Originally Posted by cap'n
I have also pondered the difference between "removing" and cancelling or not exciting a mode.
Well, the physics won't generally permit you to remove energy from the mode, unless we go to the absorbtion stuff that I still regard as way too complex for the consumer. So, you don't excite it, or you excite it in a way, using phase or delay, etc, that drives it into a different behavior.
But barring adding absorbtion to the room, or using a very complex system, it's not going anywhere unless you actually eliminate the energy storage in the room. That's what modes are all about, energy storage, when they arise from the physical room arrangement.
HOWEVER: Now we have active devices, things like loudspeakers, that can also arrange to put energy into the room, even at frequencies where the room doesn't store annoying amounts.
And, once again, modes are due to the reflections from the walls.
You can simulate reflections at farther (nearer would be cool (but my thiotimeolene doesn't work yet)
) distances by using delays. You can do this in any number of ways, some natural and some artificial. Again, I point to the method of images, as cranky and complex as it is, for more evidence that this is a valid interpretation.
There is one wart: The difference in the radiation pattern as reflected from the wall vs. emitted from a woofer.
I submit that at bass frequencies, the difference between the wall (planar reflection) and the speaker (simulated reflection from point source) is moot.
So, barring useless semantic dickering, you can add modes. No, they certainly aren't originally present.
Now, if you all wish to argue otherwise, perhaps we should start a new thread on linear algebra?
N.B. "nodes" are not part of this particular discussion, should you read "node" it's evidence that jj's finger did not move quite far enough to the right while typing "mode".