AES British Section lectures - March 2008

The Inertial Air Load of a Loudspeaker Diaphragm

John Vanderkooy, Audio Research Group, University of Waterloo & Steyning Research Centre, B&W Loudspeakers

A typical bass loudspeaker driver has an inertial air load which is about 30% of its actual cone mass. This air mass is often poorly understood, but it is significant in defining the resonance frequency, and it is the purpose of this talk to explain the concept, clarify important aspects, and present corroborative measurements. It is partly tutorial. The immediate surroundings of the diaphragm determine the low-frequency air load, and measurements on a number of test drivers with different mountings and basket conditions are made and assessed, including measurements in vacuum. In general the air load is not accurately represented by the usual approximation that applies to a piston in an infinite baffle, but it does give a rough estimate. Guidance is given so that measurements made of open drivers without a baffle can be corrected to represent the baffling in actual use in a typical loudspeaker. In this way the proper resonance frequency of the driver can be determined when it is mounted in the box.

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