Simple Sound Speed Measurement Method for Liquids and Castable Phantom Materials

The speed of sound c is an important property of materials and fluids. Ultrasonic imaging is based on the assumed values of c, and the characterization of various phantom materials includes measurements of their c values. Measurements of c, to date, have required specialized setups that are not always readily available. The development of a simple alternative allows laboratories to measure c with a minimum of specialized equipment. In this article, a method using only a polypropylene (or other plastic) test tube, fishing line, a level, a sewing or suture needle, sodium chloride, and a conventional ultrasonic B-mode imager is described that allows one to measure c readily. The measurement is based on the difference between the apparent and actual distances separating two fishing line targets embedded within a test tube. The liquid or gel being measured is poured into the test tube and B-mode ultrasonic images are obtained using a conventional clinical imager. This method was tested on seven samples of saline solutions with varying salt concentrations, pure ethanol, and a tissue-mimicking gel to verify its accuracy over a broad range (1144-1700 m/s) of reference values. Sound speed measurements were, on average, 100.4% +/- 0.86% of published values. No single measurement had a deviation exceeding 2.4%. The measurement of sound speed is simplified and may be accurately performed with minimal equipment.