B Opposing speakers and standing waves

[Israel Lucindo]

If one speaker is placed facing another speaker with the inverted phase and we reproduce an equal frequency in both, what happened?
Did the sound completely cancel out or would a standing wave be created as if it were in phase?

Is this animation valid for sound waves? https://www.compadre.org/Physlets/waves/illustration17_4.cfm

 

[BvU]

Hello Israel, !

The animation is valid and in fact shows what you can expect in your scenario: standing waves with nodes at half wavelength intervals and antinodes halfway between them.
This is for a single-frequency sound.

Normal sounds are a wild mixture of many wavelengths, so the effect is blurred.

 

[sophiecentaur]

Normal sounds are a wild mixture of many wavelengths, so the effect is blurred.

The only place that you can 'guarantee' getting a consistent interference result is half way between two co-phased speakers (or along the normal to the line of centres), where you will always get addition of the sound pressures. Anywhere else you will get frequency dependent phase differences and an uncertain interference result.

 

[Israel Lucindo]

Hello Israel, !

The animation is valid and in fact shows what you can expect in your scenario: standing waves with nodes at half wavelength intervals and antinodes halfway between them.
This is for a single-frequency sound.

Normal sounds are a wild mixture of many wavelengths, so the effect is blurred.

Thanks

 

[sophiecentaur]

Normal sounds are a wild mixture of many wavelengths, so the effect is blurred.

Very much "blurred" in most places but along the mid line of two speakers with equal amplitude and in anti-phase (easy to do by mistake) you can get a very obvious 'hole' in the sound, where all / most of the frequencies cancel out. You need to be in a room where most echos are very low level (acoustically treated) to get the effect reliably but fooling around with speaker positions and gains can give a convincing result. A listener with 'golden ears' can often identify the problem despite odd listening conditions.

 

[Herman Trivilino]

but fooling around with speaker positions and gains can give a convincing result

Hold identical drivers face-to-face so that they are touching. It's a very convincing demonstration of destructive interference.

 

[Dr Transport]

Have someone hold a pair of speakers facing forward and move from side to side, you'll hear the nulls in the sound pattern.

 

[sophiecentaur]

Hold identical drivers face-to-face so that they are touching. It's a very convincing demonstration of destructive interference.

I'm not sure what precise experiment you are proposing but with the cones facing each other, the sound pressure between them would be high. This would be additive interference. Driven in antiphase, they would produce a (near) zero net displacement of the air around them but the details of what is going on behind the speakers would affect what you actually heard.

 

[Herman Trivilino]

Driven in antiphase, they would produce a (near) zero net displacement of the air around them but the details of what is going on behind the speakers would affect what you actually heard.

Yes, but there is a very noticeable drop in the volume when they approach contact.

 

[sophiecentaur]

Yes, but there is a very noticeable drop in the volume when they approach contact.

But that would be due to a mismatch of impedance and not interference. Each cone would be preventing the other one from moving as far as the amplifier would be expected to move it. There would be a significant Mutual Impedance between the two sources.