Subwoofer as Microphone--A Question - Page 2 - diyAudio
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Old 8th June 2013, 06:24 AM   #11
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Hey Guys,
I wonder if this is a case of "missing fundamentals" (google it) if there are non-linearites in the pickup system generating harmonics......??
Its used in stuff like this : MaxxBass - Bass Enhancer Plugin | Waves
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Old 8th June 2013, 09:31 AM   #12
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Quote:
Originally Posted by didge View Post
I haven't yet, but I can open it in Ableton Live, which has a decent spectrum analyzer. Can you elaborate on what you mean when you say "If there is a pitch shift, probably some sidebands are generated (AM modulation)"?
There is no way for the signal to shift in frequency without some non-linear behavior, this non-linear behavior shows up in the frequency domain as sidebands (which are sum and difference signals of the original signal + the distortion signal). Where this distortion maybe coming from is something to be investigated if this hypothesis is confirmed. But to give an example, if the (primary) sidebands are 50 or 100Hz (60 - 120Hz in the US) from the fundamental I would take a good hard look at my (linear) power supply.
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Old 8th June 2013, 10:14 AM   #13
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Maybe the math will help clear things up, this is the basic formula for AM modulation (see attachment). taken from this Wikipedia page: http://en.wikipedia.org/wiki/Amplitude_modulation

This may also be helpful: http://en.wikipedia.org/wiki/Sideband
Attached Images
File Type: png AM modulation.png (1.3 KB, 77 views)
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Old 8th June 2013, 07:33 PM   #14
Enzo is offline Enzo  United States
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Isolate the problem. I am reading here several interesting theories, but all are based upon assuming a certain cause. May I suggest some tests. Maybe they are not what you need, but perhaps might give you an idea.

Have the instrument played, and someone listens to the speakers of the PA system. Mic the instrument with your subwoofer and then mic with an actual microphone. Does the shift happen only with the subwoofer and not the regular mic? Does the listener hear any shift at all? As it could be the hearing of the player that is affected. And on that theme, if you hear the shift while playing, do you still hear it when someone else plays? Does it matter if you are listening to amplified sound live or from a recording?

Non-linearity was mentioned. The human ear is non-linear, so it is not automatic to ignore that. This may be of interest:
http://www.cns.nyu.edu/~david/course...tch/pitch.html

I have often heard low notes that seem to shift as the volume gets high, working on systems in my pro audio shop here. It could be my ears, it could be my speakers, I doubt it is the electronics. That is a listening situation, but I see no reason to think it couldn't happen on the recording end.
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Old 9th June 2013, 09:58 AM   #15
wg_ski is online now wg_ski  United States
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The kick drum signal contains transients. These contain frequeny content below the fundamental "ringing" frequency. Remember diff eq? The solutions contain both a natural and forced response. That forced response is an impulse - do an FFT on that and see what comes up. The same is true of the sub driver itself. Hit it with a transient (ie, tap on the cone) and it rings at fs. The less damping the more it rings - short the terminals any you don't get much. So what you end up with is a hodge-podge of the kick's fundamental, harmonics, fs of the sub, and the IM products from all of them. It should sound "thicker", if you had to put a description to it. A real mic will have a low mass and very well damped resonance, to minimize such coloration.


I've always found that kick drums do not sound natural on a system that's not flat to the low 20's, especially a dry kick that doesn't have all the usual processing which doesn't sound "natural" anyway.
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Old 15th June 2013, 10:42 PM   #16
didge is offline didge  United States
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Hey thanks for all the replies. I've been busy on other things since I posted the question and so haven't had the opportunity to probe things a little further. I'm not so well-versed in the math of things, but I think I now have enough information to do some basic tests and see what I find.
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Old 22nd June 2013, 02:44 PM   #17
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Originally Posted by didge View Post
Hi, I'm using a subwoofer as a microphone for low frequencies on a didgeridoo and find that it seems to pitch shift everything slightly downward. I've read about adding a pad switch, and that it is important that impedances match, and the possibility of using a DI box. But all this is in the context of how it is a hot, unbalanced signal, prone to distortion and feedback, etc. Nothing I have read tells me why this pitch shift might be happening. I am hoping someone here might have a good explanation.

I do understand that every mic (or speaker used as a mic) is going to have a different frequency response curve. I also understand that the subjective perception of pitch is colored by the relative loudnesses of the various harmonics that make for timbre. So, perhaps my perception of pitch shift is just an illusion based on the different cones and their respective frequency response curves, but it really seems too pronounced to be just this. I've compared the sub-mic (a 4 or 5 inch Genesis subwoofer) to a Sennheiser e906 mic, and the sub-mic seems about a 1/4 tone lower in pitch.

So I'm asking, would the fact that the impedance of the sub-as-mic likely does not match the input impedance on my pre-amp be responsible for pitch shifting the signal? Is there another potential explanation? Any way to correct it (other than finding a different speaker)?

yes its a little more than that. it has to do with harmonic response. About 20 years ago, when I started to experiment with different drivers as microphones, the speakers fs is directly related to even harmonic sensitivity. as the diaphragm gets bigger, the amount of even harmonics before Fourier attenuation is less.

"o" bridge attenuator 3:1 at 150 ohm. seems to work the best for me.
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Old 22nd June 2013, 03:56 PM   #18
didge is offline didge  United States
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Quote:
Originally Posted by DavesNotHere View Post
yes its a little more than that. it has to do with harmonic response. About 20 years ago, when I started to experiment with different drivers as microphones, the speakers fs is directly related to even harmonic sensitivity. as the diaphragm gets bigger, the amount of even harmonics before Fourier attenuation is less.

"o" bridge attenuator 3:1 at 150 ohm. seems to work the best for me.
Interesting, why only the even harmonics?
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Old 23rd June 2013, 01:01 AM   #19
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the physical device sympathetically resonating verses phase response of the coil compared with the acoustical instrument being micro phoned.
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Old 2nd July 2013, 06:10 PM   #20
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In my humble opinion, a speaker isn't sufficient as a standalone device for capturing audio, but it would do nicely when paired with a traditional microphone. I've seen this done using Yamaha NS10 speakers with an SM57 in a recording studio in Tennessee. Neither input sources sounded great independently, but together, it was pretty glorious.
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