  Time domain responses of porous media have been studied by some authors, but
generally the possible descriptions have been given in the frequency domain.
The aim of this paper, limited to materials with rigid skeleton considered as
equivalent fluids, is to compare the descriptions by Johnson-Allard ($JA$%) as
well as by Pride-Lafarge ($PL$) with i) some analytical, approximate formulas,
based upon asymptotic high frequency expansion ; ii) the exact formula by
Zwikker and Kosten for the case of cylindrical pores. The paper starts with a
short summary of the statement of the different general full frequency models
($JA$ and $PL).$ The Green function in the time domain is shown to exhibit
interesting properties of materials. In particular the maximum response depends
on one dimensionless parameter only, which is denoted $\xi $ and is the ratio
of the travelled distance to the product of the \textquotedblleft
frozen\textquotedblright\ sound speed and a characteristic viscous relaxation
time. The distance $\xi $ is related to a time domain Stokes number. The
numerical computation of the Green function is done by FFT, with some
precautions, because of the importance of the higher frequencies on the
response shape. The $PL$ description is shown to be the best full frequency
general model, but some discrepancies with the exact model appear at short
times or short distances. When the distance $\xi $ increases from zero, the
asymptotic expansion shows that the maximum of the Green function decreases
first as $1/\xi ^{2}$, then exponentially.
