Latency

Every digital instrument has some latency, because it computes audio in small batches called buffers. The app fills a buffer with samples, hands it to the hardware and starts on the next one. A 256-sample buffer at 48,000 samples a second lasts about 5.3 ms, and a 128-sample buffer about 2.7 ms. Smaller buffers cut the delay but give the processor less time to finish each one, and when it can’t keep up you hear clicks and crackles.

On a phone, the biggest source by far is Bluetooth. Wireless headphones compress and buffer the audio before it plays, and in developer Stephen Coyle’s 2019 test, the time from a tap to the sound in AirPods measured 144 to 274 ms, depending on the model. At 120 BPM a sixteenth note lasts 125 ms, so on those AirPods every note you play arrives more than a sixteenth behind your finger. Distance adds a little too: sound travels about 34 cm per millisecond, so a guitarist 3 meters from an amp hears it about 9 ms late, still under the point most players notice.

You can hear the gap in ten seconds. Open the free online chord player, tap a chord with Bluetooth headphones in, then tap it again on the phone speaker.

Recording adds a second trip. The sound comes in through a microphone or audio interface, gets buffered, and reaches the recording a few milliseconds late. The trip in plus the trip out is called round-trip latency, and a DAW usually moves each recording earlier by the amount of delay the system reports. Any delay it doesn’t know about ends up in the take as late notes, and in a looper it stacks: follow a late layer and the next one lands later still.

The fix is to take Bluetooth out of the path while you play or record. Use the iPhone speaker or wired headphones, and keep wireless earbuds for listening back. AirPods latency in music apps has the measurements and the workarounds.