library("tuneR")

vol = function(w, indices, n=50) {
# w : wave object
# indices: points at which to compute the volume
# number of samples to use
# return a vector of volumes

	left = w@left
	vol = c()    # empty vector
	count = 0

	for (j in indices) {
	    count = count + 1
		from = j
		to = j + n
		vol[count] = sd(left[from:to])
    }
	print(indices)
	title = paste("skip factor= ", indices[3])
	plot(vol, main=title); lines(vol);
	return(vol)
}