scratch-audio/src/Loudness.js

const log = require('./log');

class Loudness {
    /**
     * Instrument and detect a loudness value from a local microphone.
     * @param {AudioContext} audioContext - context to create nodes from for
     *     detecting loudness
     * @constructor
     */
    constructor (audioContext) {
        /**
         * AudioContext the mic will connect to and provide analysis of
         * @type {AudioContext}
         */
        this.audioContext = audioContext;

        /**
         * Are we connecting to the mic yet?
         * @type {Boolean}
         */
        this.connectingToMic = false;

        /**
         * microphone, for measuring loudness, with a level meter analyzer
         * @type {MediaStreamSourceNode}
         */
        this.mic = null;
    }

    /**
     * Get the current loudness of sound received by the microphone.
     * Sound is measured in RMS and smoothed.
     * Some code adapted from Tone.js: https://github.com/Tonejs/Tone.js
     * @return {number} loudness scaled 0 to 100
     */
    getLoudness () {
        // The microphone has not been set up, so try to connect to it
        if (!this.mic && !this.connectingToMic) {
            this.connectingToMic = true; // prevent multiple connection attempts
            navigator.mediaDevices.getUserMedia({audio: true}).then(stream => {
                this.audioStream = stream;
                this.mic = this.audioContext.createMediaStreamSource(stream);
                this.analyser = this.audioContext.createAnalyser();
                this.mic.connect(this.analyser);
                this.micDataArray = new Float32Array(this.analyser.fftSize);
            })
                .catch(err => {
                    log.warn(err);
                });
        }

        // If the microphone is set up and active, measure the loudness
        if (this.mic && this.audioStream.active) {
            this.analyser.getFloatTimeDomainData(this.micDataArray);
            let sum = 0;
            // compute the RMS of the sound
            for (let i = 0; i < this.micDataArray.length; i++){
                sum += Math.pow(this.micDataArray[i], 2);
            }
            let rms = Math.sqrt(sum / this.micDataArray.length);
            // smooth the value, if it is descending
            if (this._lastValue) {
                rms = Math.max(rms, this._lastValue * 0.6);
            }
            this._lastValue = rms;

            // Scale the measurement so it's more sensitive to quieter sounds
            rms *= 1.63;
            rms = Math.sqrt(rms);
            // Scale it up to 0-100 and round
            rms = Math.round(rms * 100);
            // Prevent it from going above 100
            rms = Math.min(rms, 100);
            return rms;
        }

        // if there is no microphone input, return -1
        return -1;
    }
}

module.exports = Loudness;