Speaker Boundary Effects

[QED]

It seems to be common knowledge that conventional speakers sound better when placed away from the wall, and indeed my own experimentation seems to confirm this.

While it seems clear that distance from a side wall limits early reflections, I am unsure what accounts for sound differences caused by varying speaker distance from the front wall.

What is the mechanism for this? Is it resistance to air flow near the wall and the resultant air turbulence?

If so, how do in-wall speakers compensate for this? Or do they ignore it?

Just wondering...

 

[Ethan Winer]

Q,

> What is the mechanism for this? <

The mechanism is called Acoustic Interference. When sound waves strike a boundary they reflect back into the oncoming source. This creates a series of many peaks and deep nulls called "comb filtering." The distance from the source to the boundary determines the peak and null frequencies. This graph shows the frequency response measured 20 inches away from a bare sheet rock wall:

how do in-wall speakers compensate for this? <

The beauty of in-wall speakers is they avoid SBIR entirely. But again, major reflections from all around the room still combine to skew the low frequency response, and also create ringing.

--Ethan

 

[Tom351]

Here you can see how comb filtering affects sound pressure levels across a room- (see attached image)
The top image is a single speaker at 2000hz with no wall interactions, for the second image, I activated a rigid wall at the top of the prediction area.