Here's the thing: using software for Finite Element Acoustic Simulation I got the dataset of frequency response from a room; software works by solving the wave equation in the interval $`[f_i,f_f]`$ so results come directly in the frequency domain and we save them into a .csv file:
For decay time analysis, we're on the way to transform the data to time domain (using IFFT) and it has some tricks that you may know but I'll mention briefly:
(From Matlab's FFT documentation)
Space between 0 [Hz] and f_i [Hz] must be filled in (keeping the distance between points as in the results) and assigned zero amplitude, to let IFFT algorith work correctly
Do an amplitude "de-normalization"
"Reflect" the frequency spectra using the last frequency element as pivot, omitting the first one and conjugating this copied values
Apply IFFT, and done.
So far, we've made and proved a code that does this steps using the .csv file as entry. The test was realized with a .csv created from a .wav file and it resulted ok:
(Subplot 1: original. Subplot 2: after process)
Our analysis was made from 0.1 to 200 Hz with steps of 0.1 Hz. When we transformed the results to time domain, the signal was 10 secs long but we expected it to be at least 30 secs (since was a model similar to last's plot one). We tried to reduce the distance between points by interpolating it, as $`\Delta f = 1/T`$ say (with $`T`$ as signal duration). However, when applying again IFFT the plot was exactly the same!
So, how can I process my data to get a longer signal in the time domain?
Hope I have been clear enough. Thanks in advance.
EDIT: showing correctly the list before first image