First sound in superfluid helium

A description of those properties of superfluid helium which make it of special interest to acousticians will be given. Two areas of recent research will then be treated in detail. The first of these is the study of the scattering of sound by thermal waves in liquid helium. This work has led to a better understanding of the dispersion of the thermal waves. The second topic is the transmission of first sound across interfaces between solids and liquid helium. Despite much work, both theoretical and experimental, a full understanding of this problem has not yet been achieved. [Work supported by the NSF through the Materials Research Laboratory at Brown University and through Grant No. DMR75‐14761.]