- Email: [email protected]
A new microphone based on an optical measuring principle
Jounlal of Sound and Vibration (1974) 36(4), 521-526
A N E W M I C R O P H O N E BASED ON AN OPTICAL MEASURING PRINCIPLE K. BUDAL Department of Physics, University of Trondheim, The Norwegian bmtitnte of Technology, 7034 Trondheim--NTH, Norway
(Received 7 December 1973, and hi revisedform 24 April 1974) A new type of microphone is described. The measurements on a microphone prototype with a membrane diameter of 7 mm show the following results: flat frequency response from zero Hz (if required) to 20 kHz, sensitivity 60 mV/N/m 2, output can be loaded with a resistance as low as 600 s distortion satisfies requirements for a high quality microphone, the sound pressure level range is from 44 dB(C) to 131 dB(C). The measuring principle makes sensitivity rather independent of microphone size. 1. INTRODUCTION In this paper a new type of microphone based on an optical measuring principle is described. The sensing element in the microphone is a membrane coated with a metal which reflects light. This membrane is part of an optical system which is such that a membrane deflection generates light modulation on photocells. The electrical signal from the photocells is amplified by an integrated operational amplifier which delivers the output signa ! fr0m the microphone. Section 2 of this paper describes the measuring principle in detail. In section 3 a microphone prototype is presented. Measuring results on the prototype together with some comments are the content of the final section. 2. PRINCIPLE OF MEASUREMENT Figure 1 shows the principle of measurement. The light from the lamp I is transmitted through the screens 3 and 6 and is focused in two centrically opposed points 2' and 7' on the membrane 5 by means of the separated half lenses 2 and 7. The lens 4 together with the membrane 5, which is light reflecting, form an optical system in which the screens 3 and 6 are mutually formed as images on each other. The screens, which are identical, have slot apertures and are adjusted in such a way that 50~o of the reflected light from the membrane is transmitted through the screens. The light which passes through the screens is focused on the photocells 8 and 9 by means of the half lenses 2 and 7. The screens 3 and 6 are in practice made as one unit, such that one half of the screen is imaged on the other half and vice versa. When the membrane is deflected, the reflection angle for the light is changed. Thereby the light patterns on the screens will change position relative to the identical slot apertures, so that the amount of light which falls on the photocells is modulated in accordance with the motion of the membrane. The light modulations on the photocells 8 and 9 are in opposite phase and are proportional to the deflection of the membrane. The signals from the photocells may be applied directly or after amplification. 521
522
K. BUDAL
The fact that the light is reflected from small areas of the membrane has two distinct advantages. In the first place the light can be focused on the areas of the membrane where the change in the reflection angle is greatest. Thus a maximum sensitivity is obtained. Moreover, the smaller the light spots the more linear will the relationship between the deflection of the membrane and the light modulation be. 2
I
T
7
Figure I. Measuring principle for optical microphone. The advantage of using two centrically opposed points on the membrane is the following. Small movements of mechanical parts of the transducer may give a light modulation on the photocells. These inevitable light modulations are a source of noise and zero point drift. Possible sources of such noise include, for instance, a bending ofthe housing ofthe microphone which will rotate the plane of the membrane. It is also readily realized that a movement of the lens 4 or ofthe screens 3 and 6 will give rise to light modulations. Common to all these sources of noise are that the light modulations on the photocells will be in phase. Now, since a deflection of the membrane gives a light modulation 180 ~ out of phase on the photocells, it is clear that it is easy to discriminate between a membrane deflection and the noise by simply measuring the difference in the currents from the photocells. This possibility is absent when using one reflection region only. Light fluctuations of the lamp are also largely eliminated as a source of noise by the device since the light modulations on the photocells then obviously are in phase. The deflection of a circular membrane at a distance r from its center and for frequencies well below its resonance frequency is
(I)
where fl = 2.4,p is the sound pressure, a is the density of the membrane per unit area,fo is the resonance frequency and a is the radius of the membrane. Assume that the light spots have the radial position r on the membrane. Then the angular deflections of the reflected light rays are d Y = 2 / f l / 2 p 2r AO= 2 dr k4rr} afga 2"
(2)
The light pattern on the screens will then change position relative to the slot apertures by the amount As = LAO,
(3)
523
A NEW MICROPHONE
where L is the distance ofthe light pattern from the lens 4. It is easily realized that L is equal to the focal length of the lens 4. If 50Yo of the reflected light is transmitted through the screens for zero pressure, then the relative light modulation for a pressure p is Aq~ As . . . . c~o b/2
p L r 0.29 - - - - - , a f 2 ab a
(4)
where 4o is the light flux on each of the photocells for zero pressure and b is the width of the slot apertures in the screen (3 and 6). The light modulations have opposite signs on the two photocells. The difference between the currents in the photocells is then
(5)
~t=2Io ~oo = 0 . 5 8 p~ I oL , r
where lo is the current in each of the photocells for zero pressure. The maximum value for A l is 21o, corresponding to Aq~/(% = 1. 3. MICROPHONE PROTOTYPE Figure 2 shows the essential part of the transducer which consists of the membrane 5, the lens 4, the grid 3, and the lenses 2 and 7. The lamp and the photocell holder (not shown in the figure) is screwed onto the housing A. This holder also contains an integrated operational amplifier and the microphone connector. A cylindrical cover is drawn over the holder and part of the housing A.
__l
/c
/5
2
~"D ......
-I
Figure 2. Construction of optical part of microphone prototype. Components are built into a cylindrical housing A. The membrane 5 is mounted on a holder G which is screwed onto the housing A. The lens 4 is placed in one end of a cylinder B. Threads at the other end of B allows, axially, adjustment of the lens 4. Adjustment normal to the axis of the housing A is foreseen by the small screws C. The grid 4 consists of slot apertures on a photographic plate D which is glued to the end of the housing A. The lenses 2 and 7 are glued to the photographic plate by a colourless glue.
The membrane is made of mylar and has a coating of aluminium. It is glued to a holder G which is screwed onto the housing A. Parallel to the back of the membrane (not shown in the figure) there is a small plate whose function is to damp the membrane properly. The damping is continuously adjustable. The back plate covers only the central region of the membrane allowing free passage of the light to the reflecting points. The electrical components of the transducer are shown in Figure 3. The solar cells S are connected to an integrated circuit operational amplifier O. The resistance R and the
524
K. B U D A L
capacitance C determine the output signal Vout= A I . R and the upper cut-off frequency f = l/2rrRC of the amplifier. The voltage supply to the amplifier is +15 V. In addition there is a voltage supply of 1.6 V to the lamp L. The voltage of the lamp is well below its nominal value (2.5 V) in order to raise its life time to a satisfactory level.
I~--RC o+15V
i:-ioi, L
I
1,6V
Figure 3. Electrical circuit of microphone prototype. The photocells 5 are connected to an operational amplifier O. The outer dimensions of the microphone are as follows: length 13 cm, largest diameter 2.1 cm, smallest diameter 1.7 cm. The diameter of the membrane is 7 mm and its thickness is 8/am. 4. MEASUREMENTS AND COMMENTS The free field frequency response of the microphone was measured in the sound field from a loudspeaker. The sound pressure of the sound field was determined by a calibrated condenser microphone by removing the optical microphone and placing the condenser microphone in its position. Figure 4 shows the results of the measurements on a properly damped microphone for respectively 0 ~ (normal) and 90 ~ incidence of the sound waves. The frequency response is fairly flat up to 20 kHz and compares favorably with that of a good condenser microphone. This is indeed no surprise since the dynamic systems of the condenser microphone and the optical microphone are very similar. The low frequency fall offofthe frequency characteristic of the optical microphone is determined by the air leakage to the cavity behind the membrane only. If the cavity is made tight, the frequency response is flat down to zero frequency. The transducer can in fact be used for static pressure measurements. The maximum d.c. signal from the transducer can be adjusted (resistance R in Figure 3) and is for the present prototype set to ca. 10 V. "[he linearity of the microphone was measured by means of a standing wave apparatus. The microphone was introduced in the end wall of the cylindrical cavity. The signal from the microphone was compared to that of a probe microphone placed inside the cavity. The standing wave apparatus gave a sufficiently high sound level that the microphone could be excited to its maximum signal level. Figure 5 shows the true r.m.s, signal from th e optical microphone as a function of sound pressure. The harmonic distortion factor of the optical microphone was also measured, and the result is shown in Figure 5. The 1% distortion level is seen to be reached at4.7 V r.m.s, output signal or 6.6 V peak value. This should be compared to the I0 V maximum peak value. The non-linearity of the microphone is due to limitations of the optical system and most probably to irregularities on the surface of the membrane. This is made of mylar of ordinary quality. Specially prepared membranes would probably improve the linearity.
A NEW MICROPHONE
525
The sensitivity of the present microphone prototype is 60 mV/N/m 2 or -24 dB relative 1 V]N/m 2. This is by no means an upper limit. As can be seen from equation (5), the sensitivity is inversely proportional to a, the density of the membrane per unit area. Thus by using a thinner membrane the sensitivity can be increased. The only limiting factor is the stiffness of the air cavity behind the membrane. In the present case this stiffness is less than I 0 Yoof the total stiffness, leaving room for reducing a. However, such membranes had to be specially prepared since they are not available on the market.
o~ _
/
-25
~ -~o ~
-35 I
I
I III1[
I
I
I
I
I
IIIII
0.[
002
I
I
I
I
J
I
I II1[
I
Io
I
20
Frequency (kHz)
Figure 4. Frequency response of microphone prototype for respectively 0~ and 90~ incidence ofsound waves.
Equation (5) shows that the sensitivity of the microphone is inversely proportional to the diameter, a, of the membrane (with the ratio r[a kept constant). By miniaturization of the transducer the focal length, L, would also have to be reduced roughly by the same factor as a. Then it can be concluded that the sensitivity of the optical microphone is fairly independent of its size. I
I
I
I
I
I ///~///
8 7
0
)
3
2
I
20
40
60
80
Sound pressure
I00
120
140
( N / m z)
Figure 5. True r.m.s, output signal and harmonic distortion factor of microphone prototype as a function of sound pressure.
The sensitivity of the transducer can also be raised by reducing the width b of the slot apertures in the screen. In the present prototype b = 0.2 mm which is a fairly coarse grid. The advantage of having large slot apertures is that the transducer becomes rugged since it then can withstand rough treatment without coming out of adjustment.
526
K. B U D A L
The most critical factor concerning stability of calibration is a shift in the light output from the lamp. The luminous flux of the lamp is proportional to the 3.5th power of the lamp voltage. Consequently the lamp voltage must be stabilized. The aging effect of the lamp is governed by the rate of evaporation of the tungsten filament. In practice the aging effect can be made very small by operating the lamp at a low voltage since the lifetime of the lamp is inversely proportional to the twelfth power of the voltage. In the present case the lamp voltage is 1.6 V whereas the nominal voltage is 2-5 V. This means that the lifetime is raised by a factor 200 above its nominal value. The microphone was operated continuously for 300 hours without any significant shift (less than +I mV/N/m 2) in sensitivity taking place. The noise level of the microphone amounts to ca. 200/~V with the conventional filter circuits A, B and C. This noise is believed primarily to arise from shot noise in the photocells, and can only be reduced (relative to the maximum output) by raising the current, lo, through the photocells (see equation (5)): i.e., by using a more intense light source. In the present case lo ~ 10/tA. With fiat filter response from 0.02--40 kHz the noise is ca. 500/tV. Part of the high frequency noise is due to the amplifier. With a noise level of 200 pV and a maximum linear signal of 4.7 V (1 ~o distortion) the dynamic range of the microphone amounts to ca. 87 dB. Since the sensitivity is 60 mV per N / m z this covers the sound pressure level from 44 dB(C) to 131 dB(C). Because the sensitivity can be raised considerably, the upper and lower limits both can be shifted downwards correspondingly. Microphonics are primarily due to vibration of the filament of the lamp. However, if a lamp with a relatively short and thick filament (low voltage lamp) is used microphonics are of no great concern. In addition such a lamp will generally have a good shock resistance. An alternative to an incandescent lamp is of course to use a light emitting diode. It was found that the microphone output at 7 V r.m.s, signal level could be loaded with a resistance as small as 600 f2 without reducing the signal or increasing the distortion. At 10 kHz excitation frequency and 1 V r.m.s, signal level the microphone output was capable of driving a 10 000 pF load capacitance.
A N E W M I C R O P H O N E BASED ON AN OPTICAL MEASURING PRINCIPLE K. BUDAL Department of Physics, University of Trondheim, The Norwegian bmtitnte of Technology, 7034 Trondheim--NTH, Norway
(Received 7 December 1973, and hi revisedform 24 April 1974) A new type of microphone is described. The measurements on a microphone prototype with a membrane diameter of 7 mm show the following results: flat frequency response from zero Hz (if required) to 20 kHz, sensitivity 60 mV/N/m 2, output can be loaded with a resistance as low as 600 s distortion satisfies requirements for a high quality microphone, the sound pressure level range is from 44 dB(C) to 131 dB(C). The measuring principle makes sensitivity rather independent of microphone size. 1. INTRODUCTION In this paper a new type of microphone based on an optical measuring principle is described. The sensing element in the microphone is a membrane coated with a metal which reflects light. This membrane is part of an optical system which is such that a membrane deflection generates light modulation on photocells. The electrical signal from the photocells is amplified by an integrated operational amplifier which delivers the output signa ! fr0m the microphone. Section 2 of this paper describes the measuring principle in detail. In section 3 a microphone prototype is presented. Measuring results on the prototype together with some comments are the content of the final section. 2. PRINCIPLE OF MEASUREMENT Figure 1 shows the principle of measurement. The light from the lamp I is transmitted through the screens 3 and 6 and is focused in two centrically opposed points 2' and 7' on the membrane 5 by means of the separated half lenses 2 and 7. The lens 4 together with the membrane 5, which is light reflecting, form an optical system in which the screens 3 and 6 are mutually formed as images on each other. The screens, which are identical, have slot apertures and are adjusted in such a way that 50~o of the reflected light from the membrane is transmitted through the screens. The light which passes through the screens is focused on the photocells 8 and 9 by means of the half lenses 2 and 7. The screens 3 and 6 are in practice made as one unit, such that one half of the screen is imaged on the other half and vice versa. When the membrane is deflected, the reflection angle for the light is changed. Thereby the light patterns on the screens will change position relative to the identical slot apertures, so that the amount of light which falls on the photocells is modulated in accordance with the motion of the membrane. The light modulations on the photocells 8 and 9 are in opposite phase and are proportional to the deflection of the membrane. The signals from the photocells may be applied directly or after amplification. 521
522
K. BUDAL
The fact that the light is reflected from small areas of the membrane has two distinct advantages. In the first place the light can be focused on the areas of the membrane where the change in the reflection angle is greatest. Thus a maximum sensitivity is obtained. Moreover, the smaller the light spots the more linear will the relationship between the deflection of the membrane and the light modulation be. 2
I
T
7
Figure I. Measuring principle for optical microphone. The advantage of using two centrically opposed points on the membrane is the following. Small movements of mechanical parts of the transducer may give a light modulation on the photocells. These inevitable light modulations are a source of noise and zero point drift. Possible sources of such noise include, for instance, a bending ofthe housing ofthe microphone which will rotate the plane of the membrane. It is also readily realized that a movement of the lens 4 or ofthe screens 3 and 6 will give rise to light modulations. Common to all these sources of noise are that the light modulations on the photocells will be in phase. Now, since a deflection of the membrane gives a light modulation 180 ~ out of phase on the photocells, it is clear that it is easy to discriminate between a membrane deflection and the noise by simply measuring the difference in the currents from the photocells. This possibility is absent when using one reflection region only. Light fluctuations of the lamp are also largely eliminated as a source of noise by the device since the light modulations on the photocells then obviously are in phase. The deflection of a circular membrane at a distance r from its center and for frequencies well below its resonance frequency is
(I)
where fl = 2.4,p is the sound pressure, a is the density of the membrane per unit area,fo is the resonance frequency and a is the radius of the membrane. Assume that the light spots have the radial position r on the membrane. Then the angular deflections of the reflected light rays are d Y = 2 / f l / 2 p 2r AO= 2 dr k4rr} afga 2"
(2)
The light pattern on the screens will then change position relative to the slot apertures by the amount As = LAO,
(3)
523
A NEW MICROPHONE
where L is the distance ofthe light pattern from the lens 4. It is easily realized that L is equal to the focal length of the lens 4. If 50Yo of the reflected light is transmitted through the screens for zero pressure, then the relative light modulation for a pressure p is Aq~ As . . . . c~o b/2
p L r 0.29 - - - - - , a f 2 ab a
(4)
where 4o is the light flux on each of the photocells for zero pressure and b is the width of the slot apertures in the screen (3 and 6). The light modulations have opposite signs on the two photocells. The difference between the currents in the photocells is then
(5)
~t=2Io ~oo = 0 . 5 8 p~ I oL , r
where lo is the current in each of the photocells for zero pressure. The maximum value for A l is 21o, corresponding to Aq~/(% = 1. 3. MICROPHONE PROTOTYPE Figure 2 shows the essential part of the transducer which consists of the membrane 5, the lens 4, the grid 3, and the lenses 2 and 7. The lamp and the photocell holder (not shown in the figure) is screwed onto the housing A. This holder also contains an integrated operational amplifier and the microphone connector. A cylindrical cover is drawn over the holder and part of the housing A.
__l
/c
/5
2
~"D ......
-I
Figure 2. Construction of optical part of microphone prototype. Components are built into a cylindrical housing A. The membrane 5 is mounted on a holder G which is screwed onto the housing A. The lens 4 is placed in one end of a cylinder B. Threads at the other end of B allows, axially, adjustment of the lens 4. Adjustment normal to the axis of the housing A is foreseen by the small screws C. The grid 4 consists of slot apertures on a photographic plate D which is glued to the end of the housing A. The lenses 2 and 7 are glued to the photographic plate by a colourless glue.
The membrane is made of mylar and has a coating of aluminium. It is glued to a holder G which is screwed onto the housing A. Parallel to the back of the membrane (not shown in the figure) there is a small plate whose function is to damp the membrane properly. The damping is continuously adjustable. The back plate covers only the central region of the membrane allowing free passage of the light to the reflecting points. The electrical components of the transducer are shown in Figure 3. The solar cells S are connected to an integrated circuit operational amplifier O. The resistance R and the
524
K. B U D A L
capacitance C determine the output signal Vout= A I . R and the upper cut-off frequency f = l/2rrRC of the amplifier. The voltage supply to the amplifier is +15 V. In addition there is a voltage supply of 1.6 V to the lamp L. The voltage of the lamp is well below its nominal value (2.5 V) in order to raise its life time to a satisfactory level.
I~--RC o+15V
i:-ioi, L
I
1,6V
Figure 3. Electrical circuit of microphone prototype. The photocells 5 are connected to an operational amplifier O. The outer dimensions of the microphone are as follows: length 13 cm, largest diameter 2.1 cm, smallest diameter 1.7 cm. The diameter of the membrane is 7 mm and its thickness is 8/am. 4. MEASUREMENTS AND COMMENTS The free field frequency response of the microphone was measured in the sound field from a loudspeaker. The sound pressure of the sound field was determined by a calibrated condenser microphone by removing the optical microphone and placing the condenser microphone in its position. Figure 4 shows the results of the measurements on a properly damped microphone for respectively 0 ~ (normal) and 90 ~ incidence of the sound waves. The frequency response is fairly flat up to 20 kHz and compares favorably with that of a good condenser microphone. This is indeed no surprise since the dynamic systems of the condenser microphone and the optical microphone are very similar. The low frequency fall offofthe frequency characteristic of the optical microphone is determined by the air leakage to the cavity behind the membrane only. If the cavity is made tight, the frequency response is flat down to zero frequency. The transducer can in fact be used for static pressure measurements. The maximum d.c. signal from the transducer can be adjusted (resistance R in Figure 3) and is for the present prototype set to ca. 10 V. "[he linearity of the microphone was measured by means of a standing wave apparatus. The microphone was introduced in the end wall of the cylindrical cavity. The signal from the microphone was compared to that of a probe microphone placed inside the cavity. The standing wave apparatus gave a sufficiently high sound level that the microphone could be excited to its maximum signal level. Figure 5 shows the true r.m.s, signal from th e optical microphone as a function of sound pressure. The harmonic distortion factor of the optical microphone was also measured, and the result is shown in Figure 5. The 1% distortion level is seen to be reached at4.7 V r.m.s, output signal or 6.6 V peak value. This should be compared to the I0 V maximum peak value. The non-linearity of the microphone is due to limitations of the optical system and most probably to irregularities on the surface of the membrane. This is made of mylar of ordinary quality. Specially prepared membranes would probably improve the linearity.
A NEW MICROPHONE
525
The sensitivity of the present microphone prototype is 60 mV/N/m 2 or -24 dB relative 1 V]N/m 2. This is by no means an upper limit. As can be seen from equation (5), the sensitivity is inversely proportional to a, the density of the membrane per unit area. Thus by using a thinner membrane the sensitivity can be increased. The only limiting factor is the stiffness of the air cavity behind the membrane. In the present case this stiffness is less than I 0 Yoof the total stiffness, leaving room for reducing a. However, such membranes had to be specially prepared since they are not available on the market.
o~ _
/
-25
~ -~o ~
-35 I
I
I III1[
I
I
I
I
I
IIIII
0.[
002
I
I
I
I
J
I
I II1[
I
Io
I
20
Frequency (kHz)
Figure 4. Frequency response of microphone prototype for respectively 0~ and 90~ incidence ofsound waves.
Equation (5) shows that the sensitivity of the microphone is inversely proportional to the diameter, a, of the membrane (with the ratio r[a kept constant). By miniaturization of the transducer the focal length, L, would also have to be reduced roughly by the same factor as a. Then it can be concluded that the sensitivity of the optical microphone is fairly independent of its size. I
I
I
I
I
I ///~///
8 7
0
)
3
2
I
20
40
60
80
Sound pressure
I00
120
140
( N / m z)
Figure 5. True r.m.s, output signal and harmonic distortion factor of microphone prototype as a function of sound pressure.
The sensitivity of the transducer can also be raised by reducing the width b of the slot apertures in the screen. In the present prototype b = 0.2 mm which is a fairly coarse grid. The advantage of having large slot apertures is that the transducer becomes rugged since it then can withstand rough treatment without coming out of adjustment.
526
K. B U D A L
The most critical factor concerning stability of calibration is a shift in the light output from the lamp. The luminous flux of the lamp is proportional to the 3.5th power of the lamp voltage. Consequently the lamp voltage must be stabilized. The aging effect of the lamp is governed by the rate of evaporation of the tungsten filament. In practice the aging effect can be made very small by operating the lamp at a low voltage since the lifetime of the lamp is inversely proportional to the twelfth power of the voltage. In the present case the lamp voltage is 1.6 V whereas the nominal voltage is 2-5 V. This means that the lifetime is raised by a factor 200 above its nominal value. The microphone was operated continuously for 300 hours without any significant shift (less than +I mV/N/m 2) in sensitivity taking place. The noise level of the microphone amounts to ca. 200/~V with the conventional filter circuits A, B and C. This noise is believed primarily to arise from shot noise in the photocells, and can only be reduced (relative to the maximum output) by raising the current, lo, through the photocells (see equation (5)): i.e., by using a more intense light source. In the present case lo ~ 10/tA. With fiat filter response from 0.02--40 kHz the noise is ca. 500/tV. Part of the high frequency noise is due to the amplifier. With a noise level of 200 pV and a maximum linear signal of 4.7 V (1 ~o distortion) the dynamic range of the microphone amounts to ca. 87 dB. Since the sensitivity is 60 mV per N / m z this covers the sound pressure level from 44 dB(C) to 131 dB(C). Because the sensitivity can be raised considerably, the upper and lower limits both can be shifted downwards correspondingly. Microphonics are primarily due to vibration of the filament of the lamp. However, if a lamp with a relatively short and thick filament (low voltage lamp) is used microphonics are of no great concern. In addition such a lamp will generally have a good shock resistance. An alternative to an incandescent lamp is of course to use a light emitting diode. It was found that the microphone output at 7 V r.m.s, signal level could be loaded with a resistance as small as 600 f2 without reducing the signal or increasing the distortion. At 10 kHz excitation frequency and 1 V r.m.s, signal level the microphone output was capable of driving a 10 000 pF load capacitance.