Spectrogram

Spectrum, spectrogram and stereo phase, from a file or live.

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How it works

Colour is energy: from purple (silence) to yellow (loud). The frequency axis is logarithmic, like the ear.
Live uses the microphone: handy to find room resonances or a whistle to notch out.
The vectorscope shows the mix: vertical = mono, wide = stereo, horizontal = out of phase.

Everything happens on this device: no file is uploaded and nothing leaves the browser.