A Study of Low-Frequency Near- and Far-Field Loudspeaker Behavior
Low-frequency loudspeaker measurements are difficult. Room reflections, mediocre anechoic chambers, and random noise play havoc with the quest. Diffraction is different in nearfield and farfield. This paper covers a range of topics that bear on these problems, such as boundary element diffraction simulations, an approximate theory for low frequencies, methods to shorten the impulse response, and nearfield characteristics. A few points are illustrated with measurements. An earlier simplified diffraction theory of Kessel is checked for axisymmetric cylindrical and rectangular boxes by boundary-element simulations, in an attempt to pin down the diffractive 4pi>2pi transition. It turns out to have a strong connection to the acoustic centre of a loudspeaker. Some measurements are made under various conditions. Shortening methods are used to minimize the deleterious effect of truncating room reflections from the impulse response.
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