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

[John Denker <jsd@av8n.com>]

On 04/08/2014 11:07 AM, Jeff Loats wrote:
>  If we put the speakers at opposite
> ends of a tube we now do have a resonant phenomenon and we will get
> essentially 1D standing waves with displacement antinodes at the speakers.

It depends.

1) This is guaranteed to be a tricky question, because an
 ideal speaker will do one thing, while typical off-the-shelf
 speakers will do something else entirely.

2) It very much depends on the phase and frequency, and on
 the geometry of the situation.

As a particularly clear example:  Consider the low-frequency
asymptote, and drive the speakers 180° out of phase.  Then
there will be no resonance whatsoever, no nodes, and no
antinodes.  There will just be a slug of air shuttling back 
and forth.

OTOH if you /want/ to see resonance, you can drive the speakers
at one of the resonant frequencies.  
 -- You could arrange for the speaker to be "almost" a displacement
  node. Since the speaker is necessarily moving, it won't be exactly
  a node, but in a high-Q situation it will be close.
 -- You could arrange for the speaker to be "almost" a displacement
  antinode. Since the speaker is necessarily exerting some pressure, 
  it won't be exactly an antinode, but in a high-Q situation it will
  be close.

There are LOTS of other possibilities.