In this paper the cancellation of sound fields in the region exterior to a primary source by the use of a single, higher order, secondary source is discussed. The nature of the secondary source or sources required to achieve that cancellation, either globally or in particular regions, is a subject of current interest. It has recently been suggested that the sound radiation from coherent, finite size radiators may be represented as a superposition of monopole sound fields or alternatively as the sound field radiated by a single multipole. It has also been observed that the sound field radiated by a monopole source may be represented by an infinite order multipole source placed elsewhere. In the same way, multipole sources of finite order may be represented by displaced multipoles of higher order. In principle, it is thus possible to create a single multipole secondary source that could represent, and then if operated out of phase with respect to the primary source, cancel the sound field generated by an arbitrary coherent radiator. In this paper, the results of a theoretical and experimental study to determine the feasibility of such an approach are presented. Secondary sources of up to octupole order have been realized using loudspeakers and the farfield attenuation they may achieve has been established as a function of frequency, secondary source order and primary-secondary source separation distance. Excellent agreement between measurement and theoretical prediction has been obtained. It will be shown in particular that useful levels of low frequency global farfield attenuation may be obtained using secondary sources truncated at octupole order and positioned at a relatively large fraction of a wavelength from the primary source. In addition, it will be shown that the secondary multipole components’ strengths may be adjusted to achieve cancellation in specific angular sectors. It has thus been concluded that the approach suggested here may find applications in active control of low frequency exterior sound fields.
Active noise control, Multipole sources, Global control, Multipole secondary sources
Acoustics and Noise Control
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