Polar Plots for Low Frequencies: The Acoustic Centre
This paper studies some aspects of how polar plots should be carried out when measuring loudspeakers. At low frequencies the effect of the cabinet becomes simpler as the wavelength of the sound becomes large relative to the cabinet dimensions. This allows a particular point to be picked out which acoustically acts as the centre of the speaker at the lower frequencies. This concept is verified by acoustic simulation, and also theoretically by expressing the source radiation into a multipole expansion. Some general criteria are presented to give estimates of the acoustic centre for different geometrical aspects of the cabinet. Polar plots pivoted about the acoustic centre display very consistent low-frequency characteristics. The discussion includes a number of other considerations regarding the acoustic centre.
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