Permanently sealing a vented subwoofer??

I have used foam bungs to mitigate boomy bass where there was no option for repositioning or replacing speakers. It doesn't always have the desired effect, but it often improves boomy / slow bass, it costs next to nothing, and it's instantly undone if it doesn't work out.

I would imagine that the effects of the port tube will be minimal if it is bunged down a significant proportion of its length.

If you restrict airflow through the vent, but don't block it completely you will have a configuration that is sometimes good. The well regarded old Dynaco speakers were designed this way.

If you want to experiment, try filling the vent with soda straws, and see what happens.

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

Nice theoretical desctription. It is pretty obvious that the acoustic suspension of a vented box is different to that of a sealed box, and that this will affect the mechanical action of the drivers.

The only problem with the theory is that foam plugs can actually work to give the desired effect. I grant you the solution is not optimum, but if buying new speakers, or changing their room placement is not an option then plugging reflex ports can get the job done adequately.

Foam in the vent will not stop airflow, but merely change the velocity and volume of the air movement.

The original premise that sealed is superior to vented is suspect. If properly designed either type will work. It's a question of trading off different sonic and/or practical compromises.