Imagine you are riding a roller coaster. When you get to the top and the car free-falls speeding up until it reaches the bottom of the drop. And at the bottom, you would be crushed into your seat with a force equal to 1,000 billion times your weight. Obviously, more than your stomach would react to such a ride. As for the bubble, of air trapped in water, it responds to the extraordinary force by creating a flash of light only a tiny fraction of a second long. The light is mostly ultraviolet. Ultrasound wave can make the bubble repeat this wild free fall more than 30,000 times a second

Chemical effects of ultrasound enhance reaction rates because of the formation of highly reactive radical species formed during cavitation in forms of bubbles.

After being created, these bubbles are filled with vapour and gas, and can produce radicals during such an implosion. It is these implosions which are the spectacular part of sonochemistry. Each one of these imploding bubbles can therefore be seen as a microreactor, with temperatures reaching an estimated 5000°C, and pressures of several hundreds of atmospheres. Quite spectacular!

 Fig2 - Cavitation bubble  in a liquid irradiated with ultrasound implodes near a solid surface

When  ultrasound is supplied in the system, bubbles react with the compression/expansion cycle of ultrasound causing it to expand and collapse in compression , hence energy is released in term of light and heat at the point of implosion.

The high temperatures and pressures created during cavitation can even cause the bubbles to produce a flash of light (see sonoluminescence) and release enough energy to activate the reaction.

   (see Ultrasound page for more information about implosion)