beating.js

// Beating — two close frequencies create amplitude modulation.
// Run: node examples/beating.js 440 3 5s
// Run: node examples/beating.js freq=440 diff=3 -d 5s
// Keys: ←/→ ±0.5 Hz beat · ↑/↓ ±semitone carrier · q quit

import { AudioContext } from 'web-audio-api'
import { init } from './graphs/beating.js'
import { args, num, sec, keys, status, clearLine, pausedTag, help } from './utils.js'

help({
  description: 'hear beating between two nearby frequencies',
  usage: ['', '[carrier] [difference] [duration]', 'freq=440 diff=3 dur=5s'],
  options: [
    ['freq=<hz|note>', 'lower carrier frequency (default: 440)'],
    ['diff=<hz>', 'difference/beat frequency (default: 3)'],
    ['-d, --duration <time>', 'run time with optional s/m/h suffix (default: 30s)'],
  ],
  controls: [['Space', 'pause/resume'], ['← / →', 'change difference by 0.5 Hz'], ['↑ / ↓', 'move carrier one semitone'], ['Q / Esc', 'quit']],
})

let { pos, $ } = args()
let nums = pos.filter(t => /^\d/.test(t) && !/[smh]$/.test(t) || /^[A-G][#b]?\d$/i.test(t))
let f = num(nums[0] || $('freq', 440))
let diff = +(nums[1] || $('diff', 3))
let dur = sec(pos.find(t => /\d[smh]$/.test(t)) || $('dur', '30'))

let ctx = new AudioContext()
await ctx.resume()

let demo = init(ctx, { frequency: f, difference: diff, duration: dur, gain: 0.3 })
let [osc1, osc2] = demo.sources

let retune = () => {
  let t = ctx.currentTime
  osc1.frequency.setTargetAtTime(f, t, 0.02)
  osc2.frequency.setTargetAtTime(f + diff, t, 0.02)
}

let render = status()
let ui = setInterval(() => render(`${f.toFixed(2)}Hz + ${(f + diff).toFixed(2)}Hz · beat ${diff.toFixed(2)}Hz · space pause · ←→ beat · ↑↓ carrier · q quit${pausedTag(ctx)}`), 80)

keys({
  left: () => { diff = Math.max(0.1, diff - 0.5); retune() },
  right: () => { diff += 0.5; retune() },
  up: () => { f *= 2 ** (1/12); retune() },
  down: () => { f *= 2 ** (-1/12); retune() },
}, () => { clearInterval(ui); clearLine(); ctx.close() }, ctx)

console.log(`${f}Hz + ${f + diff}Hz → ${diff}Hz beating  (${dur}s)  ←→ beat · ↑↓ carrier · q quit`)

setTimeout(() => { clearInterval(ui); clearLine(); ctx.close(); process.exit(0) }, dur * 1000)