Point Sound Source Research Articles

An experimental study of transmission, reflection and scattering of sound in a free jet flight simulation facility and comparison with theory

When a free jet (or open jet) is used as a wind tunnel to simulate the effects of flight on model noise sources, it is necessary to calibrate out the effects of the free jet shear layer on the transmitted sound, since the shear layer is absent in the real flight case. In this paper, a theoretical calibration procedure for this purpose is first summarized; following this, the results of an experimental program, designed to test the validity of the various components of the calibration procedure, are described. The experiments are conducted by using a point sound source located at various axial positions within the free jet potential core. By using broadband excitation and cross-correlation methods, the angle changes associated with ray paths across the shear layer are first established. Measurements are then made simultaneously inside and outside the free jet along the proper ray paths to determine the amplitude changes across the shear layer. It is shown that both the angle and amplitude changes can be predicted accurately by theory. It is also found that internal reflection at the shear layer is significant only for large ray angles in the forward quadrant where total internal reflection occurs. Finally, the effects of sound absorption and scattering by the shear layer turbulence are also examined experimentally.

The high frequency performance of wedge-lined free field rooms

The performance of free field rooms of the type used for precision acoustical measurements has been investigated in the frequency range 1 to 20 kHz. The root-mean-square deviation of the field of a point sound source from the ideal free field value is established as a convenient index of performance. Directly comparable data based on detailed field samplings are presented for a small rectangular enclosure lined with three different wedge treatments, together with data for a large enclosure lined with one of these wedge treatments. Detailed consideration is given to transducer requirements and other factors relating to practical implementation of the test method, and to the effect of wedge imperfections on overall room performance. A theoretical analysis in which the sound scattered from each individual wedge is summed to predict the resultant field at any point within the enclosure is shown to be in good accord with the experimental observations.

Barriers and traffic noise peaks

This paper puts forward design rules for estimating the effects of long noise barriers on the propagation of motorway noise peaks. The rules take into account the effects of wind and ground absorption on barrier performance and thus cover a range of practical situations. This paper also suggests that, until the results of current research become available, these design rules may provide a useful indication of the effects of long barriers on the propagation of motorway noise in terms of L 10. The rules have been obtained by applying the results of an experimental study of barrier performance, which used a point sound source, to the spectrum of motorway noise to calculate barrier performance in dB(A). The values of barrier performance, so derived, yield a simple ‘rule of thumb’ relating the performance of long barriers, in attenuating motorway noise peaks, to barrier height.

Monopole radiation in the presence of an absorbing plane

Two theoretical solutions for the field of a point sound source above a plane boundary between air and an absorbing material are discussed. The validity of approximations based on the plane-wave reflection coefficient and on the modified image method are also examined. Results of measurements made with a source in and above slabs of absorbent material are shown to be in agreement with theoretical results. The performance of free-field rooms having plane absorbent treatment and ground-reflection effects in the measurement of aircraft noise spectra can thus be predicted.

Monopole radiation in the presence of an absorbing plane

Two theoretical solutions for the field of a point sound source above a plane boundary between air and an absorbing material are discussed. The validity of approximations based on the plane-wave reflection coefficient and on the modified image method are also examined. Results of measurements made with a source in and above slabs of absorbent material are shown to be in agreement with theoretical results. The performance of free-field rooms having plane absorbent treatment and ground-reflection effects in the measurement of aircraft noise spectra can thus be predicted.