[R] fonction PLATEAU???

[Thibault Grava]

Hello,
 
I am working on extracting data from sound recording (fee bee song of the black-capped chickadee). I obtained a two column matrix with x=time(s) and y=frequency(khz). The part of the recording that is interesting me is when the frequency is stable. Does somebody ever used a fonction that extract only constant data among a huge list of data (sample= 180000)? 
 
Thanks
 
 
Thibault Grava 
PhD candidate, 
Natural Resources and Environmental Study (NRES)
University of Northern BC, Prince George, BC
phone office: (250) 960 6050 
email address: grava at unbc.ca