three.js/src/audio/AudioAnalyser.js

class AudioAnalyser {

	constructor( audio, fftSize = 2048 ) {

		this.analyser = audio.context.createAnalyser();
		this.analyser.fftSize = fftSize;

		this.data = new Uint8Array( this.analyser.frequencyBinCount );

		audio.getOutput().connect( this.analyser );

	}


	getFrequencyData() {

		this.analyser.getByteFrequencyData( this.data );

		return this.data;

	}

	getAverageFrequency() {

		let value = 0;
		const data = this.getFrequencyData();

		for ( let i = 0; i < data.length; i ++ ) {

			value += data[ i ];

		}

		return value / data.length;

	}

}

export { AudioAnalyser };