Making a car that uses sound energy Can we use sound energy to move a car? What if, instead of an engine, we put something near the tires which make tires move with the help of sound energy? Is this possible?
 A: The problem with sound waves is that the pressures they produce are very small for any given power input. So, the forces they create would only be able to move a very small car--small as in a few millimeters.
There are devices to manipulate objects and levitate them using sound. Here's a video about one such device built by the Argonne National Laboratory: https://www.youtube.com/watch?v=3MXVSdXZzpc
At timestamp 4:44, it's explained how much power it takes to levitate beads that are just a few millimeters in diameter.

This is actually generating an awful lot of sound even though it's a pretty small device. It has about the same level of sound as a rock concert. So, you could build a bigger transducer and levitate larger objects but it would be deafening for one and also very destructive for another.

A: Sound is vibrations that propagate as acoustic waves in a medium (like air in this case) and so the question is, can we use these vibrations to power a car?
Have a read of this Wikipedia article which states " A vibration powered generator is a type of electric generator that converts the kinetic energy from vibration into electrical energy"
While such devices are possible, and indeed have been made, as per this same article:
"A miniature electromagnetic vibration energy generator was developed by a team from the University of Southampton in 2007. This particular device consists of a cantilever beam with a magnet attached to the end. The beam moves up and down as the device is subjected to vibrations from surrounding sources."
The problem that would arise, is the amount of energy required to move the mass of the car and indeed the passenger(s) would be too high. Ambient sounds would not be sufficient to power this car, and so an alternative source of energy to generate the sound to power the car would be needed, which would defeat the purpose of such a device anyway.
A: One horsepower is 746 watts. If broadcast as sound in the audible range that amount of acoustic power would be excruciatingly loud if you were anywhere near the source. If expressed as mechanical power instead, one horsepower would push a car at a brisk walk on level ground and at a crawl up a slope.
It's common for a small car to have a 100 horsepower engine. If turned into noise with 100% efficiency that's 74 kilowatts. So to match the performance of a car with a 100HP engine in it would require 74kW of acoustic power and some means of capturing it at 100% efficiency and converting it all to mechanical work.
This would make the world an unbelievably loud place, says an ex-electric bassist whose biggest rig was 400 watts (electrical) into a speaker box of about 10% efficiency.
