pressure of the diaphragm and c is sonic velocity . an amplitude sensitivity toward the sound pressure is shown in the expression ( 5 ). x . p . = sd   cos   θ c  z .  m ( 5 ) as explained above , the sensitivity of the optical microphone is in proportion to the area of the diaphragm and inversely proportional to the mechanical impedance of the diaphragm . the sensitivity is highest when the direction of the diaphragm oscillation and the direction of the sound is the same , and lowest when they are right - angled . when the mechanical impedance of the diaphragm is resistance ( the rheostatic control state that acoustic resistance and so on is put on both sides of the diaphragm ), sensitivity becomes unrelated value to the frequency . however , when the diaphragm is strained ( stiffness control ), a sensitivity rises in proportion to the frequency as much as high band . conversely , when a diaphragm is made loose ( the inertia control ), a sensitivity falls down as much as high band , because the sensitivity is inversely proportional to the frequency . in the stiffness control and the inertia control , sensitivity depends on frequency and electric correction becomes necessary . in the optical microphone device , the sensitivity toward the sound wave shows a fixed directivity response pattern as shown in fig5 . in the optical microphone device of this invention , the directivity response pattern of a sensitivity shown in fig5 is made to stretch along with the axis direction of θ = 0 , 180 [ deg ], and to be narrowed in the direction of θ = 90 , 270 [ deg ] which is vertical to the axis . fig1 is a block diagram that shows one embodiment of the optical microphone device of this invention . the same code is put to the same part with the conventional device shown in fig8 and the detailed explanation is omitted . because the structure of the microphone head part is the same as the structure shown in fig8 only the part relating to this invention is shown in fig1 . an output from the photodetector 5 is taken out through a filter circuit 8 , amplified by an amplifier 9 , and it becomes microphone output . the filter circuit 8 is used to take out a requested signal component of the frequency range . in the optical microphone device of this invention , it is composed to supply a part of the output signal from this photodetector 5 to a light source drive circuit 13 through a negative feedback ( nfb ) circuit 100 as a negative feedback signal . the light source drive circuit 13 drives this light source 3 by supplying predetermined electric current to the light source 3 . the negative feedback circuit 100 comprises a small signal amplification circuit 10 , a filter circuit 11 which takes out a signal component of the requested frequency range from the output from the small signal amplification circuit 10 , and a comparator 12 . a norm power source 14 that provides reference voltage is connected to the non - inversion - input terminal of the comparator 12 . the signal taken out through the filter circuit 11 is supplied to the reverse input terminal of the comparator 12 . only when an input signal level is less than a predetermined level , the small signal amplification circuit 10 amplifies that signal . when it is composed like this , a low output level is outputted as much as the output of the filter circuit 11 of the comparator 12 is big , and the light source drive circuit 13 is actuated by this to reduce electric current supplied to the light source 3 . as the light source 3 , led may also be used in place of the laser diode . the lens 4 , 6 can be omitted when the lens is also built in the laser diode or led . next , the circuit actuation of fig1 is explained below . fig6 is to explain the circuit actuation of the small signal amplification circuit 10 . the small signal amplification circuit 10 amplifies an input signal only when the input signal level is less than a predetermined level . in fig6 when the input signal level is beyond the b point , an output signal level doesn &# 39 ; t vary from the input signal level , and amplification degree ( gain ) becomes 0 . when the input signal level is not more than the b point , the small signal amplification circuit 10 amplifies the input signal so that amplification degree may grow high as much as the input signal level is small . as shown in fig6 the rate of increase of the output signal against the input signal rises as much as the input signal level is small . here , as the output from the photodetector 5 is in proportion to the reception sound volume , the output of the small signal amplification circuit 10 is greatly amplified as much as small sound volume . as the output of the small signal amplification circuit 10 is inputted to the reverse input terminal of the comparator 12 via the filter circuit 11 , the output level of the comparator 12 decreases conversely as much as small sound volume . as a result , the electric current supplied to the light source 3 declines as much as small sound volume . id est , it is decided as much as small sound volume that the sensitivity of the microphone declines . as a signal beyond the predetermined level is not amplified , an optical output isn &# 39 ; t restricted at the predetermined level . therefore the sensitivity of the microphone never declines . as a result , the directivity response pattern of the sensitivity when loudness was changed is shown in fig7 . in this figure , ss shows small sound , ms shows middle sound , and ls shows big sound . therefore , microphone sensitivity doesn &# 39 ; t change toward a sound beyond the predetermined level . under the predetermined level , as the level of the sound falls down , the sensitivity of the microphone becomes low . when the sound which came from the axis direction which is vertical to the diaphragm and which has a volume that does not cause the sensitivity decline of the microphone is moved from the axis direction , a sensitivity gradually declines along the original directivity response pattern curve . then , when the sensitivity becomes less than a certain level , small signal amplification circuit 10 comes to have amplification degree , and the electric current control of the light source drive circuit 13 works , and the sensitivity of the microphone declines more . as this result , with the optical microphone device that has negative feedback circuit 100 , the width of the directivity beam is more limited than the directivity response pattern of the sensitivity as shown in fig5 . here , a gain of negative feedback grows big by enlarging the amplification degree of the small signal amplification circuit 10 , and the electric current restraint of the light source 3 works toward the small sound so that a directivity response pattern may become limited more . [ 0040 ] fig3 shows an example which made the pattern of the directivity change by making a gain of negative feedback change . fig3 a shows the directivity response pattern when negative feedback wasn &# 39 ; t made . it almost becomes a circular directivity response pattern in this case . next , the directivity response pattern in which a negative feedback is made is shown in fig3 b and 3c . a gain of negative feedback is small in fig3 b , and a gain of negative feedback is big in fig3 c . as shown in these figures , the gain of negative feedback is made to change by varying the amplification degree of the small signal amplification circuit 10 . the directivity response pattern of the sensitivity can be stretched along the axis direction of the optimum sensitivity by this , or narrowed in the direction that is vertical to the axis . also , by changing the point b to begin the amplification by the small signal amplification circuit 10 shown in fig6 the directivity response pattern can be changed . this is because the point , where the sensitivity of the directivity response pattern declines , is changed . by doing like this , the directional characteristics of the sensitivity of the optical microphone may be changed . [ 0041 ] fig2 is a circuit diagram which shows an example of the small signal amplification circuit 10 . two diodes d 1 and d 2 in multiple connection are provided in opposite directions to each other between the reverse input terminal and the output terminal of the amplifier 20 . a non - reverse input terminal of the amplifier 20 is grounded . input is connected to the reverse input terminal of the amplifier 20 via impedance z 1 . in this structure , assuming the impedance of the diode d 1 , d 2 is zd , the gain a 1 of the amplifier 20 is shown in the expression ( 6 ). the impedance zd is the impedance of the diode . therefore , if the potential between the both ends of the impedance zd exceeds the threshold voltage of the diode , the impedance becomes extremely small , and thus the gain a 1 almost becomes 0 by that signal beyond the level . if the potential difference between the both ends of the impedance zd is no more than the above level , the internal impedance of the diode become high , and the internal impedance still grows higher as much as the potential difference between the both ends is low . therefore , the gain al grows higher in accordance with the expression ( 6 ) as much as output voltage is small . when the output becomes beyond a predetermined level ( beyond the threshold voltage of the diode ), the gain disappears , and an output may not become higher . therefore , amplification degree ( gain ) can be changed by changing the impedance z 1 connected to the reverse input terminal . also , the output level that amplification degree becomes 0 can be varied by changing the types of the diode d 1 , d 2 . for example , a silicon diode may achieve the level of 0 . 6 [ v ], and a ge diode may achieve the level of 0 . 2 - 0 . 3 [ v ]. a schottky diode may achieve the level about 0 . 3 [ v ]. to explain the actuation principle of this invention , as a configuration of the head portion of the optical microphone device , the structure that a sound wave enters from only one side of the diaphragm 2 was disclosed . however , in the practical viewpoint , a structure that a sound wave may enter from both sides of the diaphragm 2 is preferable . in the small , velocity type optical microphone , it is preferable that the diaphragm 2 may freely oscillate inside the head 1 by the sound wave . if a block side exists adjacent to the diaphragm 2 and a sound wave doesn &# 39 ; t enter , the oscillation of the diaphragm 2 is obstructed , and the directional characteristics don &# 39 ; t become the pattern forms stated before but become undirectional in some cases . with the optical microphone device which set up a diaphragm 2 in the center of the head 1 so that a sound wave might enter uniformly from both sides , a directivity response pattern shown in fig3 and fig7 appears in the symmetry on the opposition side as well to show the “ 8 ” character characteristics . as explained above , with the optical microphone device of this invention , a part of the output signal from the photodetector is negatively feedbacked to the light source drive circuit through the negative feedback circuit . therefore , in the small signal level , negative feedback becomes strong and the electric current to the light source becomes small and the sensitivity declines . therefore , the directivity response pattern of the sensitivity becomes a narrowed pattern more than an original directivity response pattern . therefore , the directional characteristics of the optical microphone becomes sharp and the sound wave of the specific direction can be received . therefore , there is an advantage that off site noise can be restrained .