I would think our hearing mechanisms are considerably more nonlinear than typical mic+electronics. Ever listened to a sole tweeter (hi-grade model, of course) at more than microscopic levels? Most of the perception which seems to indicate "broken tweeter" come from the now unmasked ear's own IM distortion because the lower frequencies are missing.
You can not saturate the gap (it’s air) but you can saturate the steel parts close to the air gap.
The decision about saturation state of the steel close to the air gap is not a straightforward one, because the steel there has to be :
- Deeply saturated for minimizing the effects of the local eddy currents
- Mildly to deeply saturated for minimizing or avoiding permanent magnet’s flux modulation due to varying coil’s magnetic flux
- Just at the verge of saturation but not deeper for minimizing magnetic potential loss in delivering magnetic flux from the magnet to the gap .
George
Exactly as the mathematical operation predicts, AM, FM, or simple addition.
I find it quite disconcerting that a microphone heard a 1khz signal when the two signals were from different speakers. Assuming the test was not performed at 160 to 190 dB, what is going on?
Simulation of a speaker non linearity which included a gradient in the inductance of the coil produced a difference signal. Is it possible this is also a characteristic of an electrodynamic microphone?
Q's;
PMA, was this a coil in mag gap mike.
Will an electrostatic mike produce this difference signal?
If we are going to try to measure accurately, it is very important that we trust the measurement tools.
Jn
B&K treats the distortion inherent in a diaphragm pinned at the edges vs a true piston on their tech site. I don't think you need extreme SPL to see some of this effect. The two speaker test is standard to quantify the mic's contribution.
Hi George,
In a PM speaker the magnet, pole pieces, front plate, airgaps, and VC are in a series circuit, and in most magnetic systems the part of the magnetic circuit with the lowest permeability determines the overall saturation fluxivity. Air has a permeability of 1, and the rest of the speaker magnetic circuit hundreds or thousands of times that, depending on the materials used.
It has been my experience designing switching supply inductors and transformers that any potential saturation can be basically eliminated by the inclusion of even the smallest air gap in the magnetic circuit, or a gap made of other material with low permeability.
So here is my query: Since the magnetic circuit of a speaker has two air gaps, would there not be little chance of any saturation of the rest of the magnetic structure? The wild card here is the inclusion of the field radiated by the VC in series with the PM fluxivity...Curious minds want to know!
Great discussion all!
Howie
I'm lovin that word. no matter how I try, my Ipad will NOT let me type that word beginning with flux..
By two air gaps, do you mean above and below the coil? I would actually consider that as one. If you mean the gap in a neo magnet (neo mu approaches 1), the magnet gap would dominate.
That said, the entire purpose of the iron is to get the gap field up to the 1.5 tesla range, and that is quite difficult for normal iron. It tends to saturate and then the field can just go anywhere it wants to. It's magnetic anarchy I tell ya!!!
Which is why I was so surprised at the level. In both cases it was exactly 53 dB below fundamental. It is odd that keantoken's simulation produced the same diff signal that you measured with hardware.
Then the expectation is that for any reasonable mike (defined as one that won't be damaged should I say Peter piper picked a peck....), the suspension by design gives us non linear behavior like this. And that during a multitone, we may have to ignore all spectral content at the difference frequency?
I am not happy about that. My DVC feedback design has to rely on a mike, as the amp voltage output will be distorted by design.
Edit: I'v also been thinking about two things.
1. If I take apart a full range driver, and co-wind a smaller wire that fits in the interstitial, and tinsel it out, then put it together again, I could take the "half voltage" reactance signal, put it into a gain of two, and subtract it from amp output voltage to reconstruct the diff signal of a true co-wound. That makes conversion of an existing driver easy, as it would not require winding the primary wire. Although I will admit, it will be difficult if I have to use awg 49 wire to fit the interstitials. On the spool, it looks like a brushed copper cylinder..
2. I am wondering if the pure reactance signal (historically considered velocity) could be of any use in a jndrive (sigh, nobody has come up with a good name yet). Or if both derivatives are of use with a current drive system..
Jn
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My measurements when doing my LA article showed better than that but you would be hard pressed to find a cheap capsule electret that does better than -60dB at 120 to 130dB SPL.
I mentioned 160 to 190 dB as a joke, non linearity of air and all..
With a volt into two drivers, I would not expect to see the diff at 50 dB down.
PMA didn't mention the absolute spl, but I can't imagine doing sine tests at 120 dB.
Jn
HEY!!!! Far more important than this audio stuff!!!
You in your digs? All good?
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