Yeah, I'm not completely sold on speed myself, I just think it's an additional potential source of inaccuracy in how we measure FR. I just wanted to emphasize that measured FR is lacking, so there are perceived technicalities, but those technicalities are also just un-measured FR.

It's not just the frequency of vibration that matters, but also the impulse response. Theoretically, transducers are limited by the fact that you can't replicate immediate changes in velocity while you're working with something that has mass. Even if we accept that over 20khz frequency changes aren't perceptible even in transients, FR up to that range is very inconsistent. Nonlinear in this context would be the change in FR when measuring at different amplitudes, which would include resonances for example.

I agree with this take, but the problem with this take is that it's functionally useless. FR is heavily smoothed because of how inconsistent it is with fine-grained measurements, especially in the treble, which is mostly what affects perceived detail. The way FR is measured is also just a constant volume one-tone sweep, which doesn't really catch anything nonlinear. Time-related response isn't measured either.

The more specific take would be "FR, split across all frequencies, of the sound reaching your ears, across all different speeds and volumes, is all that matters" but it's basically useless trivia at this point.

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