An automatic high‐precision ultrasonic system with the corresponding ultrasonic cell is described to measure simultaneously the speed of sound and the attenuation as a function of frequency. The technique is based on the pulse‐echo method and is designed to detect small structural changes in liquids, when an external variable is changed. The precision of the technique is limited by the stability of the temperature, which was kept constant within ±0.5 mK. The method, which permits one to obtain the absolute value of the velocity as well as the attenuation with respect to the solvent, is fully automated for the whole variation of the external variable, which added to its fast response permits easy use in dynamic processes.