The Low-Frequency Acoustic Center: Measurement, Theory, and Application
At low frequencies the acoustic effect of a loudspeaker becomes simpler as the wavelength of the sound becomes large relative to the cabinet dimensions. One point acoustically acts as the centre of the speaker at the lower frequencies. Measurements and acoustic boundary-element simulations verify the concept. Source radiation can be expressed as a multipole expansion, consisting of a spherical monopolar portion and a significant dipolar part, which becomes zero when the acoustic centre is chosen as the origin. Theory shows a strong connection between diffraction and the position of the acoustic centre. General criteria are presented to give the position of the acoustic centre for different geometrical cabinet shapes. Polar plots benefit when the pivot point is chosen to be the acoustic centre. For the first of several applications we consider a subwoofer, whose radiation into a room is strongly influenced by the position of the acoustic centre. A second application that we consider is the accurate reciprocity calibration of microphones, for which it is necessary to know the position of the acoustic centre. A final application is the effective position of the ears on the head at lower frequencies. Calculations and measurements show that the acoustic centres of the ears are well away from the head.
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