Sean,
There's no "problem" with the theory. Whether foam plugs "work"
or not depends on what you are trying to do.
If one assumes that the speaker designer has done their job
correctly and has optimized the speaker design so that the
acoustical response of the system has the proper shape -
3rd order Butterworth filter, or whatever; then putting foam
in the port is going to mess that up.
However, that's not to say that it might not do what you
want. If you are willing to sacrifice the flatness of the
frequency response; you may well get a lower extension in
your bass. You just have to realize what you are sacrificing
for that lower extension.
I also agree that acoustic suspension is not inherently
better than a ported system. It is all in the quality and
execution of the design. Ported systems are more complex
than acoustic suspension - thus it is easier to muck up a
ported design than an acoustic suspension design. Perhaps
a bunch of poorly executed ported designs is what has given
ported designs a bad name.
In fact, a properly executed ported design can be advantageous.
With a sealed box, the designer has only the volume of the
box, in addition to the driver parameters; as a design
variable. In a ported system, there are additional design
parameters related to the port that give the designer
additional "degrees of freedom". A skillful designer can
put this additional design freedom to good use in improving
the design.
If one chooses a passive radiator design - which is a
special case of ported design - one can do even better.
That's because the compliance [ "spring constant" ] of the
radiator is an additional design variable - so the designer
has one more degree of freedom, than an open port.
Thus what could have been a 3rd order Butterworth can now
be a flatter 4th order Butterworth...
The classic speaker design papers by Thiele and Small
point this out.
Dr. Gregory Greenman
Physicist
