The radiating part of circular sources

TL;DR

This paper develops an analysis linking the sound field from circular sources to their radial structure, revealing limits of radiated information and providing explicit models for tonal and random source radiation.

Contribution

It introduces a novel method connecting source radial structure to the radiated sound field without far-field approximations, using Chebyshev polynomials.

Findings

Limited information radiated, fixed by Helmholtz number

Accurate modeling of tonal and random source radiation

Explicit identification of radiating source parts

Abstract

An analysis is developed linking the form of the sound field from a circular source to the radial structure of the source, without recourse to far-field or other approximations. It is found that the information radiated into the field is limited, with the limit fixed by the wavenumber of source multiplied by the source radius (Helmholtz number). The acoustic field is found in terms of the elementary fields generated by a set of line sources whose form is given by Chebyshev polynomials of the second kind, and whose amplitude is found to be given by weighted integrals of the radial source term. The analysis is developed for tonal sources, such as rotors, and, for Helmholtz number less than two, for random disk sources. In this case, the analysis yields the cross-spectrum between two points in the acoustic field. The analysis is applied to the problems of tonal radiation, random source radiation as a model problem for jet noise, and to noise cancellation, as in active control of noise from rotors. It is found that the approach gives an accurate model for the radiation problem and explicitly identifies those parts of a source which radiate.