Brüel & Kjær Paper on Phase Distortion, and the hi fi lottery

I am always interested in references to phase distortion – the distortion that is largely unknown and ignored by audiophiles. Here is a 1974 paper from no-nonsense audio measurements people people Brüel and Kjær. They say:

A poor phase response has no influence on the reproduction of pure sines; nor on steady state music, such as a sustained chord from an organ. But it shows up in transients, such as booms from kettle-drums, or bass drums, pizzicato from strings, short blasts from horns, attack on piano and guitar and the clash of snare drums, cymbals and triangles.

More specifically, phase-related distortion is not just an academic curiosity, but is an actual colouration:

If the boom from the kettledrum is considered under such a situation, it results in a coloration of the reproduced signal — too boomy if the bass arrives first at the ear; and too sharp if the mid frequencies arrive first.

Time-alignment of the drivers is key (conventionally performed by physically moving them backwards and forwards relative to each other), combined with the phase responses of the drivers and their crossover filters. And not forgetting that passive crossovers introduce other distortions (iron cored inductor saturation? poor driver damping? etc.) and that ported enclosures are responsible for other anomalies (see The Importance of Bass). With passive crossovers, more ‘ways’ = much more complexity, and they never quite work properly and sound bad in themselves. To avoid this, the fewest number of drivers possible are usually used, and the drivers may be required to work over excessively wide frequency ranges, responsible for cone breakup, beaming and doppler distortion.T

This whole thing is mind-boggling to me: there is no wonder that speaker designers tend to go on about what a difficult job they do and how it is an “art”. There are so many interacting variables that there must always be an element of trial-and-error, and much listening to the results in order to find the least worst compromise. Measurements cannot help much when everything is partly-wrong, and the aim is simply to find the least subjectively-offensive combination of errors.

Combined with standard-issue valve/vinyl audiophile equipment that contributes its own colourations, perhaps a particular colouration partly-masks another colouration when listening to a certain recording at a certain volume in a certain room with the speakers a certain distance from the wall toed in at a certain angle, using a particular type of valve amplifier with a particular transformer tap, playing from a particular cartridge set up with a particular tracking angle and a particular pressing of the LP that has been played a particular number of times previously. You get the picture. The number of permutations goes up exponentially with the number of variables, and with a conventional setup, each completely-subjective listening trial must take a minimum of several minutes to perform. No wonder passive speakers are a lottery.

On the other hand, it seems obvious that the DSP active version with digital source and solid state amplification, reduces the number of variables drastically and allows the designer to simply dial in something close to the optimal settings first time, for much lower cost. Some audiophiles may see this as taking away much of the ‘fun’ of hi fi, but once a price tag of thousands of pounds is attached to it, I think all “fun” has gone out of the window!


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