Reading your parent, I decided to (on Android) dl and try "Spectrum View" (there's an SpectrumView for remote control of a spectrum analyzer) and "Spectrogram",

Reading your parent, I decided to (on Android) dl and try "Spectrum View" (there's an SpectrumView for remote control of a spectrum analyzer) and "Spectrogram", both from the same publisher. I would say both are useful and fun, particularly Spectrogram. Nice, barebone apps that are free and no ads, while apparently very well made. (no, I have no affiliation with the publisher). Actually, I got so mesmerized during my bus trip that I missed my stop :) Fun facts: the brakes have a very sharp 5kHz resonance sound, while the beeper ("I'm closing the doors") were spread out over a few fairly sharp bands. Very fun! When I get home I'm gonna check what the freq contents of my infant daughters laugh and yelling looks like. :)