MATLAB: Units pmtm

amplitude spectrumearthquake spectrapmtm

Hi,
I'm having trouble making some conversions because after looking through things I've realized I don't quite understand the output of pmtm. I have a time series I am feeding into pmtm with units of m on the y-axis and units of seconds on the x axis. I'm wondering what the units on the y-axis would be after being output from pmtm?
I know the manual says they are units of Power/frequency-but how does this really work out explicitly if I'm starting with units of m? I'm confused, can someone please explain this to me? Inevitably I am trying to get displacement amplitude spectra.
Cheers,
K

Best Answer

The output from pmtm() the power spectral density will be in m^2/Hz (if you are using the sampling frequency). You want to make sure you square the units (this comes from the power). So if you had an acceleration recording in time that would be in m/sec^2, the output of pmtm() for that data would be in (m/sec^2)^2/Hz (again if you are using the sampling frequency in pmtm()).
Keep in mind for statistical reasons, it is very common to plot PSD estimates in dB (in your case 10*log10(m^2/Hz))
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