What is the white noise generator?
The white noise generator is one of NumLM’s time tools with a built-in focus environment: a scenery theme for full-screen mode and an ambient-sound mixer. Noise, fan and heartbeat sounds are synthesised in your browser; rain, wind, waves, fire, stream, crickets, birds and cafe are short studio-generated loops of about 300 KB each, fetched only when you switch them on and kept for offline use. There is no time limit and nothing to sign up for. Your mix and theme are remembered on this device and carry over to the timer, Pomodoro and interval tools.
White, pink and brown noise — and why there is no file
White noise has equal energy per hertz, so it sounds bright and hissy; pink falls 3 dB per octave and sounds balanced, like steady rain; brown falls 6 dB per octave and sounds like a distant waterfall or a plane cabin, which many people find easier for sleep. All three are generated by a formula in your browser (pink is filtered white, brown is integrated white), so there is no download, no loop you can learn, and they work offline. Pick one colour in the first row; rain and wind can be layered on top.
Sleep timer with a fade-out
Pick 15, 30, 45, 60 or 90 minutes under the mixer and press Start: the sound fades out over the last 1.5 seconds and the alarm sound starts as None on this page, so nothing wakes you. Without a timer the mix runs indefinitely. On iPhone the sound stops when the screen locks (Safari suspends web audio), so dim the screen instead of locking it, or use the page on a laptop or Android phone for sleep.
Sounds that play only while you focus
With the timer link on, the mix follows the timer: it fades out one and a half seconds before the alarm, so the alarm is never buried under rain, and in the Pomodoro and Focus timers it drops to 30 % during breaks and comes back when the next focus block begins; pausing keeps the sound by default, and you can change both in the panel.
Switching a sound on plays it at once, timer or not; the link only shapes the level. Switch it off and the sound ignores the timer entirely, except that an alarm still dips it. On the white-noise and rain pages there is no link.
Background tabs, phones and iPhone
On a computer the sound keeps playing when you switch tabs: browsers do not throttle a tab that is producing audible sound, and the fade-outs are scheduled on the audio clock ahead of time. Android phones keep playing with the screen on; iPhone Safari pauses web audio when the screen locks, so keep the tab on screen and leave Keep screen on enabled (it is on by default on phones).
Themes, full screen and battery
Three themes are free (night library, rainy window, forest) and five more come with the beta pack. In the page the theme only tints the timer card; press F or the full-screen button and the scene fills the screen behind the ring, darkened so the digits stay readable. The images are still pictures — no video loops — and the optional drifting motes are off by default and stop whenever the tab is hidden or your system asks for reduced motion, which keeps the page light on laptops and phones.
Share a mix, save it, and the beta pack
Copy mix link puts your sounds and theme into the address (`?amb=rain:60,wind:20&th=night-library`), so a friend opens the same setup with one click. Logging in unlocks the beta pack: nine more sounds (waves, campfire, stream, crickets, birds, cafe murmur, fan, air conditioner, heartbeat), five more themes, a theme that follows the time of day, and named mixes. The free sounds and themes stay free; the pack only adds. Listening tip: keep the device volume at or below 60 % for long sessions.