Why do equalisers sound different

Every equaliser has its own sound, and for some time it has been suspected that this is not simply a matter of their amplitude responses. It has been believed that the ears are insensitive to the phase response of equalisers but now some people are starting to claim that, really, amplitude response is quite unimportant and most of the subjective effect of equalisers is due to their phase response. Certainly, phase response is relevant, as noted by Phil Newell in his series on monitor loudspeakers. He notes, correctly, that adjusting the polarity of speaker units to maximise flatness of frequency response can often give a much more coloured result than the other polarity, which can give a sharp dip in the amplitude response but a much smoother phase response. However, this does not prove (and neither did Newell claim) that phase response is much more important than amplitude response, only that both are important and must be carefully related to one another for the best sound. Let’s look at what determines the subjective sound of an equaliser. Unlike others, I do not claim to give definite answers. My aim, rather, is to make some tentative conjectures, report some rules-of-thumb that have often been used with some success and to raise some questions so people can give some intelligent thought to the problem and maybe eventually find some answers. We will rapidly enter the treacherous areas of hi-fl subjectivism, however, unlike the woolly-minded approach of many in the hi-fl press, I believe that ultimately one needs no magic pseudo-science to explain the mysteries. The problem with most ‘objectivists’ who demand measurable reasons for subjective differences, is that they are very narrow-minded about what kind of measurements they will consider. They often demand that measurements can easily be done on conventional audio test setups. We shall see that it is highly unlikely that some of the most audibly important aspects of equaliser response can be measured either in the amplitude or the phase response but that we shall probably have to look elsewhere. Now this is very near heresy. It is a standard mathematical result in the mathematical theory of linear filters that the behaviour of any such filter is completely determined if one knows its amplitude and phase response. This is no longer true if the filter has nonlinear distortion – and many subjective differences between equalisers are believed to be due to nonlinear distortion effects. However, I claim that even if one has a perfectly linear filter and measures both its amplitude and its phase response, one will still not, from these measurements alone, be able to predict its sound. I am a mathematician, and I do believe the theoretical mathematical result that the filter response is completely specified by its amplitude and phase response. The key words in the above are ‘if one... measures its amplitude and phase response’. The point is that real-world measurements are never exact, and what we shall see is that incredibly small changes in amplitude and phase response, supposedly quite ‘negligible’ according to objectivist ideas, can actually have large audible effects. This is not to say that these effects cannot be measured, only that measurements of amplitude and phase responses are not the way to do the measurement. If I prove to be right in my claims, we shall have to stop thinking of filters purely in terms of their amplitude and phase responses but will have to find other new ways of looking at them.