National University of Singapore
Food Science & Technology
There are three parameters that are most commonly measured in ultrasonic experiments. These parameters include:
1) ultrasonic velocity,
2) attenuation coefficient and
3) acoustic impedance
They relate to the physical properties of the food such as composition, structure and physical state.
Ultrasonic velocity:
This is the velocity (c) at which ultrasound waves go through a material. And it is dependent on the elastic modulus (E) and density (ρ):
The modulus is dependent on whether the material is a gas, solid or liquid and whether a shear wave or compression wave is used. The moduli and densities are dependent on the structure, composition and physical state; therefore, the ultrasonic velocity measurements can provide more information of these properties.
Attenuation factor:
Attenuation is a decrease in the intensity of the ultrasonic wave as it passes through a material. The attenuation coefficient of a material can expressed as nepers per meter (Np m-1).
Here, A0 is the initial amplitude of the wave and x is the distance travelled. It is sometimes also expressed as decibels per meter (dB m-1). 1 Np = 8.686 dB.
The 2 main causes of attenuation are 1) absorption and 2) scattering.
Absorption is the conversion of energy stored in the ultrasonic waves into heat during scattering. Ultrasonic waves lose their energy but this is not detected as the direction and phase have been altered.
Scattering occurs in heterogeneous materials, such as emulsions, suspensions and foams. Ultrasonic waves are incident on dispersed particles and are scattered in direction other than that of the incident wave.
Measurement of attenuation coefficient can provide information on the physicochemical properties of a material such as concentration, viscosity, molecular relaxation and microstructure.
Acoustic impedance:
It is related to the density and speed of sound in the undisturbed medium. Each food material has its own acoustic impedance. If the food material has similar acoustic impedance with its surroundings, little ultrasound waves are reflected (more waves are transmitted) whereas food materials that have very different acoustic impedance with its surroundings have more ultrasonic waves being reflected.
R represents the reflection coefficient and Z1 and Z2 represent the acoustic impedance of 2 different materials, respectively.