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This paper presents a summary and evaluation of the experimental properties of superfauid 'He as they were known in the fall of 1974. Subjects having thermodynamic significance, including specific heat, static magnetism, phase equilibria, and superfluid density, are discussed first. Then known flow properties

Certain thermoacoustic effects are described which form the basis for a heat engine that is intrinsically irreversible in the sense that it requires thermal lags for its operation. After discussing several acoustical heating and cooling effects, including the behavior of a new structure called a ‘‘thermoacoustic couple,’’ we discuss structures that can be placed in acoustically resonant tubes to produce both substantial heat pumping effects and, for restricted heat inputs, large temperature differences. The results are analyzed quantitatively using a second-order thermoacoustic theory based on the work of Rott. The qualities of the acoustic engine are generalized to describe a class of intrinsically irreversible heat engines of which the present acoustic engine is a special case. Finally the results of analysis of several idealized intrinsically irreversible engines are presented. These suggest that the efficiency of such engines may be determined primarily by geometry or configuration rather than by temperature.

We have observed the propagation of sound in liquid He 3 at 0.32 atm and at frequencies of 15.4 and 45.5 MHz down to a temperature T* of 2 mdeg on the magnetic temperature scale valid for powdered cerium magnesium nitrate (CMN) in the form of a right circular cylinder with diameter equal to height. As the temperature rises the sound attenuation increases, reaches a maximum, and then decreases. At low temperatures the attenuation is proportional to T* 2 and is independent of frequency. At high temperatures the attenuation is proportional to co 2 /T* 2 , where co is the angular frequency. The sound propagation velocity is relatively temperature independent at both high and low temperatures but near the attenuation maximum the velocity changes.In 1957 Landau 1 predicted that at sufficiently low temperatures a new type of sound, which he called zero sound, could be propagated in liquid He 3 . Based on Landau's idea, a more detailed theory of the velocity and attenuation of sound in both the hydrodynamic (first sound) and zero-sound regions was worked out by Khalatnikov and Abrikosov. 2 At temperatures sufficiently high that quantum effects are unimportant, it is predicted that the attenuation of zero sound be proportional to T 2 and independent of frequency. In the first-sound region it is predicted that the attenuation is proportional to OJ 2 /T 2 , corresponding to classical viscous attenuation with viscosity proportional to T~2. Both of these temperature and frequency dependences are observed in the present experiments. In what follows we shall show that there is quantitative agreement with theory on veloc-

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