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Undercurrent

Your own audio, looped without a seam, underneath everything else your phone does.

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An iOS instrument for ambient beds, guided tracks, and long stretches of work.

Platform
iPhone
Requires
iOS 18 or later
Price
Free
Status
Beta on TestFlight
Made by
Badwater Guidance LLC
Undercurrent's sand table mid-session: a copper ball carving a figure in the sand, the volume bead on the rim, pass 6 under the table.

The table

The ball draws while the track plays.

Each track has its own sand table. While it plays, a copper ball carves a figure in the sand, and it stops when the track stops. Tap the table to play or pause, and drag the copper bead on the rim to set the volume. Every track keeps its last drawing, so the library is a rack of plates.

Three modes set how a track plays: Loop is seamless and endless, Repeat restarts from the top, and Once plays through and stops. Leave the phone alone and the table starts to turn after 20 seconds. After two minutes it tilts. Tap it and it comes back flat.

The sand table tilted and turning on its own, the drawing's grooves catching the light.
The mode menu: Loop, Repeat, and Once, each with a one-line description.
Left, the table tilted and turning on its own. Right, the mode menu.

The blend

Plays underneath other apps.

Start a bed. Open YouTube. Both are audible. Lock the phone and it's still playing.

The blend costs you the lock-screen controls: iOS withholds them from any app whose audio mixes with other apps. Transport lives in the app.

Undercurrent's library: five sand plates, each holding its track's last drawing, one playing.
Safari playing a video on the same phone while Undercurrent continues underneath.
Undercurrent playing, and Safari on the same phone in the same session. Video: Big Buck Bunny, Blender Foundation, CC BY 3.0.

The loop

Every file loops without an audible seam.

At import, an analyzer searches the file for a splice point accurate to the sample, where the waveform can rejoin itself without a click. When no clean splice exists, an equal-power crossfade carries the join.

A ten-minute bed runs for an hour and you won't hear the join on either path. The app shows you its own analysis: which path it took, the confidence it assigned, and a control to run it again.

An arc sweeps round and passes the seam mark at the top, once a pass.

A short loop of the demo track that ships with the app. It plays only when you press play.
The loop details inspector showing the analyzer's verdict, its confidence value, and a re-analyze control.

The measurement

What a seam looks like when it fails.

A click is a broadband transient: energy across the whole spectrum in one instant, which draws a vertical line through a spectrogram. Both images below show the same bed at the same loop point, marked by the ticks on the top and bottom edges.

Spectrogram of a deliberately botched loop: a bright vertical line spans the full height of the frame at each seam.Spectrogram of the analyzer's splice at the same loop point: the ticks mark the seam and no vertical line crosses the frame.
Top, the same bed cut hard mid-waveform; the line spanning the frame is the click. Bottom, the analyzer's splice at the same point; the ticks mark the seam and the frame stays clear. Drag the handle to wipe from one to the other.
The sleep menu over the sand table: 15 to 120 minutes, under the note that it fades over the final minute.
The sleep fade. The level falls to −60 dB over 60 seconds, and the ball slows with it until the sound stops.

What it does not do

The app is offline. No accounts, no analytics, no data collection, no network requests of any kind. It never sends your files anywhere. This site counts page views. See privacy.

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