Re: [Phys-L] Acoustics: Does a speaker create a displacement antinode?

[John Mallinckrodt <ajm@csupomona.edu>]

Assuming the speakers are operating in phase at equal amplitude and that there is little acoustic reflection so that the waves traveling in each direction are simply those that would be produced by each speaker operating independently, there will be a displacement node at the midpoint of the line between the speakers.  The rest of the standing wave pattern follows from that point of reference except for the fact that the two interfering waves are not one-dimensional and, as a result, decrease in amplitude with distance from their respective speaker.  This produces a wave of decreasing standing wave ratio (SWR) as one approaches either speaker.

In general the speakers will not be at a node or antinode both because this is not a resonant phenomenon and because of the non-one dimensional nature of the waves.  If they are placed an integer number of wavelengths apart, they will be at a place that WOULD otherwise correspond to a displacement antinode.  If they are placed a half integer number of wavelengths apart, they will be at a place that WOULD otherwise correspond to a displacement node.

John Mallinckrodt
Cal Poly Pomona

On Mar 25, 2014, at 11:46 AM, Jeff Loats wrote:

> Hello,
>
> I'm having a "I thought I knew this, but I'm confusing myself" kind of
> moment. I appreciate your help:
>
> Imagine two speakers pointed at each other from some distance apart. At
> certain separations we should get a standing wave along the line connecting
> them. If we set it up to get such a standing wave and if we measure
> displacement of the air, will there be displacement antinodes at each
> speaker? Meaning a pressure node at each speaker?
>
> Something about my thinking on this is muddled...
>
> Jeff
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