public final class AnalyserNode

Exposes the recent signal and its spectrum for visualisation, passing its input through unchanged: the Web Audio AnalyserNode.

OfflineAudioContext audio = new OfflineAudioContext(2, 44100, 44100);
OscillatorNode player = audio.createOscillator();
AnalyserNode an = audio.createAnalyser();
an.setFftSize(1024);
player.connect(an).connect(audio.getDestination());
byte[] bars = new byte[an.getFrequencyBinCount()];
// in an animation callback
an.getByteFrequencyData(bars);

The input is down-mixed to mono for analysis. The frequency data is a Blackman windowed FFT of the newest getFftSize() samples, smoothed over time and converted to decibels, as the specification defines it.

Call the getters from one thread, normally the EDT: the spectrum getters share their working arrays. With a real-time AudioContext the render thread keeps writing while they copy, so a copy is the latest audio at the moment it was taken; it is meant for drawing, not for sample-exact measurement, which belongs to an OfflineAudioContext.