UI sound is narrower than an earcon — it's not a state change, but a stand-in for the tactile click your finger used to feel on a physical button. iOS's keyboard clicks, the camera shutter, and a toggle's little "clack" are classic examples, filling the tactile gap left by touchscreens.
Response speed is everything — it should play with near-zero perceived delay (ideally under 50ms) and stay quiet, around -20dB below peak, so it passes by unnoticed rather than announcing itself. For interactions that can fire many times a second (scrolling, dragging a slider), skip the sound entirely or keep it barely audible — repeated, it just becomes noise. Because silent-mode, low-vision, or deaf/hard-of-hearing users won't hear it, success always needs a visual confirmation too (color, size, position) — sound alone is never enough.
Used for keyboard clicks, toggle on/off, delete/confirm buttons, camera shutters, and unlock sounds across OS-level interactions.
When to use
Attach it to atomic, direct-touch actions like taps, toggles, and swipes. Skip it (or keep it minimal) for continuous gestures like drags and scrolls.