Abstract Here we report experimental results of propagating characteristics of ultrasonic wave and explain it in light of existing theories. First, we study the relaxation process after magnetic field built up. It is known that the particles form chain-like clusters when the magnetic field is applied. The process of the cluster for mation is a relaxation process. The ultrasonic velocity v as a function of magnetic field at field angle θ= 0 and π/2 at 4MHz were measured. We have studied the dependence of velocity on. Concentrations of the magnetic particles and frequency. The experimental results can be explain by Tarapov's theory rather than th approach used by Parsons.