Determination of the spatial structure of an acoustic field using a Fourier-Lommel double transform

Analytical and experimental techniques are defined for determining the spatial structure of an acoustic wave propagating in a cylindrical waveguide with rigid walls. The work is intended for augmenting the data base on noise generation in ducts of aircraft, ventilators, etc., so that noise suppression can be enhanced. The field structure is characterized by microphone data from which the wavenumber can be extracted and analyzed by a dispersion relation, thus defining the pressure field. Amplitude and phase data can then be derived for a modal series of a cross section of the duct. A Fourier-Lommel double transform for the pressure then permits, in combination with the orthogonality of the natural modes identified, a spectral analysis of the acoustic field. Preliminary experimental results using the technique are discussed.