Patent GB-1090582-A (GB)
Title: Method of and apparatus for computation particularly suited for microphone absolute calibration
IPC: H04R29/00
Filed: 1965-01-20
Abstract: 1,090,582. Testing microphones. GENERAL RADIO CO. Jan. 20, 1965 [Jan. 27, 1964] No. 2561/65. Heading H4J. [Also in Division G4] For measurement of microphone sensitivity (Fig. 1) a piezo-electric ring 8 of e.g. barium titanate bounds a cylindrical cavity 1 communicating with a microphone 5 to be tested and a reciprocal microphone 7. The sensitivity of microphone 5 is given by where V c  is the cavity volume, P 0  is the atmospheric pressure, j is the ratio of specific heats at constant pressure to constant volume of the cavity atmosphere, C s  is the capacitance of a sampling capacitor V 2 , V 2 <SP>1</SP>, V 1 <SP>1</SP> are voltages appearing across microphones 5, 7, and V s  is a voltage appearing across capacitor C s . A four-section nine-way switch operated by knob S (Fig. 2), controls the successive test connections, and shaft 23 carries triangular cam 20 bearing on pivoted arm 22<SP>1</SP> carrying a brake shoe 22 co-operating with knurled disc 22<SP>11</SP> on shaft 24, which drives a logarithmically calibrated read-out dial D 1 . The shaft is rotatable over slip clutch 26 by knob S<SP>11</SP>, which also rotates an inner coaxial shaft 28 driving logarithmic potentiometer P included in attenuator A (Fig. 1); a spring coupling 32 between washers 34 being interposed between shafts 24, 28. Operatively, in switch position 1<SP>1</SP>, brake 22 is disengaged and knob S<SP>11</SP> sets dial D 1  to introduce constant log {V c / j P o C s }, and in switch position 2<SP>1</SP> oscillator O energizes transducer 3, and a level meter D receives the open-circuit voltage V 2 <SP>1</SP> of microphone 5. Brake 22 is applied to shaft 24. With switches in position 3<SP>1</SP> the brake remains applied and clamps shaft 24, while oscillator O excites attenuator A, whose output is applied in series with microphone 5 to meter D, and potentiometer P is adjusted by knob S<SP>11</SP> until the reading is equivalent to that for input voltage V 2 <SP>1</SP>; without alteration of clamped dial D<SP>1</SP>. It is shown that the angular position of shaft 28 potentiometer represents log (V 2 /E) where E is the oscillator output voltage. With switches in position 4<SP>1</SP>, brake 22 is released by cam 20, so that dial D 1  is movable with knob S<SP>11</SP> and shaft 28, and oscillator 20 excites transducer 3, while the output voltage V 1 <SP>1</SP> of reciprocal microphone 7 is applied to the meter D. In switch position 5<SP>1</SP>, brake 22 is maintained released, and dial D 1  remains coupled to knob S<SP>11</SP>. Oscillator O excites attenuator A whose output is applied in series with reciprocal micro. phone 7 to meter D, and knob S<SP>11</SP> is adjusted until the reading is equivalent to that for V 1 <SP>1</SP>. The angular position of shaft 28 now represents In switch position 6, oscillator O energizes reciprocal microphone 7 in series with capacitance C, and thus excites microphone 5 to produce a voltage V 2  applied to the meter D, and in switch position 7 excites attenuator 7 to energize detector D in series with microphone 5 to read the same value. The attenuator is adjusted until the detector readings correspond; the brake being applied in positions 6<SP>1</SP> and 7<SP>1</SP>. In position 8<SP>1</SP> the brake is disengaged and the driving current set for the preceding position is sampled by capacitor C s , to read V s  at meter D. Finally in position 9<SP>1</SP> with the brake disengaged, the oscillator O excites attenuator A whose output is applied to meter D and the attenuator is adjusted by knob S<SP>11</SP> so that the meter reads V s  and the additional angular rotation of shaft 28 represents log (V 2 /V s ). It is shown that the total reading of dial D 1  represents directly. The apparatus is stated to be applicable generally to the computation of the product of the ratios of successive pairs of parameters represented by electrical analogues.