T2 – RADIATION OF SOUND IN HELIUM II*

Publisher Summary This chapter examines the propagation of sound waves in helium II. The application of the system of hydrodynamical equations to the radiation of sound in helium II shows that the propagation of sound waves in this medium can take place with two different velocities. The character of the sound radiation depends essentially on the ratio of the dimensions of the radiating body to the wavelength. Consider first the case when the wavelength of both sounds is small compared with the dimensions of the vibrating body. It can be assumed then that each part of the body surface radiates a plane wave in a direction normal to itself. It will, therefore, be sufficient to consider the radiation of sound by a plane surface vibrating as a whole in a direction perpendicular to itself. The property of super fluidity disappear sat velocities exceeding a certain critical value, which depends on the temperature as well as on the characteristic dimensions of the problem. For large dimensions of the vessel containing the helium II, the critical velocity is rather small, probably of the order of several centimeters per second.