In this study the acoustic properties of thin slices of the IEC agar-based TMM were assessed over the frequency range of 4.5MHz to 50MHz for a period of 1 year at 3 monthly intervals. To maximize the acoustical stability of the TMM it was both assessed and stored in the same fluids as used in its manufacture (water, glycerol and benzalkonium chloride). A high frequency ultrasound scanner Vevo 770® and a scanning acoustic macroscope (SAM) system were used to measure the acoustical properties. The SoS measured from the TMM slices was found to be 1544.1 ± 4.2 ms-1 and a polynomial fit to the attenuation data was calculated to be 0.4842f + 0.008373f2 (R2=0.99). Batch to batch variation in acoustic properties and repeatability was also assessed (maximum variation of 7.4ms-1 for the mean SoS and 1dB cm-1 for the mean attenuation). These results demonstrate that the IEC agar-based TMM has excellent broadband acoustical stability over a period of 1 year.