This paper focuses on the acoustic center, which represents a particular point for a normal sealed-box loudspeaker that acts as the origin of its low-frequency radiation. At low frequencies, the radiation from such a loudspeaker becomes simpler as the wavelength of the sound becomes large relative to the enclosure dimensions, and the system behaves externally as a spherical point source. Although there are near-field effects very close to the loudspeaker, the acoustic center has a clear meaning even a short distance from the enclosure, up to frequencies of about 200 Hz for typical systems. The low-frequency response of loudspeakers in rooms is determined by the position of their acoustic centers. The study is underpinned by: (1) a mathematical multipole expansion of the output of a loudspeaker, (2) an acoustic boundary-element calculation of a number of loudspeaker systems, (3) some measurements that corroborate the concept of the acoustic center, and (4) a discussion of a number of relevant concepts.
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