On Fri, 06 Aug 2004 06:49:53 +0100, Don Pearce
wrote:
On Thu, 5 Aug 2004 22:19:23 -0400, "Arny Krueger"
wrote:
Here's the results of some speaker measurements that I made tonight, based
on passing 50 Hz & 4 KHz mixed 1:1 at about 1.2 volts rms, through a
Peerless 6.5 inch woofer with about 6 mm Xmax (relatively large for a woofer
its size). The speaker is mounted in a roughly 0.4 cubic foot box with no
vent. The power amp is a QSC USA 850. This is not very loud. The mic is an
ECM8000 that is a few inches from the woofer cone.
http://www.pcavtech.com/techtalk/doppler/
The first graph shows the broadband response. The large spikes at 50 Hz and
4 KHz are clearly visible. The second and third harmonics of the 50 Hz tone
are about 30 dB down. The spike for the 4 KHz tone is about 5 dB higher than
the spike for 50 Hz because the woofer is simply that much more efficient at
4 KHz.
The second graph is taken from the same test, with the frequency scale
enlarged to show about 400 Hz on either side of 4 KHz. The first pair of
large spikes are about 50 Hz on either side of 4 KHz, the second are about
100 Hz on either side of 4 KHz, and so on. The distortion products are
probably a mixture of AM and FM distortion, with FM predominating, as the
test is contrived to focus on FM.
While I've got this set up, any other data that anyone would find
interesting?
Well, speakers generally are nonlinear, so what you are seeing here is
intermod. Doppler distortion in speakers is supposedly a "built-in"
effect - nothing to do with non-liearity - that is caused by the same
cone reproducing two frequencies simultaneously. The argument goes
that if a speaker is reproducing a 1kHz tone, but is simultaneously
moving back and forth at 50Hz, the 1kHz tone must be modulated by the
Doppler effect. Of course, if you do the maths of superposition, this
doesn't happen - the tones coexist perfectly without any doppler.
I don't think anyone intended that "superposition" be used
willie-nilly. How about an explanation of why a moving "tweeter"
does not produce doppler.
So this is simple, stright-forward intermodulation between the two
tones.
d
Pearce Consulting
http://www.pearce.uk.com