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If I were to throw a js one together, I would use these as a starting point:

[0]: http://web-apprentice-demo.craic.com/tutorials?tutorial=33&d...

[1]: https://webaudiodemos.appspot.com/input/index.html

As per my other comment - Audacity can do this. The representation is called a spectrogram.

Edit: Oh and here's a JavaScript one. Haven't tried it as I don't have a microphone. https://github.com/sebleier/spectrogram.js/tree/master

'Spek'[0] is a nice free, lightweight, open source & cross platform Spectrum Analyser.

[0] http://spek.cc/

I don't think it's really an equivalent tool. As I understood, the main point is the real-time analysis of the microphone input. Spek seems to only analyze a « static » sound file (same for SoX and Audacity if I'm correct).

Oh thanks, I'd missed the "live" part.