r - How to convert spectral density to amplitude -

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i have standard periodogram produced spectrum function call in r "stats" package. produces spectral density on y axis. wish inspect amplitude of key frequency signals.

how convert spectral density amplitude? there periodgram plot/analysis in r produces frequency vs amplitude plot automatically? appreciate advice.

maybe use different terminology do. page says value returned spectrum function list first 2 elements are:

freq     vector of frequencies @ spectral density estimated.  (possibly approximate fourier frequencies.) units reciprocal  of cycles per unit time (and not per observation spacing): see ‘details’  spec     vector (for univariate series) or matrix (for multivariate series) of  estimates of spectral density @ frequencies corresponding freq.

so $spec element calling vector of "amplitudes"? (you haven't said "key frequency signals" picked fourth frequency example in ?spectrum:

 lh.spec <- spectrum(lh)   lh.spec$freq[4] #1] 0.08333333   lh.spec$spec[4] #[1] 1.167519

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