Permanently sealing a vented subwoofer??

Chadnliz,

READ my post again!!! I choose my words very carefully.

I said a PROPERLY designed ported speaker is more difficult
to design than an acoustic suspension.

There are lots of speakers that are ported that are not
designed to properly represent the transfer function of a
high pass filter.

It's not a matter of opinion - the ported system has more
design parameters than an acoustic suspension. Both have
the driver parameters in common. Add to that the volume
of the box - and you have the parameters for the
acoustic suspension. The ported design adds box volume,
plus a number of parameters related to the port - size of
port, acoustic impedance...

Don't tell me to re-think until you re-read!!!

Dr. Gregory Greenman
Physicist

Matchstikman,

I agree with Barry Kohan of Bright Star Audio; if you want a
sealed enclosure - the drivers have to be designed with the
sealed enclosure in mind. Barry is correct that the parameters
of the drivers in a vented enclosure will be significantly
different from those of a sealed system.

Sealed systems are also known as "acoustic suspension" systems.
Some of the "springy-ness" of the driver comes from the air
that is trapped in the box. In a vented system, you don't
have that; so the "springy-ness" of the surround that
supports the cone is made to provide that.

If you seal a vented system, you will augment the spring
constant of the surround with that of the trapped air -
resulting in the WRONG spring constant for good performance.

If the air in the box has no where to go - then the tube
doesn't make any difference. In the sealed system, it is
the fact that the movement of the cone displaces air, thus
altering the volume of the box, and hence the pressure in
the box. The presence of the tube doesn't affect that.

In the vented system, air can enter and exit the box
through the vent - but must travel the length of the
vent tube before it can affect the bulk pressure in the
box. This timing delay is the purpose of the vent tube,
and that delay is designed into the system to give proper
response. The length of the vent tube was chosen in the
design of the speaker for exactly that reason.

However, given that plugging the port is a bad idea when
you have drivers designed for a vented box - the point is
pretty moot.

Dr. Gregory Greenman
Physicist

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