Undercurrent

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

Platform: iPhone
Requires: iOS 17 or later
Price: Free
Status: Beta on TestFlight
Made by: Badwater Guidance LLC

Undercurrent's library: five audio files with their play-mode indicators, one playing.

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, and the blend toggle switches to exclusive playback when you want the buttons back.

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.

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. — 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.

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.

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, how far you scroll, and how long you stay. See privacy.

Join the beta on TestFlight