The method of acoustic material signature, which has been used extensively for the study of materials, is shown to be an attractive new method of measurement for characterizing the coupling fluid in this acoustic microscope experiment. By this method, we show that sound velocity at 10 and 30 MHz in water and liquid nitrogen could be measured with a small sample volume. This method is well adapted to the study of non-linearity in liquids. Possible improvements in the instrumentation lead to the possibility of measuring relative sound velocity change of 10 ppm.