A Formant object represents formants as a function of time.
[ref: Praat help, https://www.fon.hum.uva.nl/praat/manual/Formant.html]
f$xmin ... start time (seconds)
f$xmax ... end time (seconds)
f$nx ... number of frames
f$dx ... time step = frame duration (seconds)
f$x1 ... time associated with the first frame (seconds)
f$t ... vector of time instances associated with all frames
f$maxnFormants ... maximum number of formants in frame
f$frame[[1]] to f$frame[[f$nx]] ... frames
f$frame[[1]]$intensity ... intensity of the frame
f$frame[[1]]$nFormants ... actual number of formants in this frame
f$frame[[1]]$frequency ... vector of formant frequencies (in Hz)
f$frame[[1]]$bandwidth ... vector of formant bandwidths (in Hz)