For fun this last week I made a spectrum analyzer using D3 and the HTML5 Web Audio API. Been meaning to check those out for a while, and spectrum analyzer seemed like the natural way to kill two birds.
Here it is running on Heroku with some of my music as the example track
And the github page here:
Increasing the curve setting gives the spectrum a more logarithmic display, traditionally more common for audio spectrum analyzers.
The other controls are pretty straightforward
Adding live audio input in Chrome Canary (10/4/2012)