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I'm building a PCB design that will trigger certain events when the onboard microphone picks up specific sounds outside of the range of human hearing.

The desire is to be able trigger events remotely without a wireless radio connection, for example at events with large crowds of people. This intuitively seemed like it might be a robust method.

Would anyone be able to share any insights in to things I'd need to consider? For example, is it better to go lower or higher frequency than the human range? Are there ranges that would be aggravating to dogs or other animals? Etc etc

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  • \$\begingroup\$ Already exists $$$$ lisnr.com/resources/blog/…. $$$$ $$$$ \$\endgroup\$
    – Kyle B
    Commented Jun 21, 2021 at 4:53
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    \$\begingroup\$ Vod, first thing to do is to educate yourself on the propagation of sound through our atmosphere; and educate yourself not just at sea level but for various altitudes and take note of similarities and differences. You will need to work out the transducer levels (both emitter and receiver transducers) required for your application and distance at any frequency band you choose and the power required will influence your choices, likely. \$\endgroup\$
    – jonk
    Commented Jun 21, 2021 at 9:14
  • \$\begingroup\$ @vod: "Ultrasonic" refers spefically to sounds with a frequency above normal human hearing. "Infrasonic" refers to sounds with a frequency below normal hearing. \$\endgroup\$
    – JRE
    Commented Jun 21, 2021 at 9:17
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    \$\begingroup\$ There were ultrasound remote controls for TVs. They could (and did) trigger when someone accidentally generated an ultrasonic tone of just the right frequency. Metal clinking on glass was one way to accidentally change channels. Just because you can't hear it, it doesn't mean it won't cause interference. \$\endgroup\$
    – JRE
    Commented Jun 21, 2021 at 9:23

3 Answers 3

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It is better to go to higher frequency as microphone will have a hard time picking up lower ones.

You have to make sure your microphone is sensitive on the range you choose.

As for filtering and amplifying the signal, you can use op-amp based high pass filter and an non-inverting amplifier, and a comparator, search these terms on Google and you will get a lot of examples.

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First of I would build a mic/mic pre combination that was actually sensitive in that range, then I would do some experiments to see what ambient sound already exists there. Obviously we don't perceive sounds up there, but that doesn't mean there aren't any.

And you could have fun listening to bats on a quiet night, if you demodulated the received signal into the audible range.

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  • \$\begingroup\$ There are ultrasonic microphones available. An ordinary audio microphone has poor or no sensitivity to ultrasonic frequencies but is easily overloaded with noise "at events with large crowds of people". \$\endgroup\$
    – Audioguru
    Commented Nov 1, 2023 at 18:35
  • \$\begingroup\$ There is a huge range of "audio microphones" available, of many different types, such as ribbon, condenser (various diaphragm sizes), electret and so on, and an absolute plethora of designs of each type. I think it rather an assumption to make a blanket statement about the frequency range of all of them. As for overloading, well, that's what filter circuits are for. \$\endgroup\$
    – danmcb
    Commented Nov 6, 2023 at 7:28
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There are a bunch of "bat detector" circuits on the web. Some of them are very simple and work decently well in practice. For instance, this one from http://batdetector.freevar.com/ ...

enter image description here

Here this ultrasonic transducer does a lot of filtering for you. It's the same transducer that you find in those Arduino ultrasonic range finder modules. Amplitude information is lost, but it should work okay as a signalling device.

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