Ultrasonic waves degradation:
When a dilute solution of a high molecular weight polymer is subjected to ultrasonic wave, the polymer begins to degrade, there-by reducing its molecular weight. The ultrasonic degradation is a special case of mechanical degradation as the polymer here is subjected to very high vibration, which are only mechanical forces.
Frequency of ultrasonic sound waves are sometimes above than 20,000 hertz, which is beyond the available range of the human ear.
Ultrasonic waves pass through the solution. The localised shear gradients produced tear-off polymer molecules leading to chain scission.
As a result, the average molecular weight of the sample goes down. But the decrease in the molecular weight of the polymer doesn't go beyond a certain value even after a long time exposure to ultrasonic or mechanical stresses.