ios - 关于从 AVAudioPCMBuffer 中提取声压级

Question

我对信号处理几乎一无所知，目前我正在尝试在 Swift 中实现一个函数，当 sound pressure level 增加时触发一个事件。 (例如，当人尖叫时)。

我正在使用这样的回调进入 AVAudioEngine 的输入节点:

let recordingFormat = inputNode.outputFormat(forBus: 0)
inputNode.installTap(onBus: 0, bufferSize: 1024, format: recordingFormat){
 (buffer : AVAudioPCMBuffer?, when : AVAudioTime) in 
    let arraySize = Int(buffer.frameLength)
    let samples = Array(UnsafeBufferPointer(start: buffer.floatChannelData![0], count:arraySize))

   //do something with samples
    let volume = 20 * log10(floatArray.reduce(0){ $0 + $1} / Float(arraySize))
    if(!volume.isNaN){
       print("this is the current volume: \(volume)")
    }
}

将其转换为 float 组后，我尝试通过计算平均值来粗略估计声压级。

但这给我的值(value)波动很大，即使 iPad 只是放在一个安静的房间里也是如此:

this is the current volume: -123.971
this is the current volume: -119.698
this is the current volume: -147.053
this is the current volume: -119.749
this is the current volume: -118.815
this is the current volume: -123.26
this is the current volume: -118.953
this is the current volume: -117.273
this is the current volume: -116.869
this is the current volume: -110.633
this is the current volume: -130.988
this is the current volume: -119.475
this is the current volume: -116.422
this is the current volume: -158.268
this is the current volume: -118.933

如果我在麦克风附近拍手，这个值确实有显着增加。

所以我可以做一些事情，比如在准备阶段首先计算这些体积的平均值，然后比较在事件触发阶段差异是否显着增加:

 if(!volume.isNaN){
    if(isInThePreparingPhase){
        print("this is the current volume: \(volume)")
        volumeSum += volume
        volumeCount += 1
     }else if(isInTheEventTriggeringPhase){
         if(volume > meanVolume){
             //triggers an event
         }
      }
 }

在从准备阶段到触发事件阶段的过渡期间计算 averageVolume:meanVolume = volumeSum/Float(volumeCount)

....

但是，如果我在麦克风之外播放响亮的音乐，似乎没有显着增加。在极少数情况下，volume 大于 meanVolume，即使环境音量没有显着增加(人耳可听见)也是如此。

那么从AVAudioPCMBuffer中提取声压级的正确方法是什么？

维基百科给出了这样一个公式

p 是均方根声压，p0 是引用声压。

但我不知道 AVAudioPCMBuffer.floatChannelData 中的浮点值代表什么。 The apple page只说

The buffer's audio samples as floating point values.

我应该如何与他们合作？

Answer 1

感谢@teadrinker 的回复，我终于找到了解决这个问题的方法。我分享了输出 AVAudioPCMBuffer 输入音量的 Swift 代码:

private func getVolume(from buffer: AVAudioPCMBuffer, bufferSize: Int) -> Float {
    guard let channelData = buffer.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
    }
}