## Direction of phase shift when rotating sound source clockwise

DSP, Plug-in and Host development discussion.
mystran
KVRAF
5001 posts since 12 Feb, 2006 from Helsinki, Finland
stratum wrote:Just curious, is an omni-point source actually possible in practice?
Omni-directional point-source in acoustics is basically the same thing as a "point light" in computer graphics and it's certainly not a physically plausible model of anything (eg. if you tried to solve for the required energy at the point source, you'd run into a singularity). Rather it's just a simplification that makes the mathematics a lot easier to deal with, based on the assumption that if the source is reasonably small (ie. "looks close enough to a point from a distance") and reasonably omni-directional (although adding directionally dependent attenuation factor is not exactly complicated) then hopefully the error introduced isn't too big of a deal.
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Aleksey Vaneev
KVRAF
3529 posts since 7 Sep, 2002
Thanks for your replies. OK, common logic suggests that at least for lower frequencies the phase incidence is not a thing to be considered. However, I still can't dump my actual practice with sound recording. Most of us know that all-pass filters shift frequencies considerably irrespective of wavelength. I do not insist that all-pass filtering happens when soundwaves hit microphone at different angles, but maybe it's a thing to consider, it's just not as impossible as it seems, simple local feedback loops.

The problem with the "state of art" is that it relies on polar plots and HRTF, without much attention to phase. That's why I've posted my message - I was unable to find a clear answer to my question on the web. While answers given on this thread contradict my studio practice.

vortico
KVRist
191 posts since 19 Jul, 2008
I think your question could be summed up as "What do the measured polar phase plots of certain sources and microphones look like?" We were initially answering your question "If you have a given polar plot of a source and microphone, do their relative angles have an additional effect on phase?" in which the answer is no.

If I had access to a better damped room, I would record some complex-valued polar plots for you, but my guess is that for all microphone types and all frequencies with wavelength under a few cm (>10kHz), a constant phase polar plot is likely a good enough approximation for any modeling application I can think of. By constant phase, I mean that if A(theta) is the complex gain at the angle theta, Im(A) is approximately everywhere 0, meaning that arg(A) jumps from 0 to pi with essentially a discontinuity.

"The problem with the "state of art" is that it relies on polar plots and HRTF"

Polar plots and HRTF both consider phase, as everything is complex-valued. You might notice that only the real part of the gain is plotted, which is evidence for my above claim that Im(A) is approximately zero (otherwise they would include that in their plots.
Last edited by vortico on Tue Jun 26, 2018 2:53 pm, edited 1 time in total.
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Aleksey Vaneev
KVRAF
3529 posts since 7 Sep, 2002
vortico wrote:Polar plots and HRTF both consider phase, as everything is complex-valued. You might notice that only the real part of the gain is plotted, which is evidence for my above claim that Im(A) is approximately zero (otherwise they would include that in their plots.
Well, you are probably "assuming" that. If not, please give some link where it's not an assumption, but a measurement.

stratum
KVRAF
1934 posts since 29 May, 2012
JCJR, mystran: thanks for your replies.

Aleksey: This mentions an HRTF measurement with complex spectra: http://pcfarina.eng.unipr.it/Public/Pre ... naural.pdf
~stratum~

vortico
KVRist
191 posts since 19 Jul, 2008
Yes, I am saying that the fact that I've never seen a microphone polar phase plot is evidence, not an implication, that the polar phase plots are omitted because they are too boring/useless as they are near-constant. Like I said, I'm not going to measure this for your company unless you offer me compensation, but you're free to ignore my educated guess and go down a rabbit hole I'm trying to stray you from for your own benefit. Now your question just seems to be "Someone please measure a polar phase plot for me for free." I'm happy to do this research for you, but for a fee. An alternative is to find the dataset of IRs mentioned in the PDF above by stratum and compute the phase for certain frequencies yourself. It would be valuable for you to post a plot of the results here.
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Aleksey Vaneev
KVRAF
3529 posts since 7 Sep, 2002
Ah, money talk. I never planned to ask YOU or anybody else here to do the measurement. I asked for a link to scientific evidence much like polar plots and HRTF measurement data which is plenty on the net. Without any hard evidence your opinion is just an opinion, moreover you place yourself in the position of "not believing" in a phase shift, and that's a bias making you a bad candidate for the work even if I can pay.

vortico
KVRist
191 posts since 19 Jul, 2008
"I asked for a link to scientific evidence much like polar plots and HRTF measurement data which is plenty on the net"

To my knowledge, there are no published plots or summarized data of the phase dependence of sources and microphones for various off-axis angles, but the raw data is certainly out there in the form of published IRs, from which you can easily extract its frequency/phase information. Try with stratum's link above, if they've published their data.
Last edited by vortico on Tue Jun 26, 2018 3:26 pm, edited 1 time in total.
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Aleksey Vaneev
KVRAF
3529 posts since 7 Sep, 2002
stratum wrote:JCJR, mystran: thanks for your replies.

Aleksey: This mentions an HRTF measurement with complex spectra: http://pcfarina.eng.unipr.it/Public/Pre ... naural.pdf
Of course, HRTF carries the phase information, and that is considered in the method described, unfortunately there's no "polar plot", but for phase which I was looking for without much success.

kryptonaut
KVRian
725 posts since 25 Apr, 2011
The original question asked if the phase shift would be positive or negative when rotating a source clockwise around a microphone. Given the physical symmetry of the microphone, there can logically be no difference in phase shift to the left or right of the microphone's axis. So if the proposed phase shift is in a particular direction when rotating clockwise, it will be in the same direction when rotating anti-clockwise.

If there is any noticeable phase shift between on-axis and off-axis response, then it is going to depend on the physical characteristics of a particular microphone body and the route that sound waves take before being converted into an electrical signal. It would likely be frequency-dependent, and I don't think it's something that could be generalised for all microphones.

As mentioned earlier, the signal inversion from one side of a figure-8 to the other is not the same as a 180-degree phase shift in a positive or negative direction, and you certainly can't simply interpolate between 0 and 180 to get a phase shift for some arbitrary orientation between the front and back directions for such a device.