Refrigerators are the source of much joy in the world, and the fridge is made possible by continually-cycled, selectively-pressurized gas. But someday, sound waves may replace this system in your fridge.
In your modern refrigerator, the gas (called a refrigerant) starts its journey outside the fridge, where it is pumped through a series of coils which compress it. The compression heats it up, and it bleeds heat into the air around it. After it has shaken off enough heat, and is still under a great deal of pressure, it condenses into a liquid.
The liquid refrigerant is then passed through a valve which allows the liquid, a bit at a time, to move from a very high pressure chamber to a very low pressure one. The refrigerant goes from liquid to gas, and the gas expands rapidly.
When liquid evaporates, it cools. When gas expands, it cools. The refrigerant has moved from liquid to gas and the gas has expanded quickly. The result is a lot of extremely cold gas, which is pushed up into metal coils inside of the fridge. This cold gas rushing through the coils cools the inside of the fridge. When the gas is pushed up through the fridge and back out, it’s compressed once again, and cycle starts over.
Current compressors are mechanical. It may not always be that way. Thermoacoustic compressors may be on the way. Instead of mechanics, these will use loud sound waves at resonant frequencies to generate compression of the gas. Not only will this save power, but many tests have been done using air, instead of refrigerants. Considering that many refrigerants are ecologically damaging, like chlorofluorocarbons, or dangerous for humans to inhale, like ammonia, sound waves may be a great option.