It has been established that structural changes in a ferrofluid (formation of clusters) upon the effect of an external magnetic field can be effectively studied by ultrasound spectroscopy. In particular, the resonance absorption of ultrasonic waves enables detection and determination of the size of spherical clusters. This paper reports results of a study of changes of the size of clusters formed in a magnetic field as a function of the rate of the magnetic field intensity changes, the ferrofluid temperature, the frequency of the ultrasound wave and the angle between the magnetic field vector and the direction of ultrasound wave propagation.