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A very useful real application that demonstrates this idea was the development of impulse response [[loudspeaker]] testing in the 1980s which led to big improvements in loudspeaker design. Loudspeakers suffer from [[colouration]], a defect that has nothing to do with the normal measured properties like [[frequency response]] because it is the result of small delayed sounds that are the result of resonance, or energy storage in the cone, the internal volume, or the enclosure panels. These 'smear' the sound, giving reduced 'clarity' or 'transparency' to the sound. Measuring the impulse response, which is a direct plot of this '[[time-smearing]]' provided a tool for use in reducing resonances by the use of improved materials for cones and enclosures. Initially, short pulses were used, but the need to limit their amplitude to maintain the linearity of the system meant that the resulting output was very small and hard to distinguish from the [[noise]]. Later techniques therefore moved towards the use of other types of input, like [[maximal length sequence]]s, and using computer processing to derive the impulse response. Recently this led to the very graphic three dimensional [[waterfall plot]]s that can often be seen in test reviews, of delayed response shown against time for each frequency.
=== Digital
Impulse response is a very important concept in the design of [[digital filters]] for audio processing, because these differ from 'real' filters in often having a [[pre-echo]], which the ear is not accustomed to.
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