How End-of-Utterance Detection Works
End-of-utterance detection (sometimes called endpoint detection or turn detection) identifies the moment a speaker completes a conversational turn. In voice AI systems, accurate detection is critical: respond too early and you interrupt the user; respond too late and the interaction feels sluggish.
Core Signals Used
Acoustic features: Silence duration, falling pitch contour, reduced energy, and final lengthening of syllables all signal a speaker is wrapping up.
Linguistic context: Syntactic completeness, discourse markers ("so," "anyway"), and sentence-final intonation patterns help distinguish a finished thought from a mid-sentence pause.
Semantic turn detection: More advanced systems analyze the meaning of what has been said so far. If the utterance forms a semantically complete request or statement, the model can confidently predict the turn is over, even before a long silence occurs.
Approaches and Models
Traditional systems rely on a fixed silence threshold, often called the "end-of-utterance delay." If no speech is detected for a set period (commonly a few hundred milliseconds), the system assumes the turn is complete. While simple, this approach struggles with natural pauses that occur mid-thought.
Modern turn detection models use neural networks trained on conversational data. These models fuse acoustic, lexical, and timing features to predict turn boundaries in real time. Platforms such as LiveKit and Deepgram offer configurable turn detection that lets developers tune sensitivity and latency tradeoffs. LiveKit's implementation, for example, allows adjustment of the end-of-utterance delay alongside model-based predictions.
Practical Considerations
Latency vs. accuracy: Shorter detection windows feel snappier but risk false positives (cutting off the speaker). Longer windows are safer but add perceived lag.
Domain tuning: A dictation app tolerates longer pauses than a fast-paced voice assistant. Configurable thresholds and model fine-tuning help match behavior to the use case.
Multimodal cues: In video or embodied agents, gaze and gesture can supplement audio-only detection for even more reliable turn-taking.
End-of-Utterance Detection Example
Consider a caller saying, "I'd like to book a flight to... hmm... Chicago." A naive silence-based detector might trigger after the pause following "to," cutting the user off. A semantic turn detection model recognizes the utterance is syntactically incomplete and waits for the destination, yielding a smoother experience.