. The Bell System technical journal . Fig. -i- lOO 1000 FREQUENCY IN CYCLES PER SECOND Pressure calibration of the 394 type condenser microphone. first becomes noticeable in the region of 1200 cycles and reaches amaximum of 6 db at approximately 2200 cycles. The second factorwhich causes a difference between the pressure and field calibrations isacoustic resonance in the shallow^ cavity in front of the causes the pressure actuating the diaphragm to be higher thanthat of the incident sound wave in the frequency region of 1500 to5500 cycles. The maximum increase in pressure occur
. The Bell System technical journal . Fig. -i- lOO 1000 FREQUENCY IN CYCLES PER SECOND Pressure calibration of the 394 type condenser microphone. first becomes noticeable in the region of 1200 cycles and reaches amaximum of 6 db at approximately 2200 cycles. The second factorwhich causes a difference between the pressure and field calibrations isacoustic resonance in the shallow^ cavity in front of the causes the pressure actuating the diaphragm to be higher thanthat of the incident sound wave in the frequency region of 1500 to5500 cycles. The maximum increase in pressure occurs at approx-imately 3500 cycles. If the sound source is so located relative to the. I -55 100 1000 10000 FREQUENCY IN CYCLES PER SECOND Fig. 5—Field calibration of the 394-type condenser niicrophone for a direction ofapproach of sound normal to the diaphragm. microphone that the waves approach from a direction normal to the diaphragm and reflection from surrounding walls and objects is negligible, the combined effect of diffraction and resonance is to produce a maximum departure from flatness of approximately 12 db as is shown by the field calibration Fig. 5.^ If the sound wave travels 9 These curves are taken from unpublished work of P. B. Flanders of the BellTelephone Laboratories, Inc. 54 BELL SYSTEM TECHNICAL JOURNAL along the diaphragm the effective pressure is reduced at the higherfrequencies due to difference in phase. Hence, if the direction ofapproach of the sound wave is parallel to the plane of the diaphragm,the departure from flatness is materially reduced. This is broughtout quite clearly by the field calibration for sound approaching froma direction p
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