Transmission medium, ultrasound waves

The other important consideration concerns the transmission of ultrasound (and other forms of energy) from one medium to another and the importance of impedance matching . When wave energy is transferred from one medium to another then a part is transmitted and the rest reflected. The ratio of reflected to transmitted energies depends on the characteristic impedances of the two media and the transmission is total if these are matched. In the case of acoustic waves the specific impedance (Z) of a medium is given by the product of the density p and the velocity of sound v. that is... 

The use of air-bome ultrasound for the excitation and reception of surface or bulk waves introduces a number of problems. The acoustic impedance mismatch which exists at the transducer/air and the air/sample interfaces is the dominant factor to be overcome in this system. Typical values for these three media are about 35 MRayls for a piezo-ceramic (PZT) element and 45 MRayls for steel, compared with just 0.0004 MRayls for air. The transmission coefficient T for energy from a medium 1 into a medium 2 is given by... 

High-intensity ultrasound has the ability to alter a material s characteristics due to interactions between the sound wave and the food product at a structural level. Transmission of the ultrasonic wave takes place by the oscillation of medium particles, and the damping of ultrasound energy in deeper tissue layers forms the basis of the modification of tissue structures, leading to an impact on simultaneously occurring mass and heat transfer processes during drying. 

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