In spite of the multitude of medical applications of ultrasounds, the interactions of ultrasonic waves on living tissues are not completely described and, in last years, the necessity of a guideline for a safe application of ultrasounds on patients is emerged. Thus, simulations of ultrasound field interactions with tissues are become essential. Simulations are usually conducted on phantom made of Tissue Mimicking Materials (TMMs), assuming that their thermophysical and mechanical properties are similar to those ones of living biological samples. In this work the experimental apparatus realized for the characterization of semi-solid materials at the Thermophysical Properties Laboratory of INRiM is presented. In order to obtain a standard phantom, useful to define ultrasound dose for therapeutic ultrasound, accurate speed of sound measurements and frequency dependence attenuation coefficient on specific gels produced at INRiM have been carried out.