从AVAudioPCMBuffer提取声压级时

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，该函数会在声压级增加时触发事件（例如，当人尖叫时）。

我正在通过这样的回调进入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)")
    }
}

将其转换为浮点数组后，我尝试通过计算平均值来大致估算声压级。

但这给我带来了很大的价值波动，即使iPad只是坐在一个很宽敞的房间里也是如此：

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，此值的确会大大增加。

因此，我可以做一些类似的事情，首先在准备阶段计算这些数量的平均值，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
         }
      }
 }

其中，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，如果我除了麦克风之外播放响亮的音乐，似乎并没有明显增加。而且在极少数情况下，甚至volume比meanVolume环境中音量没有明显增加（人耳听得见）的情况下更大。

那么从AVAudioPCMBuffer提取声压级的正确方法是什么？

维基百科给出了这样的公式

其中p是均方根声压，p0是参考声压。

但是我不知道float值AVAudioPCMBuffer.floatChannelData代表什么。苹果页面只说

缓冲区的音频采样为浮点值。

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
    }
}

Answer 2

I think the first step is to get the envelope of the sound. You could use simple averaging to calculate an envelope, but you need to add a rectification step (usually means using abs() or square() to make all samples positive)

More commonly a simple iir-filter is used instead of averaging, with different constants for attack and decay, here is a lab. Note that these constants depend on the sampling frequency, you can use this formula to calculate the constants:

1 - exp(-timePerSample*2/smoothingTime)

Step 2

When you have the envelope, you can smooth it with an additional filter, and then compare the two envelopes to find a sound that is louder than the baselevel, here's a more complete lab.

请注意，检测音频“事件”可能非常棘手且难以预测，请确保您有大量调试帮助！