Abstract Two frequency scanning ultrasonic spectrographs utilizing time-delay signal processing techniques have been developed. These permit measurement of time delays (and hence velocity changes) in a sample in the order of 10 −8 s. The present instruments allow such measurements to be made continuously over a frequency range of 1–10 MHz. These instruments have been used to monitor changes in velocity as a function of frequency during coagulation in purified human fibrinogen solution and human plasma. It was found that velocity dispersion occurred at discrete frequencies during the coagulation process. The frequency dependent data is reproducible, although this depends to some extent upon the samples used. The instruments which were calibrated using a fixed length of coaxial cable, exhibit a high degree of reproducibility.