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In Reply to: RE: JA has commented previously, posted by A.Wayne on June 20, 2015 at 20:17:40
>Then why no counter measurements from magnepan, can i request
>imp/mag-phase and FR graphs from magnepan....?Magnepan's Wendell Diller has stated that he doesn't want the company's
speakers measured because they will will reveal proprietary information
regarding the crossovers. This may or not be correct. However, in my
conversations with Wendell, it emerges that he does not like the waterfall
plots that I publish, feeling that they paint Magnepans in a bad light.With panel speakers, cumulative spectral-decay waterfall plots do tend
to look hashy. This may be due to the multiple arrivals at the microphone,
due to the physically large radiator. Or, as I have conjectured in the
magazine, it might indicate Chaotic diaphragm behavior, ie, while the
average position of the diaphragm responds pistonically to the driving
force, individual elements vary in their response, "shimmering," as it
were.I don't currently know how to best investigate either conjecture.
John Atkinson
Editor, Stereophile
Edits: 06/21/15 06/22/15Follow Ups:
Like anyone with an LCR meter can't take apart the xo, trace the circuit, and measure the individual components?
John, you've measured a number of dipolar planar speakers over the years, including electrostats, magnetic planar, and "true" ribbons. Have you found any "typical" measured behaviors that characterize these and distinguish them from monopolar box speakers?
> Have you found any "typical" measured behaviors that characterize these
> and distinguish them from monopolar box speakers?
The dipolar radiation pattern of course, and the fact that other than in
larger rooms, not just the measuring but also the listening takes place
with more of a nearfield effect than with a physically small speaker. And
the hashy-looking waterfall plots. These are all common factors with large
panel speakers.
On the plus size, there are no cabinet resonances. However, depending on
the skill and insight of the designers, there will be some effect from the
stretched diaphragm's "drumskin" resonance.
John Atkinson
Editor, Stereophile
Any rectangular membrane (such as those in Maggies and other panel speakers) is going to produce modes that are even less pretty than the classical drumhead resonance, due to the asymmetry of the diaphragm. Circular membranes produce fairly predictable reflections/resonances; rectangular membranes, less so, with some ugly crush-points at corners.
I can imagine that it wouldn't look pretty under most test procedures.
If I were making a measurement for the purpose publishing in a review and I didn't know what the measurement I was taking actually represented, I might be reluctant to publish it until I did.
Or at minimum, attempt to correlate the measurement with something I heard.
But WTFDIK about publishing?
I would agree with your final conjecture , chaotic behavior of the diaphragm......
> I would agree with your final conjecture, chaotic behavior of the > diaphragm...
In a lecture I attended in the early 1990s, the mathematician Manfred
Schroeder (who passed away a few years back) mentioned that one signature
of a chaotic audio system is that it generates subharmonics.
I have investigated this with a Magnepan speaker. Yes, it does indeed
generate a strong subharmonic at exactly half the signal frequency, ie,
feed the panel a 1kHz signal and as well as the usual harmonic distortion
harmonics at 2kHz and 3kHz, you see a tone appear at 500Hz. But this is
not really conclusive evidence for Chaotic behavior.
John Atkinson
Editor, Stereophile
| But this is not really conclusive evidence for Chaotic behavior.If it were a system running in a truly chaotic regime, then a periodic input would create a non-periodic chaotic "noise" out. It would be really obvious acoustically: think about a self-feedback oscillation 'squeal' but which then turns into a horrible broadband seeming noise.
A nonlinear system can be chaotic with some parameters and non-chaotic with others.
Subharmonic generation requires a nonlinearity, and chaos requires a nonlinearity as well, but they aren't necessarily the same.
What your observation shows is that the mechanisms of nonlinearity in a panel speaker driver may be different (well it's obvious) from those of a conventional driver and amplifiers which tend to make typical higher-frequency harmonics instead of subharmonics.
My guess is that there is resonance/vibration in the panel's supporting structure which naturally doesn't vibrate very well at 1kHz , but with some weak nonlinear coupling can 'keep up' with vibration and resonate and couple to energy every other cycle.
Subharmonics often happen with nonlinearly coupled resonant systems, superharmonics often happen with nonlinearities/static input-output functions which are worse when amplitude is higher {extremes of signals}, e.g. the magnetic field isn't fully homogeneous at the limits of excursion or the transistor's transfer function isn't fully linear.
Edits: 06/22/15
Wow, you certainly said a 'mouthful' there!
'I don't currently know how to best investigate either conjecture.'
Since no one can really confirm either hypothesis it would be premature to jump to any conclusions.
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