I’m sure some people have one or more of those plug-in mouse repellers that claim to keep your home rodent free. Sure they have an LED or even flashy lights to indicate operation, but that does not mean it’s actually emitting any sound. Some fakes sold online (e.g. eBay) actually have nothing other than a few LEDs inside.
While generating test tones with the Audacity (an open source sound editor), I was quite surprised to see just how high tones my phone can pick up. The spectrum analyser App I’m using below is Audizr and the following screenshot shows what happens when I play a 40000Hz test tone in Audacity, well above the human hearing range. For comparison, 16000Hz is the highest tone I can hear:
To get Audizr to sample that high, go into Settings, then Microphone and change the Sampling Range to 96000Hz. Close out of the App and relaunch it. If there’s no 96000Hz option, the phone probably does not sample that high, in which case choose the 48000Hz option.
As most ultrasonic repellent devices operate around 20kHz up, I walked up to our solar powered unit outside our kitchen window. This is motion activated, which keeps our cat from catching birds off the window feeder and is surprisingly effective. Being solar powered, I often wondered if it was still working since our solar garden lights barely work in the winter. Sure enough, there were plenty of tones being emitted above the 20kHz range, not surprising given that our cat clearly avoids that area. :