# Can I propel a boat

HappyEngineer

## Homework Statement

How do I calculate the propulsive force produced by a speaker?

## The Attempt at a Solution

According to the wikipedia page on sound (http://en.wikipedia.org/wiki/Sound) a jackhammer 1m distant will generate 2 Pa of pressure. A Pascal is 1 N/m^2. So, a speaker that had 1 m^2 of surface area would produce 2 N of force away from the speaker if it produced that sound.

Is that correct? If I attached a speaker like that on the back of a boat, it should produce a continuous 2N of force to push the boat forward.

Last edited:

HappyEngineer
Oops. I meant for the subject to be "Can I propel a boat using a speaker?".

physixguru
The above report by wikipedia is only valid for ideal situations i.e. where air resistance,viscosity,friction,wave tension, etc. are absent.
In real life situations ,the above experimental theory can't be realized.

gendou2
A speaker functions by moving a electromagnetic coil back and forth.
I think the pressure you refer to is the wave peak.
For a typical sound wave, the wave valley will be the same amplitude pressure in the opposite direction.
I'm not an expert in acoustics, but I suppose that sound waves which had wave forms that look like a half dome could apply repulsive force. This is because the slow relaxed intake would be drown out by friction, while the quick pule out would overcome friction and push objects away. Sounds like a tricky but fun experiment to do.

However, if you used the same electromagnetic coil to pump water in one direction, with a valve to insure unidirectional flow, you could pump water from the bow to the stern and move your boat.

HappyEngineer
@physixguru, do you mean air resistance on the other side of the boat and friction which keeps the boat in place? If so, couldn't this be addressed by instead putting the speaker on a trolly (with greased axles) on greased train tracks? If the sound has any propulsive effect at all then it's just a matter of cranking the volume up high enough so the force overcomes friction.

@gendou2, I think your objection is perhaps more serious. I'm imagining a simplification of this. Imagine a 0.1Hz wave made by a speaker the size of a sail boat sale. It would puff out in one direction, thus pushing air away and moving the vehicle in the opposite direction. Then the sail (encased in a speaker box) would puff in the opposite direction thus pulling the vehicle back in the opposite direction.

The sound would need to be generated such that the push away was fast and the pull back was slow. If I took normal sound produced by a speaker and somehow doubled the length of time the speaker took to pull back in, what would that do to the sound it produced?

After taking a normal sinusoidal tone at some frequency (let's say 60Hz) and then performing that alteration to the wave (the troughs would be made twice as long as the peaks), what would it sound like? What would happen to a normal song if we made that alteration to the sound waves produced by a song?

If I produced a sound of that sort, is there some way to calculate the amount of propulsive force it produces? The above method would presumably move forward 2 steps for every one step back.

physixguru
How would you create a vacuum my friend?
I could have easily gone into the technical part of it to trouble you like 'gendou2', did but i thought that nothing is more specific than basics.
Wave resistance can't be eliminated until specific devices are used in 'ideal situations'.