I have an occupancy sensor connected to a lighting control system. It uses a 40Khz ultrasonic mic to detect sound to determine if people are still occupying a room.

I'm wondering whether the mic could be used to record normal conversation?

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    \$\begingroup\$ The datasheet of that microphone will tell you. \$\endgroup\$ – Jonathan S. Feb 23 '18 at 18:24
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    \$\begingroup\$ I would be surprised if it can. \$\endgroup\$ – Eugene Sh. Feb 23 '18 at 18:42
  • \$\begingroup\$ are you talking about a motion sensor that also emits an ultrasonic pulse? \$\endgroup\$ – jsotola Feb 23 '18 at 18:45

It really depends on 1. the microphone and 2. the circuit they've designed.

To take two randomly selected ultrasonic mic's from DigiKey:

Knowles SPU0410LR5H:


Invensense ICS-41350:


As you can see, the Knowles part's response to 40kHz is only ~2db less than the 1kHz response. This means that the microphone is as good at picking up "normal sounds" as ultrasonic sounds.

The Invensense part, on the other hand, is much better at picking up ultrasonic sounds. The chart only shows to 20kHz, but the datasheet talks about higher frequencies. Note that, even though the "normal sounds" are attenuated, the mic can still pick them up.

However, this is only the microphone. Even if the mic can hear the sounds, it doesn't mean that the circuitry can. For example, your sensor is likely filtering out lower-frequency sounds to make it easier to use ultrasound to determine occupancy. You can't really tell without looking at the design and the code.

Having designed purpose-built sound sensors, I can tell you that recording and transmitting speech is much, much more resource-intensive than only looking for occupancy. "Occupancy" is a simple "yes/no" signal. Speech would need to be sampled, recorded, buffered, and streamed.


Depends on the microphone and what you mean by "record normal conversation"...

Will it be able to pick something up that you can tell someone is speaking, probably, will you be able to understand it, maybe, will it sound like a normal audio mic... not so much.

Most of the frequencies under 1 - 10kHz will be significantly attenuated.

  • \$\begingroup\$ Even the cheapest 38/40 kHz ultrasonic sensors are actually very flat down to at least 1 kHz. They typically will require quit high gains of course, so their use as a microphone will depend on their physical structure. Some have very heavy aluminum cover plates and their sensitivity sucks ...but the non waterproof variety actually work quite well (though not hi-fi by any means) \$\endgroup\$ – Jack Creasey Feb 23 '18 at 19:16
  • \$\begingroup\$ @JackCreasey thanks for the info.. The ones I found did not list under 10... Even then though 1K is pretty high for speech. But with enough gain... \$\endgroup\$ – Trevor_G Feb 23 '18 at 19:22

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