# Voice activity detection Canonical page: https://flit.fyi/glossary/voice-activity-detection [Glossary](https://flit.fyi/glossary) / [Talking to it](https://flit.fyi/glossary#input) Also: VAD Updated October 4, 2026 Definition Voice activity detection (VAD) is the technique of detecting when someone is speaking versus when there is silence or background noise. It lets software start and stop recording automatically. A VAD model is small and fast, and runs continuously on the audio stream without transcribing anything: its only job is a yes/no answer to "is this speech?". Dictation tools use it for hands-free modes (start when speech begins, stop at a pause) and to trim silence before the recognizer runs. A well-behaved implementation only arms the microphone when you ask it to, and discards audio that never became a dictation. ## How it works The classic approach is energy-based: if the microphone signal is louder than a threshold for long enough, it is speech. That breaks in any room with a fan, a keyboard, or a second person. Modern VAD is a tiny neural network trained to tell human speech from everything else, and it runs in a few milliseconds per chunk of audio, cheap enough to run all the time on a laptop without anyone noticing. The output is a stream of speech and non-speech segments. A dictation tool uses the first transition to start the recognizer and a long enough non-speech stretch to decide that you have finished. ## Why it matters for privacy A hands-free mode means a tool is listening before you have said anything. What separates a trustworthy implementation from a creepy one is what happens to audio that turns out not to be speech, or speech that never becomes a dictation. The right answer is that it is never stored, never sent anywhere, and evaporates as soon as the detector says no. A tool that runs VAD on-device can make that promise; a tool that streams to a server to decide whether you are speaking cannot. ## Where VAD falls short Detecting a pause is not the same as detecting the end of a thought. People pause mid-sentence to think, and a VAD-driven stop will cut them off; set the pause longer and the tool feels sluggish. That is why hands-free modes suit steady, fluent dictation and push-to-talk suits everything else, and why a good tool offers both. ## How to see silence detection at work on your Mac You do not need special software to watch a speech detector make a decision. Apple’s Dictation stops automatically when no speech is detected for 30 seconds, and you can trigger that rule yourself. 1. Choose Apple menu → System Settings, click Keyboard, go to Dictation, and turn it on. 2. Click into an empty note or document. 3. Press the Dictation shortcut and say one sentence. 4. Stop talking and leave the Mac alone. 5. Watch dictation end without a key press once 30 seconds pass with no speech. 6. Repeat the test with music or a fan running nearby to see whether background sound changes the result on your setup. ## Why does hands-free dictation cut me off or miss my first word? A speech detector can only judge the signal it is given. If the wrong microphone is selected, or the input is very quiet, your voice and the room start to look alike, and the start and stop decisions get worse. Before you blame the tool, check what the Mac is actually listening to. - In System Settings → Sound, click Input and select the microphone you are really speaking into. - On the same screen, drag the input volume slider up if your voice is too quiet, or down if it is too loud. - For Apple’s Dictation, check Microphone source under System Settings → Keyboard → Dictation. Automatic lets the Mac pick the device you are most likely to use. - Move closer to the microphone or use a headset, so your voice is clearly louder than the room. - If you pause to think in the middle of sentences, use a held key or a toggle for those passages instead of a hands-free mode. ## How to check whether your microphone is open Any tool that waits for speech has to keep the microphone open while it waits. macOS shows you when that is happening and lets you decide which apps may do it at all. 1. Open Control Center and look for the Recording Indicator. It shows when the microphone is in use or was used recently. 2. Choose Apple menu → System Settings, then click Privacy & Security in the sidebar. 3. Click Microphone. 4. Read the list of apps that have asked for the microphone. 5. Turn access off for any app that should not be listening. In Flit Flit’s hands-free mode is armed by you, starts the moment it hears speech, stops when you pause, and records nothing until it hears you. [Hands-free in Flit](https://flit.fyi/#start) - [The three activation modes](https://flit.fyi/#start) - [Dictating without touching the keyboard](https://flit.fyi/use-cases/accessibility) - [What Flit records and when](https://flit.fyi/privacy) ## Questions people ask. ### What does VAD do in a dictation app? It decides whether the microphone is picking up speech. In a hands-free mode it starts recognition when you begin talking and stops when you pause, so you never touch a key. ### Is a hands-free mode always listening? Only once you arm it, in a well-designed tool. Even then the detector is deciding "speech or not" rather than transcribing, and audio that never becomes a dictation should be discarded immediately. Ask where that decision runs; on the device is the answer you want. ### What does VAD stand for? VAD stands for voice activity detection. It is the part of a voice tool that decides whether the incoming sound is speech or not, without working out what the words are. ### How long does Mac dictation wait before it stops listening? Apple’s Dictation stops automatically when no speech is detected for 30 seconds. You can end it sooner by pressing the Dictation shortcut again or pressing Escape. ### How can I tell if an app is using my microphone on a Mac? Open Control Center and look for the Recording Indicator, which shows when the microphone is in use or was used recently. To see which apps are allowed to use it, go to System Settings → Privacy & Security → Microphone. ## Guides that use it - [Mac dictation keeps stopping](https://flit.fyi/mac-dictation-keeps-stopping) - [Transcribe an audio file](https://flit.fyi/transcribe-audio-file-mac) ## On the blog - [Flit 1.5: transcribe recordings on your Mac](https://flit.fyi/blog/flit-1-5) ## Use cases - [Therapists & clinicians](https://flit.fyi/use-cases/therapists) - [Hands-free & RSI](https://flit.fyi/use-cases/accessibility) ## Related terms - [Push-to-talk](https://flit.fyi/glossary/push-to-talk) - [Wake word](https://flit.fyi/glossary/wake-word) - [Streaming transcription](https://flit.fyi/glossary/streaming-transcription) - [On-device processing](https://flit.fyi/glossary/on-device-processing) ## Your voice was always faster. 7 days free, then $19 USD once. No account, no subscription, nothing uploaded. 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