The purpose of the split ring commutator and slip rings are updated here.
Students tend to think that ultrasound is a totally different kind of wave, or it is part of the electromagnetic spectrum, which is wrong!
The different kind of waves below are just different kind of sounds. So the properties of sound apply to all. Ultrasound is also sound wave, just that the frequency is greater than 20 kHz which is outside the range of audible frequencies for us.
Other examples in our daily lives:
In some supermarket, the seafood are placed outside of air-conditioned place. The seafood is kept cold by putting crushed ice covering the seafood to keep the them cold and fresh.
Refer to this Sci Physics question N2008P2Q6(b)
For the solid that does not melt, when thermal energy is absorbed from the surrounding food, its temperature starts to rise. So it is not so effective at keeping the food cool.
For ice-pack, when thermal energy is absorbed from the surrounding food, it starts to melt. During melting process, a much larger quantity of thermal energy is absorbed from the food to melt per unit mass of ice, the temperature remains constant at 1oC, and the melting process is long. Hence ice-pack is more effective at keeping the food cool.
Related posts about ice: