The analysis, interaction, and measurement of loudspeaker far-field polar patterns

Abstract

This paper discusses some issues related to the measurement and representation of loudspeaker far-field polar pattern data (also known as directivity patterns). The mathematical framework of this method is based on approximating the time-harmonic far-field complex acoustic pressure field using the sum of spherically weighted acoustic point sources. One issue that is discussed in detail is a method where the far-field polar pattern is estimated by rotating a loudspeaker on a turntable and measuring the response at equally spaced intervals. It is often asserted that in order for the far-field data to be most useful, this ‘‘point of rotation’’ must correspond to an ‘‘effective acoustical center.’’ However, this location is often not known a priori, and often a single ‘‘effective acoustical center’’ does not exist or varies with frequency. A simple mathematical transform is developed to move the ‘‘point of rotation’’ of a measured far-field polar pattern to a different location, simplifying the choice of a rotation point. Experimental evidence showing the usefulness of this transform is presented from careful measurements of a model loudspeaker rotated in an anechoic chamber.