Measurements have been made of the acoustic impedance (
Z
) of liquid
3
He under its saturated vapour pressure in the temperature range 0·035 to 0·6°K. A 1000 Mc/s sound wave is propagated along the axis of an
X
-cut quartz crystal, and undergoes many reflexions from each end of the crystal. Values are obtained for the reflexion coefficient at an interface between quartz and liquid
3
He by comparing the rate at which the signal decays in the crystal, with and without liquid
3
He present on the ends. These values then lead directly to the acoustic impedance (
Z
) of the liquid. Usually,
Z
/
ρ
(where
ρ
is the density) should be equal to the velocity of sound, and above 0·1°K this is found to be so. However, at about 0·1°K the value of
Z
/
ρ
increases abruptly, and at lower temperatures has a value about 10% greater. This result is in good agreement with Landau’s theory of a Fermi liquid, which relates the change in impedance with the propagation of a new mode of sound, the so-called 'zero-sound’.
Competing solutions of Landau’s kinetic equation for zero sound and first sound in thin arbitrarily polarized Fermi-liquid films