Knock your phone to skip a song

What each knock does

• Right edge, twice: jump forward (5, 10, 15 or 30 seconds, your choice).
• Right edge, three times: next track.
• Left edge, twice: jump back.
• Left edge, three times: previous track.
• Top edge, three times: pause, or play again.

Left and right are the phone's own sides when it's upright, the way you'd hold it to read. A short buzz confirms each gesture, so you know it worked without looking.

When it listens

• Off until you switch it on. Settings, Bump gestures.
• Only while music is playing. If a knock paused the music, the top-edge knock keeps working for five minutes so you can start it again. After that the sensor is switched off completely.
• Screen off by default. That's when it's most useful: phone on the desk, in a car mount or in your hand on a walk. You can allow it with the screen on too.
• Not in your pocket. Before acting on a knock, the app takes one reading from the proximity sensor (or the light sensor, on phones without one). If the phone is in a pocket or bag, the knock is ignored, so walking doesn't skip songs.
• Not while battery saver is on. Gestures give way when the phone is saving power.

Setting it up

1. Open Settings and switch on Bump gestures.
2. Tap Calibrate. The screen draws the motion sensor live and marks every knock it recognises, with the side it thinks you hit.
3. Knock each edge a few times, both ways: your hand into the phone, and the phone into your palm.
4. If light knocks are missed, lower the sensitivity. If ordinary handling sets it off, raise it.

Phones carry different motion sensors, so the calibration screen exists to tune it to yours. Some phones stop sending motion readings once the screen is off. The app checks for this each time it starts listening. If your phone does it, Settings tells you, and gestures work with the screen on only.

How it tells a knock from everything else

The app reads the accelerometer about 200 times a second while it's listening. It filters out gravity and slow movement such as walking or turning the phone. A knock is a sharp jolt that pushes the phone sideways, mostly along one axis. The app judges the side from the strongest moment of the jolt, not the first reading, because the first reading is often the swing before the impact or the bounce after it.

Two or three knocks on the same edge within about half a second make a gesture. One knock, or four and more, does nothing, so bumping the phone by accident is ignored.

Privacy

Motion readings are handled in memory on the phone and thrown away straight after. They're never stored, and the app has no internet permission, so they can't be sent anywhere. The calibration screen can record a trace to a file on the phone only if you tap the record button yourself.

Get Flex Music Player · Gestures don't react?