Loudspeaker acoustics

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Loudspeaker acoustics are important when attempting to reproduce sound realistically (with good sound quality).

Engineers make measurements to address different aspects of sound quality, usually in an anechoic chamber or outdoors, in order to avoid the effect of reflected sounds (reverberation). While various methods exist for minimising the effect of a room, such as delayed gating, they tend to be unsatisfactory, especially at low frequencies.

Designers use models to predict the performance of drive units in different enclosures, and this has become more sophisticated with computer modelling, which usually involves Thiele Small parameters relating to the drive unit properties (magnet strength, free-air resonance etc, as defined by the Australian engineer Neville Thiele).

Important characteristics are:

Frequency response
Off-axis response (dispersion pattern)
Sensitivity (dB SPL for 1 Watt input)
Maximum power handling
Non-linear distortion
Colouration (delayed resonance).

In the final analysis it is the performance of a loudspeaker/listening room combination that matters, as the two interact in various ways. There are two approaches to good reproduction. One says that the listening room should be reasonably 'alive' with reverberant sound at all frequencies, in which case the speakers should ideally have equal dispersion at all frequencies in order to equally excite the reverberant field created by reflections off side walls and ceiling in particular. The other says that the listening room should be 'dead' acoustically, in which case the dispersion of the speakers need only be suffient to cover the listening positions.

A dead acoustic has to be the ideal, but only when accompanied by true surround reproduction, so that the reverberant field of the original space is reproduced realistically. This is currently very hard to achieve, and so loudspeakers for normal stereo need uniform dispersal. Listening in a dead room is stressful and unnatural, and so normal stereo reproduction requires a good uniform acoustic in the room plus a uniform angular dispersal from the speakers.

It is the varying directional properties (with frequency) of speakers that make them sound so different even when they measure fairly flat on-axis. That and the delayed resonances of cones, cabinet, and internal volume.