i remember in the rhodesia war as well as other situations [Apocalypse now movie] intimidating sounds were projected at the enemy… very effective at times [although i love wagner… but he’s not very popular in vietnam 
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Or this one (spooled up to the right spot):
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herewith results of layman sunday afternoon scientific experiment 
i purchased this gadget via takealot [aka payalot] and downloaded a frequency analyzer app on my mobile to test it as my ears are shot beyond 10Khz - result is that the unit projects a sound of 19659Khz at about 30db at a distance of about 100mm for the duration of about 10sec [pir triggered] - the pir detection is about 4m…
there is a little knob [right front], which if turned can adjust the frequency between around 13Khz to about 21Khz.
i believe the secret is to find the highest db, which at around 5m distance from the speaker was still 30db at 21Khz, not bad i guess
enjoy your sunday nap folks
Do you have a link to the one on Takealot?
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Google tells me kids can hear up to 20kHz.
In practice, I found kids I tested did up to around 16kHz. ( Although they were all teenagers).
30db is not very much at all. It’s like a whisper.
My phone app was louder than 30db in the range I could hear it.
yip @Phil.g00 , 30db is not much for us but how would cats or dogs feel about it… wil try and Google… on the other hand what will it require to up the dB in a simple circuit 
Just check if that phone app shows what sample rate it is using.
If the sample rate is less than 2x the frequency of the signal you are trying to measure, it will appear as a lower frequency than it actually is. i.e. To measure a 20kHz signal, the sample rate needs to be 40 or more k samples/sec.
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it seems like dogs can hear 30db at high frequencie pretty well at about 10m… makes them think, it seems
Very unusual, I sometimes shout at my children for not thinking, but it doesn’t help.