# 9 properties of ultrasonic waves

1. The following are the main properties of ultrasonic waves:
1. The ultrasonic waves cannot travel through vacuum.
2. These waves travel with speed of sound in a given medium.
3. Their velocity remains constant in homogeneous media.
4. These waves can weld certain plastics, metals etc.
5. These can produce vibrations in low viscosity liquids.
6. The ultrasonic waves are reflected and refracted just like light waves. i.e.

(a) Angle of incidence is equal to angle of reflection.

(b)   Incident ray, reflected ray and normal lie in same plane.

(c)    If i is angle of incidence, r is angle of refraction, V1 is velocity of ultrasonic waves in incident medium and  V2 in refracted medium then

Sin i/ sin r = V1 / V2

This is called Snell’s law of refraction.

7.    The speed of ultrasonic waves/acoustic waves is more in more dense media                    i.e. Vg < Vl < Vs. If ultrasonic waves enter from rarer medium to dense medium, then V1 < V2  so equation (5) gives sin i / sin r < 1 => i<r. Thus ultrasonic waves will bend away from normal. Similarly when ultrasonic waves enter from denser to rarer medium, then they bend toward normal. This property is just opposite to that of light.

8. As ultrasonic waves cannot travel through vacuum, therefore if these waves travel through a non- homogeneous medium, then at each discontinuity like crack or change in density or presence of impurity etc., the amplitude and thus intensity of ultrasonic waves decreases by some amount. This decrease in intensity of ultrasonic waves as these travel through a medium is called Attenuation. The vacuum in the material causes strong reflection of ultrasonic waves while impurities or discontinuity cause the scattering of ultrasonic waves leading to net decrease in intensity. The attenuation is increased with increase in frequency of ultrasonic waves for a given medium. The intensity of ultrasonic waves decreases exponentially according to the relation

I = I0 e-ax

where                                   I0 = Intensity at surface

I = Intensity at depth x inside the sample.

α is called Monochromatic Attenuation coefficient. Its value is different for different media and for a given medium, its value is different for different frequencies/wavelengths.
With increase in frequency of ultrasonic waves, value of α also increases.

The above are the major 9 properties of ultrasonic waves.