On the Audibility of Midrange Phase Distortion in Audio Systems
The current state of our knowledge regarding the audible consequences of phase nonlinearities in the audio chain is surveyed, a series of experiments is described which the authors have conducted using a flexible system of all-pass networks carefully constructed for this purpose, and some conclusions are drawn regarding the audible effects of midrange phase distortions. It is known that the inner ear possesses nonlinearity (akin to an acoustic half-wave rectifier) in its mechanical-to-electrical transduction, and this would be expected to modify the signal on the acoustic nerve in a manner which depends upon the acoustic signal waveform, and so upon the relative phase relationships of the frequency components of this signal. Some of these effects have been known for over 30 years, and are quite audible on even very simple signals. Simple experiments are outlined to enable the readers to demonstrate these effects for themselves. Having satisfied ourselves that phase distortions can be audible, the types of phase distortions contributed by the various links in the audio chain are surveyed, and it is concluded that only the loudspeaker contributes significant midrange phase nonlinearities. Confining the investigation to the audibility of such phase nonlinearities in the midrange, circuitry is described which enables such effects to be assessed objectivbely fo their audible consequences. The experiments conducted so far lead to a number of conclusions. 1) Even quite small midrange phase nonlinearities can be audible on suitably chosen signals. 2) Audibility is far greater on headphones than on loudspeakers. 3) Simple acoustic signals generated anechoically display clear phase audibility on headphones. 4) On normal music or speech signals phase distortion appears not to be generally audible, although it was heard with 99% confidence on some recorded vocal material. It is clear that more work needs to be done to ascertain acceptable limits for the phase linearity of audio components-limits which might become more stringent as improved recording/reproduction systems become available. It is stressed that none of these experiments thus far has indicated a present requirement for phase linearity in loudspeakers for the reproduction of music and speech.
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