Some calculations have been made of the acoustic impedance of liquid ³He, in order to analyse the experimental results of Keen, Matthews and Wilks and of Betts, Keen and Wilks According to the theory of a Fermi liquid, due to Landau, the propagation of sound in liquid ³He should show an anomaly when the period of oscillation is made less than the relaxation time of the liquid. Landau predicts that a new type of wave motion, called zero sound, should propagate in this case The calculations reported here show that there is good agreement between the measured value of the acoustic impedance and that expected for a Fermi liquid supporting zero sound It is concluded that the experimental results provide strong evidence for the existence of zero sound. The agreement between theory and experiment, while close, is not perfect, so we have examined the approximations made in calculating the acoustic impedance in the hope of obtaining further information. It seems that the sequence of parameters describing the interaction of quasi-particles, F₀, F₁, F₂,., which is usually carried only as far as F₁, should be extended to include F₂ as well Rough estimates of F₂ are made by two different methods, the agreement is not very close, but is considered satisfactory