Sound-Reception and Flow-Noise of Simple Lines and Perforated Pipes in Motion

摘要

Extended acoustic receivers that sample a pressure-field by means of a simple propagating wave and their realization in the form of a microphone-terminated perforated pipe are studied. Motion through the medium subjects a receiver to apparent changes of frequency, to loss of surface reactance, and to flow-noise. It is found that at subsonic velocities the receiver-gain generally decreases with increasing velocity. In the perforated pipe, the phase-speed of sound can exceed the speed of sound in the free medium; true end-fire reception, at least at one frequency, becomes possible through the Doppler-effect. The flow-noise for a number of receiver-lengths and axial velocities is calculated on the basis of several models of the homogeneous and inhomogeneous turbulent boundary-layer pressurefield. The experimental flow-noise, whether obtained in the wind-tunnel or in free-fall through the atmosphere, lies generally higher. (Author)