The more interesting case is that for higher frequencies, in which
case it is now understood (Berryman et al., 2000, 2002; Berryman and
Wang, 2001)
that the simple Biot-Gassmann theory is actually
inadequate because there can be dependence of 
on liquid saturant
properties at high frequencies, such as f > 1 kHz. The precise
frequency at which this becomes important is material dependent, but it
is generally observed that for ultrasonic frequencies f > 20 kHz some
deviations from Biot-Gassmann predictions are normally found.
So it is in this regime that the distinctions between air-filled and
water-filled pores become important for the torsional motion of a
cylinder.
