# What happens to the energy in case of destructive interference ?

#### jonnyk

What happens to the energy in case of "destructive interference"?

Hi everyone,

I'll mention a few cases here.

1) In case of sound suppose we have two speakers. One gives off a certain sound and the other speaker gives off the exact inverted phase of the other speaker's sound, both with the samer power. No sound will be heard BUT my qs now where does the energy go? I suspect heat but am not sure.

2) Two water waves with exact opposite phases come across each other. Again they cancel each other out and the water is still. BUT where does the energy go here? Again heat or something else?

3) Two photons as far as im aware would form a pair of elementary particles BUT here I'd like to know what happens if two photons of the SAME PHASE come together i.e. wht happens in case of CONstructive interference in case of photons? Does it turn into one single photon of higher energy?

Thankyou very much!

Related Other Physics Topics News on Phys.org

Staff Emeritus
Re: What happens to the energy in case of "destructive interference"?

Destructive interference only occurs at a set of points, not everywhere. Constructive interference also occurs at a set of points, not everywhere. When you add up the total energy, you'll see that it is conserved.

#### jonnyk

Re: What happens to the energy in case of "destructive interference"?

Destructive interference only occurs at a set of points, not everywhere. Constructive interference also occurs at a set of points, not everywhere. When you add up the total energy, you'll see that it is conserved.
JK- In my first example if we have two loudpeakers, one producing the sound and the other the anti sound of euqal amounts and both are switched on at say 500 Watts, no sound will be heard or will it? If not what happens to the energy in that case? Thanks.

#### jtbell

Mentor
Re: What happens to the energy in case of "destructive interference"?

At some points in the room, the intensity of the sound (energy per volume) will be twice what it would have been without the interference. At other points the intensity will be zero. The total energy integrated over the entire room will be the same as it would have been without interference.

#### jonnyk

Re: What happens to the energy in case of "destructive interference"?

@jtbell

At some points in the room, the intensity of the sound (energy per volume) will be twice what it would have been without the interference. At other points the intensity will be zero. The total energy integrated over the entire room will be the same as it would have been without interference.
JK- AHA so the sound is not totally cancelled out with a phase inverter/anti sound device. BUT then i wonder why not use a device which turns the sound completely into heat, which is way less disturbing at tht power rate to the environment, instead of more disturbing sound at other places? Thanks.

#### Buzzworks

Re: What happens to the energy in case of "destructive interference"?

Acoustic padding convert sound to heat but they are very porous so they are ill-suited for outdoor use. Large scale use will also make them extreme fire hazard.

#### jonnyk

Re: What happens to the energy in case of "destructive interference"?

@crapworks

Acoustic padding convert sound to heat but they are very porous so they are ill-suited for outdoor use. Large scale use will also make them extreme fire hazard.
JK- Thanks.

### Physics Forums Values

We Value Quality
• Topics based on mainstream science
• Proper English grammar and spelling
We Value Civility
• Positive and compassionate attitudes
• Patience while debating
We Value Productivity
• Disciplined to remain on-topic
• Recognition of own weaknesses
• Solo and co-op problem solving