Ultrasound Propagation in the Normal State of Liquid $^{3}$He/ 98{\%} Aerogel.

We studied the propagation of longitudinal sound in the normal state of liquid $^{3}$He/ 98{\%} aerogel at 9.5 MHz. The absolute attenuation and sound velocity were determined by direct propagation of sound pulses through the medium. Our measurements cover a wide range of temperatures from 2 mK to 200 mK at three different pressures (10, 21 and 29 bars). As reported by Nomura \textit{et al}., the sound mode remains in the hydrodynamic limit down to 2 mK due to the impurity scattering off the aerogel. However, we observed a new feature in the high temperature range that the attenuation shows a minimum and increases at high temperature. The minimum (T$_{M})$ occurs around 60 mK at 10 bars and moves to 40 mK at 29 bars. For T $>>$ T$_{M}$, the attenuation at high temperature shows a T$^{0.7}$ dependence for all pressures. We will discuss our observations in the framework of theories proposed by Higashitani \textit{et al.} and Biot.