What is voice activity detection and how does it work?

Voice activity detection (VAD) classifies short audio frames, typically 10-30 milliseconds, as containing speech or not. It returns a yes-or-no decision that tells downstream tools such as speech-to-text, LLMs, and turn planners whether to process or wait. VAD does not transcribe words or decide when a speaker has finished, which is the job of endpointing systems.