Experimental investigation of the parameters effecting the pressure profiles of finite amplitude standing waves in air-filled tubes

Abstract

The finite-amplitude standing wave behavior of air within several tubes was experimentally investigated near standard conditions of temperature and pressure for frequencies below 3000 cps. The sound pressure levels obtained were large enough to generate shock waves of 0.10 atmospheres. The air in the tube was driven by means of a vibrating piston. The motion of the piston was measured by an accelerometer while the pressure at the rigid end was measured with a condenser microphone. It was found that the wave forms for different tubes would be the same if the following quantities were made equal: 1) the ratio of the driver acceleration to the acoustic attenuation constant, and 2) the phase angle between the acceleration and the pressure at infinitesimal amplitudes. Values of the attenuation constant were determined by several different methods including measuring the decay of pressure after clamping the driver piston and by determining the ratio of acceleration to pressure. Observed attenuation constants were of the order of 10)-4) cm (-1). The resonant frequency for infinitesimal amplitude was observed to be the frequency maximizing both the average rectified pressure and the shock strength only for the stronger shocks. However, for weak shocks the shock strength became a maximum at lower and lower frequencies as the pressure decreased. This tendency, as indicated by the amount of second harmonic distortion, continued in the weak finite amplitude region.