question about noise-cancelling technology

[lostatlantis]

I've always been fascinated by the noise-cancelling technology found in many headphones, but I've always wondered - do our ears actually receive less energy when the noise is being "cancelled" with a wave 180 degrees out of phase? I am wondering because I'd like to get one for my mom so she can more comfortably mow her lawn and vacuum (she just won't let me buy her a new vacuum, that thing is incredibly loud).

 

[FrankSchwab]

It does seem that energy just kind of disappears here, doesn't it? I mean, an audio wave traveling through air has a certain amount of energy associated with it. If you expend even more energy to launch another wave 180 out of phase, you now have twice as much energy traveling through the air, but the energy is indetectable. Where did it go?

Anyone?

From a practical perspective, yes, it's good for the ears. The eardrum converts the incoming pressure waves to mechanical motion; if portions of the incoming wave are cancelled, the eardrum will move less, reducing damage to the inner ear.

From what I've heard, the noise-cancelling headsets work best for noise sources that have significant discrete frequencies - for example, a private prop plane has most of it's noise at stationary frequencies related to the current RPM of the engine. The headsets can filter that out OK. For something like a vacuum, where most of the noise may be essentially "white noise", they don't tend to work very well.

BTW, be a good kid and mow your mom's grass for her.

/frank

 

[toastyghost]

Not to over simplify but a plus coming this way is the same as a minus going that way. Waves do just cancel.

 

[Fencer128]

Originally posted by: toastyghost
Not to over simplify but a plus coming this way is the same as a minus going that way. Waves do just cancel.

Whilst you are correct - it is possible to examine this on a more physical level also. Sound waves are basically mechanical waves of high and low pressure air. By moving the anti-noise out of phase you are ensuring that a point of high pressure is somewhat coincident with a point of low pressure. This causes a rough equalisation of the local air pressure and "cancels" out your sound.

Cheers,

Andy

 

[lostatlantis]

So with the destructive interferrence, the noise wave is neutralized and the ear actually receives less energy... correct? My main point is to confirm that noise-cancelling technology decreases the amount of energy the ear receives, hence protecting the ear...

anyone?

 

[AEB]

i would say yes since its canceling the noise, since its the exact inverse of the wave propagating from whatever, they cancel its like a ripple in a pond . the ripples at the edge of the pond(if the rock was dropped in the middle) are like after noise cancelation small and weak

 

[syberscott]

Originally posted by: lostatlantis
So with the destructive interferrence, the noise wave is neutralized and the ear actually receives less energy... correct? My main point is to confirm that noise-cancelling technology decreases the amount of energy the ear receives, hence protecting the ear...

anyone?

I think you're getting confused by thinking of sound as energy instead of mechanical waves.

 

[Fencer128]

Originally posted by: lostatlantis
So with the destructive interferrence, the noise wave is neutralized and the ear actually receives less energy... correct? My main point is to confirm that noise-cancelling technology decreases the amount of energy the ear receives, hence protecting the ear...

anyone?

Hi,

Yes, I inferred this in my previous post when I spoke about pressure equalisation. When the pressure is equalised to atmospheric pressure there will be no effect on the ear drum --> no "energy" absorbed by ear drum.

Cheers,

Andy

 

[sgtroyer]

Yeah, the ear recieves less energy. Also, a vacuum cleaner is probably more like an airplane engine than you think. It's not white noise: most of the noise will be at multiples of 60Hz because the motor rotates at a fixed frequency (related to the AC power frequency).

 

[lostatlantis]

Thanks for all the inputs! Do you guys have any recommendations for a good set of noise-cancellation headphones?

 

[Smilin]

Sennheiser's

Check crutchfield online for em.

 

[tweeve]

BOSE make a great pair of noise cancelling head phones, I know I have a pair, they work great while mowing the lawn.

 

[Kntx]

It does seem that energy just kind of disappears here, doesn't it? I mean, an audio wave traveling through air has a certain amount of energy associated with it. If you expend even more energy to launch another wave 180 out of phase, you now have twice as much energy traveling through the air, but the energy is indetectable. Where did it go?

Probably just gets dissipated as heat. Cause in physics, if you don't know where it goes... heat is the answer! Of course you'd never notice the increase in temperature as the actual amount of energy in sounds waves is miniscule. There is not enough energy in the sound created by a NFL stadium full of screaming fans to boil a mug of coffee over the course of a game.

 

[GridOner]

Well there goes my 'power from sonic waves' idea from the other topic...back to the drawing board.