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In Reply to: RE: "extending the lower bass of the panels" ... posted by Satie on March 19, 2017 at 11:56:53
They spec'd the measurement at 100", though. That isn't near field. And if they tried to gate it at that distance to make it quasi-anechoic, they screwed up *unless* they were outside or in a huge space in which the reflections were outside the window.
Some other wrinkles -- if the gating is too short, you won't get a contribution from the full height of the line source or the floor and ceiling reflections that you do want. And if you do it outside, you won't get a contribution from the ceiling reflections.
Just very hard for these reasons to do meaningful quasi-anechoic or even anechoic of a planar. What counts is the in-room response -- and then you have to accept that bass levels vary widely between rooms and speaker/listening position.
Sounds like an interesting white paper. I remember Pass saying that he tries to mimic the harmonic distortion of the air but I'm afraid I'm missing the effect of device linearity -- that's what feedback is for. Of course you'll hear effects of feedback such as the adverse effect on the harmonic spectrum, but I'd expect the feedback to take care of obvious effects of the magnitude I was hearing. That I think is too high for the harmonic distortion spectrum of a modern amp, usually what I hear of harmonics is subtle except for crossover notch distortion and this didn't sound like that to me.
They say it is nearfield right on the caption. That is how they get "quasi anechoic" in the bass. Atkinson details these measurements somewhere on the Stereophile website.
Part of Pass' philosophy is to avoid using too much feedback, so he can't rely on it to entirely flatten out non linearities in the output devices. He is mainly avoiding high order odd harmonics so far as I understand it and that relates to the HD spectrum in air - which does not contain those. And his DIY projects going into the First Watt concept also try to avoid time domain issues due to overly aggressive feedback, which is something he also worked into his "regular" AB and Class A amps.
He's wrong, though. For the typical planar, 100" isn't near field, it's medium field -- on the border of the near and far field, which is to say it's near field at some frequencies, far field at others. This is very apparent when you look at a plot of dispersion as a function of both distance and frequency.
"Quasi anechoic" just means the measurement has been gated to suppress room reflections and at that distance in the typical room you *can't* gate out the reflections. Unless he measured outdoors, but then it would likely be described as an outdoor measurement and would include the reflections from the ground -- which are actually crucial to the LF response of a dipole since they effectively double the baffle size.
I seem to remember having read Pass on the harmonic distortion of the air some years back. The man is an artist. On the other hand, high order harmonics sound more dissonant and have to be weighted in a harmonic distortion measurement because they're a good deal more audible. One of the main arguments against high levels of negative feedback is that the distortion spectrum changes to emphasize higher-order harmonics. They really should dispense with THD and use the formulas for audibility to create a meaningful weighted measurement. It would take into account both the relative audibility of the harmonics and the Fletcher-Munson curves.
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