noise_meter

Noise Meter

A noise meter package for iOS and Android.

Install

Add noise_meter as a dependency in pubspec.yaml. For help on adding as a dependency, view the documentation.

On Android you need to add a permission to AndroidManifest.xml:

<uses-permission android:name="android.permission.RECORD_AUDIO" />
<uses-permission android:name="android.permission.WRITE_EXTERNAL_STORAGE" />

On iOS enable the following:

• Capabilities > Background Modes > Audio, AirPlay and Picture in Picture
• In the Runner Xcode project edit the Info.plist file. Add an entry for 'Privacy - Microphone Usage Description'

Usage

Initialization

Keep these three variables accessible:

bool _isRecording = false;
StreamSubscription<NoiseReading> _noiseSubscription;
NoiseMeter _noiseMeter = new NoiseMeter(onError);

Start listening

The easiest thing to do is to create a new instance of the NoiseMeter every time a new recording is started.

void start() async {
    try {
      _noiseSubscription = _noiseMeter.noiseStream.listen(onData);
    } on NoiseMeterException catch (exception) {
      print(exception);
    }
}

On data

When data comes in through the stream, it will be caught by the onData method, specified when the subscription was created. The incoming data points are of type NoiseReading which have a single field with a getter, namely the db value of type double.

void onData(NoiseReading noiseReading) {
  this.setState(() {
    if (!this._isRecording) {
      this._isRecording = true;
    }
  });
  /// Do someting with the noiseReading object
  print(noiseReading.toString());
}

On errors

Platform errors may occur when recording is interupted. You must decide what happens if such an error occurs. The [onError] callback must be of type void Function(Object error) or void Function(Object error, StackTrace).

void onError(Object error) {
    print(error.toString());
    _isRecording = false;
}

Stop listening

To stop listening, the .cancel() method is called on the subscription object.

void stopRecorder() async {
  try {
    if (_noiseSubscription != null) {
      _noiseSubscription.cancel();
      _noiseSubscription = null;
    }
    this.setState(() {
      this._isRecording = false;
    });
  } catch (err) {
    print('stopRecorder error: $err');
  }
}

Technical documentation

Sample rate

The sample rate for both native implementations is 44,100.

Microphone data

The native implementations record PCM data using the microphone of the device, and uses an audio buffer array to store the incoming data. When the buffer is filled, the contents are emitted to the Flutter side. The incoming floating point values are between -1 and 1 which is the PCM values divided by the max amplitude value which is 2^15.

Conversion to Decibel

Computing the decibel of a PCM value is done as follows:

db = 20 * log10(2**15 * pcmValue)