[R] power in a specific frequency band

[Giuseppe Pagnoni]

Dear R users

I have a really simple question (hoping for a really simple answer :-):

Having estimated the spectral density of a time series "x" (heart rate 
data) with:

x.pgram <- spectrum(x,method="pgram")

  I would like to compute the power in a specific energy band.

Assuming that frequency(x)=4 (Hz), and that I am interested in the band 
between f1 and f2, is the power in the band simply the following?

sum(x.pgram$spec[(x.pgram$freq > f1/frequency(x)) & (x.pgram$freq <= 
f2/frequency(x))])

If it is so, are the returned units the same units as the original time 
series, but squared (if x is bpm, then the power is in bpm^2)?

I own a copy of Venables and Ripley (MASS 2003), but I was not able to 
extract this information from the time series chapter....


thanks for any help
(please cc to my e-mail, if possible)


         giuseppe





Giuseppe Pagnoni
Dept. Psychiatry and Behavioral Sciences
Emory University
1639 Pierce Drive, Suite 4000
Atlanta, GA, 30322
tel: 404.712.8431
fax: 404.727.3233