• WANTED: Happy members who like to discuss audio and other topics related to our interest. Desire to learn and share knowledge of science required as is 20 years of participation in forums (not all true). Come here to have fun, be ready to be teased and not take online life too seriously. We now measure and review equipment for free! Click here for details.

DIY 3D Speaker Scanner - the Mathematics and Everything Else

Joined
Nov 19, 2019
Messages
57
Likes
68
Location
Canberra, Australia
Hi Dave, these impressive noise levels have been pretty common now for years in MEMS mics.
Thanks for the links, the noise is better than the ones I remember and indeed impressive.

...you must appreciate about a MEMS data sheet is that it is completely spec'd. Few conventional mics are....
1) https://www.invensense.com/download-pdf/ics-52000-data-sheet/
2) https://www.invensense.com/products/analog/ics-40618/
Better spec'ed in some areas, a bit short on details in others, no polars?
Should be close to omni if it can be mounted in a sufficiently tiny probe so probably not an issue..
The TDK in your second link is low noise but much worse frequency response compared to the AD devices plots you showed.
Practically +-10 dB from 20 Hz to 20 kHz.
I know this can be included in a calibration file but it doesn't fill me with enthusiasm.
Spec'd for tolerance at 1 kHz but no frequency band tolerance?
Probably very repeatable but it would be nice to have a spec.
I do think phase match will be a problem for an array version of this project but I am really happy to have learned about the current state of the art in MEMS microphones.

Best wishes
David
 
Last edited:
Joined
Nov 1, 2019
Messages
13
Likes
11
The MEMS mic idea raises a practical positive if a single one was used in that such a small microphone would allow very lightweight hardware with very minimal acoustic reflection. Along those lines, I have been thinking a Panasonic WM-61a capsule at the end of some light tubing for the same reasons. Nevertheless, given the comments, a "real" measurement microphone may be required.

@No. 5 I still haven't had the time to send you the sketches but "Real Soon Now"!
No problem, I'm patient. :)
 

QAMatt

Member
Manufacturer
Joined
Mar 1, 2019
Messages
29
Likes
80
Hi @Dave Zan

MEMS are all omni. Invensense has a plot in their beamforming document (see fig 2 in the link below). The final mechanics has a much larger impact on the pattern, of course

> The TDK in your second link is low noise but much worse frequency response compared to the AD devices plots you showed.

As I understand it, the peaking comes from internal resonances given the very small size. The silicon inside the MEMS has a very flat response well beyond 20k. But once the silicon goes into its package, you get some high-end peakiing and some low frequency roll-off. And then when you put that package into your own mechanical design, you get more variation.

But yes, these would be dialed out in the software.

> I do think phase match will be a killer for an array version of this project

Since MEMS mics are overwhelmingly used in beamforming and noise cancelling applications, the phase match has to be excellent otherwise those technologies don't work. There's an issue at lower frequencies related to assembly tolerance (eg putting the MEMS silicon into its package). This can result in variations around 30-40 Hz that causes problems for beam forming and noise cancelling. That could be an issue at very low frequencies (sub 40 Hz)

> Spec'd for tolerance at 1 kHz but no frequency band tolerance?

Yes, that would be preferred for sure. It's out there for some mics. My guess is that it's so consistent it's not tested. There's a lot of parameters on semiconductors that are like that too. I did take a look this afternoon at what kind of consistency would be needed. NTK's review of the literature suggests 3 mm of positioning accuracy is needed. The plots below are a measurement of a large (~500W) powered speaker in the near field. The amplitude changes about 0.1dB when the mic (Earthworks M23R) is moved forward about 3mm. So, I'll take that is the required amplitude matching would be required from a mic array, and on first thought, yes, that looks very tight for MEMS

https://www.invensense.com/wp-content/uploads/2015/02/Microphone-Array-Beamforming.pdf
 

Attachments

QAMatt

Member
Manufacturer
Joined
Mar 1, 2019
Messages
29
Likes
80
The MEMS mic idea raises a practical positive if a single one was used in that such a small microphone would allow very lightweight hardware with very minimal acoustic reflection.
Take a look at delta pick and place machines. If the thing you are moving is very light, the delta machines are unbelievable simple and they scale easily to very large sizes. Every axis is identical. There are a lot of hobbyists going this route for large 3D printers and so there's a fair bit of ground already broken there in terms of linkages. These are 3D printers that a person can fit inside.

The math might seem difficult, but it's not. You have 3 motors driving a carriage on a rail, and the location of the head is completely constrained by the location of the motors. Plus, the head stays in a constant orientation during all movements.


 
Joined
Nov 19, 2019
Messages
57
Likes
68
Location
Canberra, Australia
MEMS are all omni...The final mechanics...on the pattern, of course
Yes, that was more or less my point, even a 6 mm electret capsule has to allow for the physical mount to achieve accurate free field response.

As I understand it...internal resonances
The very reliable JCX (John Cox) on DIYaudio wrote that it's the Helmholtz resonance, consistent with what you understand.

> I do think phase match will be a killer for an array version of this project
...There's an issue at lower frequencies related to assembly tolerance...This can result in variations around 30-40 Hz that causes problems...That could be an issue at very low frequencies (sub 40 Hz)
After I posted I decided that "killer" was excessive and edited it to "problem" but we must have cross-posted.
Unfortunately low frequencies is where we need phase match.
Also the response is about 10 dB down at 20 Hz so we will need substantial boost, maybe makes the problem worse?

NTK's review of the literature...3 mm...accuracy is needed.
Actually I think it was more that 3 mm is what Klippel claim.
Presumably it's in the ballpark- it is adequate, and they would make it cheaper and sloppier if it was wildly more than needed.
I suspect it's more that we need a certain rotational accuracy, say 0.01 radian combined with certain radial position accuracy, say 3 mm, or whatever.
Not sure we need your 0.1 dB accuracy match, so maybe it doesn't constrain MEMS mics?
I hated to work out this kind of error sensitivity analysis at university!
Maybe just do a few Monte Carlo simulations to arrive at reasonable values

I still think a movable mic may be simpler but thanks for more links, always fun to learn.
Looks like MEMS would be the sensor to measure compression driver wavefronts.

Best wishes
David
 
Last edited:

NTK

Active Member
Forum Donor
Joined
Aug 11, 2019
Messages
213
Likes
372
Location
US East
A quick update. I got some preliminary results with the double concentric spheres sound field separation. It looks like it can outperform my previous method by a pretty large margin. I'll need to run more tests, clean up my code, and write the report. May take another couple of days (or more).

Then I'll go onto error sensitivity tests (using Monte Carlo simulations). For that I'd like to try using regularized regression to improve the robustness of the numerical method. But first I'll need to spend sometime reading up on regularization and learning how it works.
 

NTK

Active Member
Forum Donor
Joined
Aug 11, 2019
Messages
213
Likes
372
Location
US East
Take a look at delta pick and place machines. If the thing you are moving is very light, the delta machines are unbelievable simple and they scale easily to very large sizes. Every axis is identical. There are a lot of hobbyists going this route for large 3D printers and so there's a fair bit of ground already broken there in terms of linkages. These are 3D printers that a person can fit inside.

The math might seem difficult, but it's not. You have 3 motors driving a carriage on a rail, and the location of the head is completely constrained by the location of the motors. Plus, the head stays in a constant orientation during all movements.
I have thought about using a standard configuration robot too (I was thinking of a 5 revolute axes one). The problem I couldn't figure how to overcome is reach. It will take a pretty huge robot (or a robot with lots of joints) to be able to position the mic(s) all around the speaker.

May be there are cleverer ways I don't know about.
 

Similar threads

Top Bottom