Sound velocity of liquid He-4 in aerogel

The sound velocity and attenuation of liquid He-4 in 93.7% and 94.6% porosi ty aerogels were measured for a frequency of 10 MHz. The sound velocity in 93.7% aerogel was larger than that in bulk He-4, whereas that in 94.6% was smaller. A model of the aerogel-He-4 composite gave satisfactory agreement with the dependence of the sound velocity on the temperature and the aeroge l density. This shows that the normal fluid is clamped to aerogel strands, although the viscous penetration depth is comparable to the typical size of a void in the aerogel. (C) 2000 Elsevier Science B.V. All rights reserved.