java – 使用频率从android获取超声波

我希望从任何Android设备获得超声波,例如频率介于18KHz和19KHz之间的超声波.

我使用下面的代码来计算频率,但它似乎没有让我得到正确的频率.我的频率保持在11 KHz和13KHz之间.

private void        calculateFrequency()
{
    // 1 - Initialize audio
    int channel_config = AudioFormat.CHANNEL_CONFIGURATION_MONO;
    int format = AudioFormat.ENCODING_PCM_16BIT;
    int sampleRate = 8000;
    int bufferSize = 2048;

    if (bufferSize < AudioRecord.getMinBufferSize(sampleRate, channel_config, format))
        bufferSize = AudioRecord.getMinBufferSize(sampleRate, channel_config, format);
    AudioRecord audioInput = new AudioRecord(AudioSource.MIC, sampleRate, channel_config, format, bufferSize);

    // 2 - Get sound
    byte[] audioBuffer = new byte[bufferSize];
    audioInput.startRecording();
    int nbRead = audioInput.read(audioBuffer, 0, bufferSize);
    audioInput.stop();
    audioInput.release();

    // 3 - Transform to double array
    double[] micBufferData = new double[bufferSize];
    final int bytesPerSample = 2; // As it is 16bit PCM
    final double amplification = 100.0; // choose a number as you like
    for (int index = 0, floatIndex = 0; index < nbRead - bytesPerSample + 1; index += bytesPerSample, floatIndex++) {
        double sample = 0;
        for (int b = 0; b < bytesPerSample; b++) {
            int v = audioBuffer[index + b];
            if (b < bytesPerSample - 1 || bytesPerSample == 1) {
                v &= 0xFF;
            }
            sample += v << (b * 8);
        }
        double sample32 = amplification * (sample / 32768.0);
        micBufferData[floatIndex] = sample32;
    }

    // 4 - Create complex array
    Complex[] fftTempArray = new Complex[bufferSize];
    for (int i=0; i<bufferSize; i++)
    {
        fftTempArray[i] = new Complex(micBufferData[i], 0);
    }

    // 5 - Calculate FFT
    Complex[] fftArray = FFT.fft(fftTempArray);

    // 6 - Calculate magnitude
    double[] magnitude = new double[bufferSize / 2];
    for (int i = 0; i < (bufferSize / 2); i++)
    {
        magnitude[i] = Math.sqrt(fftArray[i*2].re() * fftArray[i*2].re() + fftArray[i*2].im() * fftArray[i*2].im());
    }

    // 7 - Get maximum magnitude
    double max_magnitude = -1;
    for (int i = 0; i < bufferSize / 2; i++)
    {
        if (magnitude[i] > max_magnitude)
        {
            max_magnitude = magnitude[i];
        }
    }

    // 8 - Calculate frequency
    int freq = (int)(max_magnitude * sampleRate / bufferSize);

    ((TextView) findViewById(R.id.textView1)).setText("FREQUENCY = " + freq + "Hz");
}

我正在使用两部手机：一部分用this app发送超声波,另一部用于超声波.
我用这个question作为起点,我用FFT和复杂类.

我的代码出了什么问题？

解决方法:

为了获得正确的非混叠频率估计,必须使用更多的采样率(可能比滤波器滚降更大10％到20％),而不是音频输入中最高频率的两倍,因此超过两倍你想要找到的最高频率.

这是由于采样定理所需的奈奎斯特速率.

因此,如果要查找19 kHz信号,则需要接近48000的采样率.