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What to know ehen choosing/searching a Ultrasonic microphone

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Conrad Hoffman:
I suspect when you get up that high the wavelengths are so short that the response is all over the place, depending on the mounting and surroundings. At 100 kHz we're only talking a few mm.

SiliconWizard:
Absolutely. And likewise, most app notes for those microphones only mention tips for mounting them for audio purposes (so up to 20kHz or so), and do not help for anything else.

That said, I remember Knowles used to have a pretty nice tech support (hope they still do), so do not hesitate to contact them.

mark03:
There aren't many MEMS mics fully specified in the ultrasonic range, but according to the Knowles support rep I talked to, almost any bottom-port device will have a similar frequency response as the ones advertised for ultrasonic coverage.  I've been using, e.g., ICS-40730 in a bat receiver with no problems.

As someone already pointed out, the frequency response of these mics above 20 kHz is definitely not flat.  If anyone has an idea for DIY calibration I would be very interested.  If you have another microphone with known response it would be relatively easy, but that means $$$.  I wondered if a very small spark might be a good broadband (flat) source.

PS  it sounds like you are planning to digitize the signal.  Why would you go to all of the effort of building an analog filter when you could just do it in software?

Detzi:

--- Quote from: Conrad Hoffman on August 06, 2019, 12:17:24 pm ---My guess is the Knowles curves stop because that's as high as they can measure. I know one of their mics has a bandwidth approaching 200 kHz, but I can't remember which one. I used a small piezoelectric disk to excite it and the bandwidth was far beyond what they claimed.

--- End quote ---
The Application Note linked above let's me guess the same, they just picked three well suited ones and measured them above the regular 10kH.

--- Quote ---While in most cases Knowles microphones are used
for receiving audible signals, several models are
also used in ultrasonic applications. Knowles
production line testing typically takes place at
several frequencies up to 10kHz. Therefore the
microphone specifications are also written with
performance data up to 10kHz but not beyond.
While this is sufficient for ensuring the quality of
the outgoing microphone, it does not mean that
the microphone is limited to 10kHz in practice.
--- End quote ---


--- Quote from: mark03 on August 06, 2019, 05:25:38 pm ---There aren't many MEMS mics fully specified in the ultrasonic range, but according to the Knowles support rep I talked to, almost any bottom-port device will have a similar frequency response as the ones advertised for ultrasonic coverage.  I've been using, e.g., ICS-40730 in a bat receiver with no problems.

As someone already pointed out, the frequency response of these mics above 20 kHz is definitely not flat.  If anyone has an idea for DIY calibration I would be very interested.  If you have another microphone with known response it would be relatively easy, but that means $$$.  I wondered if a very small spark might be a good broadband (flat) source.

--- End quote ---
Thank you that is some very usefull piece of information, i have already contacted the knowles support to assist. I am very courious what they have to say but have no answer yet.


--- Quote from: mark03 on August 06, 2019, 05:25:38 pm ---PS  it sounds like you are planning to digitize the signal.  Why would you go to all of the effort of building an analog filter when you could just do it in software?

--- End quote ---
Connecting the ΔΣ-ADC directly to the microphone would not be very robust neither will it give best results in terms of signal quality. Which means i will have to do some signal conditioning to get best results out of the ADC. So when i'm at it why would i pass the "noise" through the adc when i can cut it off with a few cents worth of extra components. This way i can safe money on the µC and time programming it. I honestly don't feel like building a active filter is much of an effort either so i guess it is personal preference too.

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