To verify the accuracy of the instrument, a test measurement was performed on an aluminum plate to measure its speed of sound. Transmission and pulse-echo measurement were performed on an aluminum plate 6.54 mm thick (measured by a micrometer) immersed in water using 2.25 MHz ultrasonic pulses (Figure ). Speed of sound is calculated by determining the ToF through the plate alone by separating the ToF through the water by: ToF(Al plate) = ToF(Transmission) -ToF(forward echo)/2 -ToF(reverse echo)/2 The waveforms used to calculate these values are shown in Figure and their corresponding ToF was calculated by the envelope method with a 10% threshold. The speed of sound is then calculated by dividing the plate thickness by ToF(Al plate). This gives a value of 6230 m/s, close to the literature value of 6320 m/s. This method is more accurate than directly measuring the transmission ToF using transducers directly clamped to the plate because it eliminates near surface interference effects as well as contributions from the instrument response time (e.g. wave transit time within the transducers themselves).