Collective Modes in Aerogels of Different Porosity Filled with Helium II

The velocities of fast and slow collective modes of superfluid He II in 90%, 94% and 98% porosity aerogels were measured by means of low-frequency resonant technique using piezoelectric transducers. The measured temperature dependences of velocities of both modes are compared with the theory based on approach of Biot (J. Acoust. Soc. Am. 28:168, 1956; J. Acoust. Soc. Am. 28:179, 1956) for porous medium filled with superfluid helium with taking into account of tortuosity of an acoustic way and porosity (Kekutia and Chkhaidze in Fiz. Nizk. Temp. 28:1115, 2002). The coefficients of tortuosity are obtained for the aerogel samples studied. Evolution of the fast and slow modes with the temperature and porosity variation is established.