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Many of these are why I like panel speakers despite their shortcomings. The surface sound intensity is very low versus cones. It was thought they drove the entire surface evenly though modern measures indicate something more like chaotic shimmer of the panels which evens out at distance. Still easier to get low distortion inherent to the design.A very interesting thread that focuses the mind. What it tells me:
If you test at a fixed frequency, and you assume the room is linear, you know that any harmonics you measure are distortion. Of course the room may not be linear: the harmonics may come from vibrating objects or even flexing panels within the room.
Room reflections will mix with the fundamental and harmonics from the speaker and affect their measured amplitude, sometimes boosting the amplitude and sometimes reducing it. Any single measurement that includes reflections cannot tell you the proportion of distortion products relative to fundamental.
Multiple tests averaged with different mic positions and/or slightly different fundamental frequencies will probably improve the accuracy, but only by some unknown amount.
Near field measurement would be good because it reduces room effects but would only work properly at low test tone frequencies and low harmonic frequencies (wavelengths long compared to driver diameter), otherwise the fundamental and/or distortion products are attenuated (but possibly by a predictable and therefore compensate-able amount..?). To work best it would need a specialised microphone designed for high SPLs, otherwise it just trades off room effect uncertainty for higher microphone distortion.
There are several speaker distortion mechanisms, including cone break-up and Doppler distortion.
Distortion products caused by cone break-up will have nonlinear 'onset' and 'hysteresis' and their own peculiar dispersion characteristics. Shifts in test frequency and/or amplitude may cause these characteristics to change in unknown ways. These characteristics may be temperature and humidity dependent.
Conclusions:
Accurate measurements can only be made in properly anechoic conditions, but as to whether they are meaningful given the nature of cone break-up type distortion...
Never mind the difficulties in making meaningful measurements. What this says to me is that a notion I have often had about pre-distorting the signal using a neural network to compensate for speaker distortion is pie in the sky and wouldn't work, not least because I couldn't meaningfully create the training data and measure the results.
By far the most significant conclusion is that the speaker should be designed to avoid distortion in the first place. Designing a low distortion speaker would be much easier than measuring a speaker for distortion!
There are obvious methods for reducing speaker distortion. These methods may clash with traditional speaker design rules of thumb e.g. "Never place a crossover in the middle of the vocal range". Thankfully active DSP allows you to do this far more transparently than was ever possible previously. You probably also need to make space in your living room for a box that's a bit bigger than is fashionable.
OTOH, the Harman results that are most intriguing to me are the bookshelf and monitor speakers which basically have nothing below 100 hz. Their correlation coefficient was nearly 100% (was it like 99.3% IIRC) for tests vs listener preference. Yet the bass is about 30% of a person's evaluation of sound quality.