Showing papers on "Microphone array published in 1981"

Acoustic near field and local flow properties associated with broadband shock noise

Abstract: Shock noise associated with unheated supersonic jets were investigated using a near field microphone array and a single sensor wedge shaped hot-film probe. Both over and underexpanded cases were investigated using Mach 1.5 and 2.0 convergent-divergent nozzles. Correlation measurements through each shock cell of a single underexpanded case with the Mach 1.5 nozzle were obtained between the hot-film probe and microphone array. The results show for the Mach number cases selected that the probe's response is primarily sensitive to velocity. The results of the hot-film near field microphone correlations show general agreement with certain theoretical models as to the location for shock noise production, although they demonstrate the existence of some large perhaps turbulent structure that collectively interacts and phases the motion of the downstream shocks. The near field microphone correlations demonstrate that downstream shocks dominate shock noise production, and suggests the existence of a Doppler effect in near field of the sources. In addition broadband shock noise is found to also propagate at small angles to the jet axis.

OTS propagation measurements during thunderstorms

Abstract: Otspropagation measurements are described which comprise a complete determination of the incoming polarizations for both linearly and circularly polarized beacons. Auxiliary equipment includes radars, E-field sensors and a microphone array. General trends in the propagation data are discussed and two basic models are proposed for the observed correlations between electrostatic field and cross-polarization. A detailed analysis, with radar data, is now given for one thunderstorm. A method for the prediction of the linear cross-polarization from the circular is shown to work well for this ice-dominated event. Fast lightning-induced, cross-polar jumps are described, and by relating them to acoustic thunder location data, the physical models for particle alignment are discussed.

Improvement of the imaging of moving acoustic sources by the knowledge of their motion

Abstract: To obtain, as a function of frequency, the directivity and spatial distribution of an acoustic source in motion, we make it pass, at the velocity V, in front of a linear microphone array. In classical methods, as the numerical processings of the signals concern T - duration windows, there results that the spectral resolution is limited by the Doppler spreading effect on spectral lines, and also that the source imaging is deteriorated by the blurring effect (VT) whose order of magnitude may reach the dimensions of the source. The purpose of this paper is to propose a method which takes advantage of the knowledge of the source motion to suppress the Doppler and blurring effects through an appropriate choice of sampling times. We present the mathematical formulation, which is illustrated by real applications on vehicles (motor car and air cushion train).