Adaptive Band pass filter for extracting respiratory waveform from accelerometer data
Under the extremely controlled conditions required to measure respiratory rate using this method, you can try to use the autocorrelation (which is more robust in the presence of noise) to determine the "dominant" frequency in your signal.


  2) The red line is spectra after filtering. Why has the amplitude of certain frequencies increased? I am using the 'spec' function in R: uses the hanning window for computing.


If that is not a mere rescaling of the waveform as it gets plotted over the same figure (?), then it would be safe to assume that the filter included some overall gain.

Hope this helps.