# Making an Acoustic Propulsion System at Home

###### By Anupum Pant

Imagine you’ve got 2 PET bottles coupled to the ends of a stick and the centre of the stick is suspended using a piece of thread. Now, if you could spin this contraption, using the low frequencies from just a sub-woofer, you could give this phenomenon a fancy name – acoustic propulsion. Sounds like rocket science? It’s far from that…

Everyone must have tried making low whistles by blowing air into an empty coke bottle. The sound that comes out happens due to something called Helmholtz resonance. When you blow in air, the central column of air inside moves much faster than the air that is touching the inside of the bottle  (the air that surrounds the column of air which you blow in). This difference in velocities creates several vortexes inside the bottle, which make the air move inside in a periodic fashion. And a resonant sound frequency is generated.

I’d explain the fancy word – acoustic propulsion – like this:

In simple words, an empty coke bottle in this little sound experiment acts as a box which converts air you blow into sound. Think of it as an engine with a twist that uses heat to turn a crank. Heat being the air you blow, and the crank turning being the sound that comes out. Just that this engine also works the other way. Turn the crank and heat is generated from the other side.

Likewise, what if you put in the sound? Instead of blowing into the bottle, leave it suspended and make it vibrate with an external source of sound, a sub-woofer! If you get the right frequency, you’d generate a resonant condition inside the bottle, generate vortexes and air would come out of the bottle’s opening. This would propel the bottle further! The video below shows you how…