DIY 3D Speaker Scanner - the Mathematics and Everything Else

[Tom Kamphuys]

I gave you the hypothesis, some kind of glitch.

I've got an hypothesis:
The bands in the directivity mean that we have large (reconstructed) sound levels in every direction. That means that the omnidirectional N=0 mode (Monopole) is large. Indeed the Monopole is large around 5800Hz.
The fit error is not worse than nearby frequencies, so the incoming Monopole has to compensate the large outgoing Monopole. It is, shifted 180 degrees.
Now it gets more hypothetical. The distance between the two spherical measurement surfaces is 3 cm, half the 5800Hz wavelength. I can image that results in a situation where the difference between incoming and outgoing sound is known, but not the incoming sound strength alone. That degree of freedom is not controlled/measured.
I plan on doing a measurement with a uniform distribution of r for the measurements, but that needs a good night sleep, some programming, bug fixing,...

 

[NTK]

Good observation, Tom! That reminded me of a paper I read during the early part of my research regarding singularities in the field separation math under certain conditions.

Separation theory of the incident and scattered sound fields in spherical coordinate

By the spherical wave spectrum transform, the sound pressures on the two spherical surfaces surrounding the scattering objects with arbitrarily-shaped surfaces are decomposed into spherical wave components that propagate in a known manner, the relationships between the spherical wave components...

www.sciengine.com

Klippel sidesteps the problem by using time gating (to remove room reflections rather than using sound field separation) above ~1-3 kHz when poor frequency resolution is no longer a problem.
One can compute the impulse response response from the frequency sweep. Impulse response = inverse FFT of the transfer function (i.e. frequency response with both magnitude and phase). Then the impulse response is time gated to extract the direct sound, and the FFT of the gated response is the free field/anechoic transfer function.

https://www.klippel.de/fileadmin/klippel/Bilder/Our_Products/R-D_System/NFS/Logan_Klippel%20Near%20Field%20Scanner_2015.pdf

 

[Tom Kamphuys]

I'm thinking of putting the hole setup on a speaker stand in a large barn to start off with a less 'echoic' situation.

I finally relocated the setup.





I used a small dedicated router as the barn has no wifi of its own. Internet is not necessary, but using my phone as a remote control is very handy. At first I tried to use the pi as a router, but I wasn't able to get it working. I decided I didn't want to fight that war and ordered a very small router.

It's running now as a first test. Sometimes the motion system just stops. I can't reproduce it on my test Arduino.

 

[NTK]

I finally relocated the setup.

View attachment 392873
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I used a small dedicated router as the barn has no wifi of its own. Internet is not necessary, but using my phone as a remote control is very handy. At first I tried to use the pi as a router, but I wasn't able to get it working. I decided I didn't want to fight that war and ordered a very small router.

It's running now as a first test. Sometimes the motion system just stops. I can't reproduce it on my test Arduino.

The place looks like a reverb chamber. You are brave!

 

[Tom Kamphuys]

Just like the previous room, only bigger. I hope to apply some gating for the higher frequencies.

 

[Tom Kamphuys]

It's back, unfortunately without measurements. Somehow it freezed everytime quite early in the measurement sequence. It has done that every now and then before, but now it seems to do it every time. I changed the arduino and some cabling, but it didn't help. I've been trying to reproduce it with my HIL test, but that never shows the problem.

Now I need to somehow solve this...

 

[Justin Zazzi]

This is a lot of fun and I watch. Thank you for sharing as you go.

 

[Tom Kamphuys]

I found the problem. When I left the barn, my phone lost the connection to the wifi router. Then the ssh connection was closed, terminal closed, processes started from that terminal were stopped...

I want to improve the cable management of the setup a bit and I need to make the horn standard a bit stronger. Give me some time and I will hopefully report back with some measurements

 

[somebodyelse]

I found the problem. When I left the barn, my phone lost the connection to the wifi router. Then the ssh connection was closed, terminal closed, processes started from that terminal were stopped...

In case you don't know already, something like screen will keep the session running when you disconnect, and let you reconnect to it later.