How Greyscale makes its noise and sound mixes

One engine, three modes

Every sound plays through the same engine in every mode; a mode only changes the defaults. The noise colours and steady sounds are generated live on your phone. The recorded sounds are bundled files that the engine joins with crossfades at random points, and thunder and birdsong add their events at random moments.

The 19 sounds

White noise has the same energy at every frequency, so it sounds bright and hissy. Pink noise turns the high end down and sounds softer. Brown noise turns it down twice as steeply, into a deep rumble. Greyscale also has grey noise, shaped to sound even across pitches, brighter blue and violet, and green, centred on the mid range.

Noise colours

White, pink, brown, grey, blue, violet, green

Steady

Fan, wind, ocean surf, womb

Nature and places

Light rain, heavy rain on a roof, distant thunder, stream, fireplace, night crickets, forest birds, café murmur

Each recorded sound is built from three recorded passages. The engine plays a stretch of 10 to 22 seconds, then crossfades over 2 to 4 seconds into a random point of another passage, so there is no fixed loop point.

The mixer

A mix holds up to five sounds, each with its own volume. Tone tilts the whole mix darker or brighter, and Drift slowly varies each sound's level so the mix moves a little over time. Saved mixes can be renamed and put in your own order. Editing a built-in mix saves a copy and leaves the original as it is.

Sleep

Rain on the roof, Slow surf, Crickets by the stream, Bedroom fan

Baby

Womb, Soft shush, Quiet waves, Humming fan

Focus

Deep brown, Café corner, Rain at the window, Forest stream

Four built-in mixes come with each mode.

Fades and timers

Starts fade in and timers fade out. By default Sleep and Baby fade in over 15 seconds and Focus over 10, and you can change both lengths per mode in Settings: fade-in from 0 to 60 seconds, fade-out from 30 seconds to 10 minutes. Cancelling during a fade-out brings the sound back up over 2 seconds.

Timer choices: Sleep suggests 45 minutes; Baby offers 30, 60 or 120; the notification's Timer button cycles off, 15, 30, 45, 60 and 90 minutes; the night screen adds +15 min. Focus sessions: work 15 to 90 minutes, break 3 to 20 minutes, 1 to 8 rounds, starting at 25, 5 and 4. During a break the sound dips to 30% over 5 seconds.

Controls outside the app

You don't need to open the app to start or stop the sound. The Quick Settings tile toggles your last mix, the home-screen widget has Play, Stop and the three modes, and the notification on the lock screen has Play and Pause, a Timer button and Next mix. Headset and Bluetooth buttons work once a mix has played.

The tile shows the mix name when it is on; a long press opens the app. The widget shows the current or last mix. After a restart, the last mix is offered in Android's media controls; nothing plays by itself.

Around the sound

A phone call pauses the mix, and it fades back in after the call. Unplugging headphones pauses it instead of moving it to the speaker, unless you turn on Keep playing on speaker. With Mix with other audio off, the default in Sleep and Baby, another app's sound pauses Greyscale; in Focus both play.

Keeps playing with the screen off. Some phones stop background apps to save battery; Battery help shows the steps for your phone's brand.

If headphone volume is above 60%, a one-time hint suggests a lower volume; it doesn't change the volume itself.

Inputs and limits

• The volume cap limits Greyscale's own sound, not the phone's volume or a connected speaker.
• Greyscale doesn't measure how loud it is in the room. It has no microphone permission.
• Some phones stop apps in the background to save battery. Battery help shows the steps for your phone's brand; Greyscale never asks to be exempted from battery optimisation.
• Each recorded sound is built from three recorded passages joined by 2 to 4 second crossfades at random points. Over a long night the same passages come back; no 30-second stretch repeats exactly within 10 minutes.
• Headset and Bluetooth buttons work once a mix has played.
• With Mix with other audio off (the default in Sleep and Baby), another app's playback pauses Greyscale. In Focus it is on, so both play.
• Unplugging headphones pauses the sound unless you turn on Keep playing on speaker.
• After a restart nothing plays by itself.
• Android 8.0 or later, English only. Tablets and wide screens get a two-pane layout.
• Greyscale plays sound. It isn't a baby monitor, sleep tracker, sound meter or alarm clock.

Where the sounds come from

The 11 noise colours and steady sounds are made in code. The 8 recorded sounds are cut from CC0 (public domain) recordings on Freesound.org; the app's About screen credits each recording's title, author, licence and edits.