MEMS microphone (ultrasonic?)

eve

Dear Forum,

For a project, we'd like to use the following MEMS microphone with Bela: product 2716 from Adafruit
https://www.adafruit.com/product/2716.

Is this one compatible with the Bela board?
Could someone estimate the sound quality of the microphone? We don't have experience with MEMS microphones.

A second question: we were looking at ultrasonic microphones (not sensors) to use with Bela. So far, I didn’t find anything yet, except for a digital one which seems to be incompatible. Is ultrasonic recording even possible with the Bela board, or is the sampling rate of 44.1Hz too low?

I hope it’s ok that I added these two questions in one discussion.

giuliomoro

eve Is this one compatible with the Bela board?

sure, you should be able to plug it into the audio in using the AC output or the analog in using the DC output.

eve Could someone estimate the sound quality of the microphone?

they tend to be noisier than dynamic mics I think. You can see the specs for yourself here https://www.knowles.com/docs/default-source/model-downloads/spw2430hr5h-b.pdf
59dB SNR, but decent THD (0.1%) at 94dB so it should work fine if you are not at a rock concert.

eve A second question: we were looking at ultrasonic microphones (not sensors) to use with Bela. So far, I didn’t find anything yet, except for a digital one which seems to be incompatible

Could you post a link?

eve Is ultrasonic recording even possible with the Bela board, or is the sampling rate of 44.1Hz too low?

It depends on the range of frequencies you are interested in. The analog ins can run at 88.2 kHz and so can the audio codec (with some work) so you should be able to record frequencies up to 44.1kHz. Anything above that could go through the analog inputs if you have narrow enough filter on it. Can you specify the range of frequencies you need?

eve

Thank you!

giuliomoro Could you post a link?

The microphone I was referring to is the SPH0641LU4H-1 from Knowles: https://www.mouser.de/ProductDetail/Knowles/SPH0641LU4H-1?qs=ueBQf%252BU%252Bi5RrGzOCJCKX0Q%3D%3D.

I will answer your other question soon.

giuliomoro

eve The microphone I was referring to is the SPH0641LU4H-1 from Knowles: https://www.mouser.de/ProductDetail/Knowles/SPH0641LU4H-1?qs=ueBQf%252BU%252Bi5RrGzOCJCKX0Q%3D%3D.

That has a PDM digital out, so it's not easy to use it in combination with Bela. A technically-keen person could write some PRU code to drive the PDM clock line and read the data back from the microphone.. but it's not a simple endeavour and unfortunately we don't have the bandwidth to deal with it ourselves right now.

Example PDM on PRU : https://github.com/dinuxbg/beaglemic/blob/master/pru/pru0-8ch.S

giuliomoro

I mean, you could try to send out a 4.8MHz clock, then apply an analog low-pass filter to the signal on the data line and feed it into the analog input running at 88.2kHz. This would easily give you a decent input with bandwidth up to 44.1kHz. However, in that case a better option is probably to use an I2S microphone as explained here https://forum.bela.io/d/2678-bela-i2s-microphone-connection/22 , though you'll need one that supports higher sampling rate than the INMP441 used there.

eve

The application is to use echolocation or time-difference-triangulation with multiple microphones. So the frequencies should be a bit above 20kHz (human hearing), but not extremely high.

I will look into those I2S microphones.

robinm

eve Hi! that sounds super interesting, care to share some updates about that specific endeavor you were onto? I'd be curious to know more about which solution you adopted for the task of recording ultrasounds for echolocation on Bela 😃