code: func LowPass(buf *audio.PCMBuffer, cutOffFreq float64) (err error) {
  	if buf == nil || buf.Format == nil {
  		return audio.ErrInvalidBuffer
  	}
  	s := &filters.Sinc{
  		// TODO: find the right taps number to do a proper
  		// audio low pass based in the sample rate
  		// there should be a magical function to get that number.
  		Taps:         62,
  		SamplingFreq: buf.Format.SampleRate,
  		CutOffFreq:   cutOffFreq,
  		Window:       windows.Hamming,
  	}
  	fir := &filters.FIR{Sinc: s}
  	buf.Floats, err = fir.LowPass(buf.AsFloat64s())
  	buf.SwitchPrimaryType(audio.Float)
  	return err
  }
docs: // LowPass is a basic LowPass filter cutting off
  // CutOffFreq is where the filter would be at -3db.
  // TODO: param to say how efficient we want the low pass to be.
  // matlab: lpFilt = designfilt('lowpassfir','PassbandFrequency',0.25, ...
  //         'StopbandFrequency',0.35,'PassbandRipple',0.5, ...
  //         'StopbandAttenuation',65,'DesignMethod','kaiserwin');