AVAudioPCMBufferExtension.swift

//
//  AVAudioPCMBufferExtension.swift
//
//  Created by Atsushi Nagase on 2018/05/14.
//  Copyright © 2018 LittleApps Inc. All rights reserved.
//

import Speech

extension AVAudioPCMBuffer {

    // https://stackoverflow.com/a/50237644

    func volume(for bufferSize: Int) -> Float {
        guard let channelData = floatChannelData?[0] else {
            return 0
        }

        let channelDataArray = Array(UnsafeBufferPointer(start:channelData, count: bufferSize))

        var outEnvelope = [Float]()
        var envelopeState:Float = 0
        let envConstantAtk:Float = 0.16
        let envConstantDec:Float = 0.003

        for sample in channelDataArray {
            let rectified = abs(sample)

            if envelopeState < rectified {
                envelopeState += envConstantAtk * (rectified - envelopeState)
            } else {
                envelopeState += envConstantDec * (rectified - envelopeState)
            }
            outEnvelope.append(envelopeState)
        }

        // 0.007 is the low pass filter to prevent
        // getting the noise entering from the microphone
        if let maxVolume = outEnvelope.max(),
            maxVolume > Float(0.015) {
            return maxVolume
        } else {
            return 0.0
        }
    }
}