One of the sounds of my childhood was the 15.6 kHz noise of the vibrating television flyback transformer, caused by a phenomenon known as magnetostriction. It has since disappeared – not because I grew old enough not to hear it (phew, not yet), but because cathode ray tubes have been replaced by modern display technologies that don't use magnetic deflection coils.
The piercing, ubiquitous sound would easily reveal if someone had a TV on in the house. It would also change a little when channels were changed. But due to its short wavelength, it would also reveal when someone was moving around in the room the TV was in. This was apparent as a modulation that could even be heard through a closed door and in another floor. Listening to it was much like using a passive sonar.
Faking it a little
To get a stable tone at the flyback transformer, the TV apparently has to be tuned to a channel. This poses a problem: all analogue TV broadcasts in this country were discontinued in 2007. I can use my RF modulator to generate a TV channel, which stabilizes the sound. But it's still quite weak for scientific purposes, coming from such a tiny portable TV.
So I ended up sampling the sound and then using a digitally generated version, after confirming that it is indeed a pure sinusoid. I'll play the sound through the speakers and use the laptop's microphone to record the soundscape in the room. Of course, this is different from the setup where the moving person is between the sound source and the listener; but nevertheless, it should give us some insight.
It's a Doppler shift!
Blocking the sound source obviously modulates the signal amplitude to some extent. But turns out even a slight movement anywhere in the room causes noticeable frequency modulation in the echoed tone. This is obviously due to Doppler shift, since the sign of the shift correlates with the direction of the movement in respect to the laptop.
In this video, the zero shift frequency has been filtered out of the spectrogram.
Another childhood mystery settled.