Temperature and frequency dependence of longitudinal sound in hcp<mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" display="inline"><mml:mrow><mml:mmultiscripts><mml:mrow><mml:mi mathvariant="normal">He</mml:mi></mml:mrow><mml:mprescripts/><mml:mrow/><mml:mrow><mml:mn>4</mml:mn></mml:mrow><mml:mrow/><mml:mrow/></mml:mmultiscripts></mml:mrow></mml:math>

Data are presented for the longitudinal sound velocity of single crystals of hcp $^{4}\mathrm{He}$ at a molar volume of 17.4 ${\mathrm{cm}}^{3}$/mol from 0.25 to 3.0 K The velocity was measured at 5, 15, and 25 MHz on each crystal. Most, but not all, crystals show a strongly anomalous velocity and attenuation below about 1.8 K. The anomaly moves to lower temperatures with increasing frequency. Very large dispersion is found in all crystals, the dispersion being normal in most crystals, but anomalous in two crystals in the 15 to 25 MHz range. Various possible explanations are discussed.

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