> ## Documentation Index
> Fetch the complete documentation index at: https://docs.runanywhere.ai/llms.txt
> Use this file to discover all available pages before exploring further.

# Voice Activity Detection

> Detect when users start and stop speaking

Voice Activity Detection (VAD) automatically detects speech in audio streams, enabling hands-free voice interfaces.

## Overview

VAD determines when a user is speaking vs. silent, which is essential for:

* Knowing when to start/stop recording
* Triggering transcription at the right time
* Building push-to-talk or hands-free interfaces

The SDK provides energy-based VAD with Silero VAD for accurate speech detection.

## Automatic VAD with Voice Session

The easiest way to use VAD is through the Voice Agent pipeline:

```dart theme={null}
final session = await RunAnywhere.startVoiceSession(
  config: VoiceSessionConfig(
    speechThreshold: 0.03,    // Audio level threshold
    silenceDuration: 1.5,     // Seconds of silence before processing
  ),
);

session.events.listen((event) {
  switch (event) {
    case VoiceSessionListening(:final audioLevel):
      // Show audio level meter
      updateAudioMeter(audioLevel);
    case VoiceSessionSpeechStarted():
      print('User started speaking');
    case VoiceSessionProcessing():
      print('Processing speech...');
    default:
      break;
  }
});
```

## VoiceSessionConfig for VAD

| Parameter         | Type     | Default | Description                             |
| ----------------- | -------- | ------- | --------------------------------------- |
| `speechThreshold` | `double` | 0.03    | Audio level to trigger speech detection |
| `silenceDuration` | `double` | 1.5     | Seconds of silence before processing    |

```dart theme={null}
// More sensitive (quieter environments)
final quietConfig = VoiceSessionConfig(
  speechThreshold: 0.01,
  silenceDuration: 1.0,
);

// Less sensitive (noisy environments)
final noisyConfig = VoiceSessionConfig(
  speechThreshold: 0.1,
  silenceDuration: 2.0,
);
```

## Building a Voice Level Indicator

```dart theme={null}
class VoiceLevelIndicator extends StatefulWidget {
  final Stream<double> audioLevelStream;

  const VoiceLevelIndicator({required this.audioLevelStream});

  @override
  _VoiceLevelIndicatorState createState() => _VoiceLevelIndicatorState();
}

class _VoiceLevelIndicatorState extends State<VoiceLevelIndicator> {
  double _level = 0.0;
  StreamSubscription? _subscription;

  @override
  void initState() {
    super.initState();
    _subscription = widget.audioLevelStream.listen((level) {
      setState(() => _level = level);
    });
  }

  @override
  Widget build(BuildContext context) {
    return Container(
      height: 10,
      child: LinearProgressIndicator(
        value: _level.clamp(0.0, 1.0),
        backgroundColor: Colors.grey[300],
        valueColor: AlwaysStoppedAnimation(
          _level > 0.03 ? Colors.green : Colors.grey,
        ),
      ),
    );
  }

  @override
  void dispose() {
    _subscription?.cancel();
    super.dispose();
  }
}
```

## Voice Session with VAD Events

```dart theme={null}
class VoiceAssistant extends StatefulWidget {
  @override
  _VoiceAssistantState createState() => _VoiceAssistantState();
}

class _VoiceAssistantState extends State<VoiceAssistant> {
  VoiceSessionHandle? _session;
  double _audioLevel = 0.0;
  bool _isSpeaking = false;
  String _status = 'Ready';

  Future<void> _startSession() async {
    _session = await RunAnywhere.startVoiceSession(
      config: VoiceSessionConfig(
        speechThreshold: 0.03,
        silenceDuration: 1.5,
        autoPlayTTS: true,
        continuousMode: true,
      ),
    );

    _session!.events.listen((event) {
      setState(() {
        switch (event) {
          case VoiceSessionListening(:final audioLevel):
            _audioLevel = audioLevel;
            _status = 'Listening...';
          case VoiceSessionSpeechStarted():
            _isSpeaking = true;
            _status = 'Speaking detected';
          case VoiceSessionProcessing():
            _isSpeaking = false;
            _status = 'Processing...';
          case VoiceSessionTranscribed(:final text):
            _status = 'You: $text';
          case VoiceSessionResponded(:final text):
            _status = 'AI: $text';
          case VoiceSessionSpeaking():
            _status = 'AI speaking...';
          case VoiceSessionTurnCompleted():
            _status = 'Ready';
          case VoiceSessionStopped():
            _status = 'Stopped';
          default:
            break;
        }
      });
    });
  }

  void _stopSession() {
    _session?.stop();
  }

  @override
  Widget build(BuildContext context) {
    return Column(
      mainAxisAlignment: MainAxisAlignment.center,
      children: [
        // Audio level indicator
        Container(
          width: 100,
          height: 100,
          decoration: BoxDecoration(
            shape: BoxShape.circle,
            color: _isSpeaking ? Colors.green : Colors.blue,
            boxShadow: [
              BoxShadow(
                color: (_isSpeaking ? Colors.green : Colors.blue)
                    .withValues(alpha: 0.5),
                blurRadius: _audioLevel * 50,
                spreadRadius: _audioLevel * 20,
              ),
            ],
          ),
          child: Icon(
            _isSpeaking ? Icons.mic : Icons.mic_none,
            color: Colors.white,
            size: 48,
          ),
        ),

        SizedBox(height: 24),
        Text(_status, textAlign: TextAlign.center),

        SizedBox(height: 24),
        ElevatedButton(
          onPressed: _session == null ? _startSession : _stopSession,
          child: Text(_session == null ? 'Start' : 'Stop'),
        ),
      ],
    );
  }
}
```

## VAD Tuning Tips

<AccordionGroup>
  <Accordion title="Threshold Adjustment" icon="sliders">
    Lower thresholds (0.01) detect quieter speech but may trigger on background noise. Higher
    thresholds (0.1) require louder speech but are more robust to noise.
  </Accordion>

  <Accordion title="Silence Duration" icon="clock">
    Shorter duration (0.5s) feels more responsive but may cut off pauses mid-sentence. Longer duration
    (2.0s) allows natural pauses but feels slower.
  </Accordion>

  <Accordion title="Environment Calibration" icon="gauge">
    Consider measuring ambient noise level and adjusting threshold dynamically.
  </Accordion>
</AccordionGroup>

## See Also

<CardGroup cols={2}>
  <Card title="Voice Agent" icon="robot" href="/flutter/voice-agent">
    Complete voice pipeline
  </Card>

  <Card title="Speech-to-Text" icon="microphone" href="/flutter/stt/transcribe">
    Audio transcription
  </Card>
</CardGroup>