AES British Section lectures - April 2006

Polar Plots at Low Frequencies: The Acoustic Centre

John Vanderkooy

This talk studies some aspects of how polar plots should be carried out when measuring loudspeakers. It arose as part of a study that was started last year when the author was visiting the B&W Research Centre in Steyning. At low frequencies the effect of a loudspeaker cabinet becomes simpler as the wavelength of the sound becomes large relative to the cabinet dimensions. This allows a particular point to be chosen 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 as 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.

Experiments being carried out now to support the simulations will be presented. The discussion includes a number of other considerations regarding the acoustic centre.

Lecture recording - mp3 format (18.1 MB)

Lecture slides - pdf format (1.6 MB)

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