Loudspeaker acoustics

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Loudspeaker acoustics is a subfield of acoustical engineering concerned with the reproduction of sound and the parameters involved in doing so in actual equipment.

Engineers measure the performance of drivers and complete speaker systems to characterize their behavior, often in an anechoic chamber, outdoors, or using time windowed measurement systems -- all to avoid including room effects (eg, reverberation) in the measurements.

Designers use models (from electrical filter theory) to predict the performance of drive units in different enclosures, now almost always based on the work of A N Thiele and Richard Small.

Important driver characteristics are:

Frequency response
Off-axis response (dispersion pattern)
Sensitivity (dB SPL for 1 watt input)
Maximum power handling
Non-linear distortion
Colouration (ie, more or less, delayed resonance).

It is the performance of a loudspeaker/listening room combination that really matters, as the two interact in multiple ways. There are two approaches to high quality reproduction. One ensures the listening room 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 fields created by reflections off room surfaces. The other attempts to arrange the listening room to be 'dead' acoustically, leaving indirect sound to the dispersion of the speakers need only be sufficient to cover the listening positions.

A dead acoustic may be best, but only when properly filled with correct '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 psychologically (and for some physically) stressful and unnatural, and so normal stereo reproduction requires a reasonable reflecting acoustic in the room plus a uniform angular dispersal from the speakers.

It is in large part the directional properties of speaker systems, which vary with frequency that make them sound so different even when they measure well on-axis. Acoustical engineering in this instance is concerned with adapting these variations to each other.