I guess frequency range of operation should be dependent on the grain-size too. However factors like pore shape and pore fraction become dominant factors at low frequency. See this, http://www.virginia.edu/ms/research/wadley/Documents/Publications/Modeling_the_Interaction_Ultrasound_Pores.pdf

Mr. Vikash Pandey,

Thank you for your response and good material!!

Typical pore size is important. If I assume a typical pore size of 10 µm and a ultrasonic wavelength of 1 mm (as for lower MHz frequencies), we are in the range where Rayleigh scattering of ultrasound at pores is operative. Ultrasound attentuation for Raileigh scattering depends on frequency as f to the power of 4! Therefore, lower frequencies such as 1 MHz are preferable. At much higher frequency, attenuation may be too high for your application (non-destructive testing, pulse-echo technique?), you might not be able to detect echoes. On the other hand, you need larger samples at lower frequency because of longer duration of the ultrasonic pulses. Larger samples might not be available. Clearly, you need to find a compromise which depends on the details of your application.