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I have a standard periodogram produced from the spectrum function call in the R "stats" package with frequency vs spectral density. The spectral analysis relates to long monthly ocean water level time series data that are composed of many signals and noise.

I wish to determine the amplitude of the sinusoidal signal corresponding to peaks in the spectral density at frequencies of 0.071 (14 month cycle) and 0.0094 (8.85 year cycle). How do i convert the ordinate of the spectral density into signal amplitude? Is there a relatively quick conversion?

Is there a recommended function call/package in R to extract the estimate of the real sinusoidal signal corresponding to a known frequency from the periodogram?

Many thanks Phil

Phil W
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  • Firstly can you provide a [reproducible example](http://stackoverflow.com/questions/5963269/how-to-make-a-great-r-reproducible-example) - i.e. the steps you've taken to produce the standard periodogram. Secondly is it possible to make some sample data available to illustrate your aim, or alternatively frame your question in more generic statistical terms? – geotheory Aug 11 '13 at 13:09
  • possible duplicate of [How to convert spectral density to amplitude](http://stackoverflow.com/questions/18161789/how-to-convert-spectral-density-to-amplitude) – IRTFM Aug 11 '13 at 18:07

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