Increased emphasis toward the development of new microphones for measurements at high acoustic pressure occurring in sonic-boom phenomena, flight-vehicle noise, and shock-wave studies has necessitated continued development of devices and methods to measure and calibrate these transducers. Since each method has its limitations with regard to pressure, frequency, and accuracy, it is necessary to apply several calibration methods for evaluation and calibration of pressure transducers. From the great number of calibration devices available, the pistonphone is the apparatus that delivers an absolute calibration of high accuracy in the pressure range up to 185 dB (re 10−4 dyn/cm2). Its application at high pressure and frequency is limited owing to nonlinearity characteristics of the gas, which leads to a distortion of the waveform applied. A high-intensity pistonphone has been developed; its theoretical aspects are described and compared with experimental results. It operates in the range from 5 to 200 cps at sound-pressure levels from 134 to 185 dB. A number of measurements showing nonlinearity, hysteresis effects, and repeatability of sensitivity are described.

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Transitional ballistics involves the events that occur at the muzzle of the weapon prior to, during, and immediately after the uncorking of the projectile. Shock waves precede and follow the projectile during this period of the ballistics cycle. Theoretical and experimental examination of this phase provides details regarding the near noise field associated with a weapon. Shadowgraphs of the nearfield shock waves and space-time diagrams for both supersonic and subsonic projectiles reveal the development of the acoustic and ballistic flow fields. The nonlinear nature of the near noise field is examined in detail by application of Whitham's theory of propagation in nonuniform channels. [Work supported by the Rock Island Arsenal, Illinois.]

Measurement of boundary-layer noise, shock-wave phenomena, and ballistic pressures are interpreted in terms of the pressure response of a microphone. These measurements require a flush-mounted transducer capable of providing accurate information well above audio frequencies. Current methods of microphone pressure calibration are inadequate above frequencies of the order of 20 kc/sec. A useful approach to microphone calibration is to determine the microphone's steady-state response from its transient response to an appropriate pressure input. The shock tube has been used to provide a “step-function” pressure input simulating sound-pressure levels in excess of 180 dB. The microphone response to this “step function” provides a measure of high-frequency resonance characteristics, damping, and an absolute measure of sensitivity. The technique provides a practical method of microphone pressure calibration approaching a frequency of 1 Mc/sec. The difficulty in this approach lies in interpreting transient-response characteristics in terms of the desired steady-state response of the microphone. This paper discusses various shock-tube techniques and methods for determining steady-state response from transient response.

Considering the emphasis being placed on studies of community reaction to sonic-boom phenomena, the simulation of these pressure transients has increased in economic and scientific importance. The expansion of shock waves produced by burst diaphragm techniques similar to those used in the shock tube provides waveforms that approximates the subjective pressure experienced during the onset of a sonic boom. Conventional shock-tube analysis, combined with Whitham's theory concerning the gas dynamics of a nonuniform channel, provides the basis for predicting the pressure time history of the waves generated in the expansion tube. Two shock tubes in a unique arrangement are attached to one acoustic horn. When the two burst diaphragms are subsequently punctured by an electrodynamically driven plunger, a pressure signature is generated which closely resembles the waveform subjectively experienced by an observer of a sonic boom. Experimental data are presented indicating the wide range of pressure amplitudes and pulse durations achievable utilizing this tool. [Work supported by the National Aeronautics and Space Administration, Langley Research Center.]