Sound: Destructive interference

[(+) Influence]

I'm no scientist so pardon my ignorance on this topic. Is it true that when two signals of the same frequency meet, it's possible for them to have "destructive interference" and become 0? If that's the case, would I be able to put two speakers together (in calculated distances apart) so that they each produce a sound, but together the sound cancels?

 

[Sophie 378]

Yes, but only if they're producing a constant note at one pitch (frequency), and you have to calculate the position to the nearest <mm, and it's only silent at particular points. I'll see if I can find a diagramme for you before tonight. The sound waves spread out spherically - in circles - and you can get constructive (louder) and destructive (quieter) interference. I'm a girl who did A-level Physics BTW.
PS I don't think it has to be the same frequency, just the wavelength must be shorter than the distance between the speakers (not usually a problem).
Meanwhile, read http://en.wikipedia.org/wiki/Interference - if that doesn't help you I'll try to explain more.

 

[Timko]

I think if you want them to cancel entirely at some point in space then the two speakers would have to be playing exactly the opposite thing (although one can be delayed), and then you just need to look at the distance from each speaker to your desired point in space, and the speed of sound.

 

[Sophie 378]

No, it just has to be in antiphase for the sound to cancel at a node.

 

[brennan]

The two speakers both put out their own sound waves. At no point do the waves actually destroy each other - so that there is no sound after a certain point. It is just that the 2 waves have an equal magnitude, with an opposite sign, that cancels out the sound at a certain point(s). This is called destructive interference, similarly the two waves can both add up at other points to make a stronge signal (constructive interference).

The dark blue and purple waves combine to produce the light blue. Things get a lot more complicated if the two waves have different frequencies etc...

 

[Mr. Blonde]

In a technology magazine they showed active sound dampening systems based on destructive interference (years ago). The problem was to find a fast algorithm to compute the proper output (from the dampening speakers) from the input (the incoming noise) to actually kill the noise (you have to solve the problem in 3D - several microphones and speakers needed).

 

[Mise]

Waves are a function of 2 variables, time and space. Hence:
- you CAN stand at some POINT where the waves interfere destructively and cancel out.
- you CAN stand at some TIME " " " " " " " " " " " " " " " " " " " " " " " " " " " " " " " " " " " " " " .
- you CANNOT create a wave that at ALL TIMES and at ALL POINTS produces no sound.

 

[Truronian]

The same thing happens with light (as long as the sources are in phase). If you shine a laser at a piece of card with a double slit in it the resultant pattern looks like a series of lights (this comes from one laser beam):

The black bits between the white areas where the two waves cancel out

 

[Tank_Guy#3]

I know how to cancel the sound form speakers, turn off the stereo. I have no idea, I'm a computer guy, not a physic (is it physics?) guy.

 

[Timko]

No, it just has to be in antiphase for the sound to cancel at a node.

I didn't spot that single frequency remark, and was referring to general sound from a speaker.

- you CANNOT create a wave that at ALL TIMES and at ALL POINTS produces no sound.

Unless the two speakers were at the same point in space. I tried that with my speakers by means of a sledge-hammer but I think the testing apparatus may have unduly biased the results.

 

[Erik Mesoy]

I think there are some headphones for use on construction sites that use destructive interference to further dampen sound beyond what normal earmuffs do. Will have to see if I can find the article...

 

[Enkidu Warrior]

I'm no scientist so pardon my ignorance on this topic. Is it true that when two signals of the same frequency meet, it's possible for them to have "destructive interference" and become 0? If that's the case, would I be able to put two speakers together (in calculated distances apart) so that they each produce a sound, but together the sound cancels?

Yes, you can get them to cancel at a particular point in space or time, but if you're expecting to cancel out the sound at all positions (i.e. so you don't hear anything) then I'm afraid you'll be disappointed.

 

[Perfection]

In a technology magazine they showed active sound dampening systems based on destructive interference (years ago). The problem was to find a fast algorithm to compute the proper output (from the dampening speakers) from the input (the incoming noise) to actually kill the noise (you have to solve the problem in 3D - several microphones and speakers needed).

As Erik Mesoy says, they have that now.
http://en.wikipedia.org/wiki/Noise-cancelling_headphone

 

[Fetus4188]

Waves are a function of 2 variables, time and space. Hence:
- you CAN stand at some POINT where the waves interfere destructively and cancel out.
- you CAN stand at some TIME " " " " " " " " " " " " " " " " " " " " " " " " " " " " " " " " " " " " " " .
- you CANNOT create a wave that at ALL TIMES and at ALL POINTS produces no sound.

If the two waves originate from the same point and are perfectly out-of-phase they well be cancelled out everywhere. Although you can't really get two things in the exact same point. Theoretically possible, practically impossible.

 

[Perfection]

But if that happened, would they really be waves to begin with?

 

[Mise]

@Perfection, Fetus and Timko:
Something tells me that that is not possible, since a wave is propagating, and energy dissipating, but no physical effects are observed.

 

[Perfection]

@Perfection, Fetus and Timko:
Something tells me that that is not possible, since a wave is propagating, and energy dissipating, but no physical effects are observed.

Indeed, there would be nothing characteristic of waves. There'd be no amplitude, so no energy transfer.

Though the function z=0 works for pretty much any differential equations you see with harmonic waves and dampened harmonic waves.

Methinks it's a trivial solution.

(We just happen to be in the wave unit in my physics course)