Why permanent? Why not plug the vent? If it does not work out well, remove the plug. That is what the Hsu woofer has, i.e. foam to plug and unplug the port. If it works well, you can seal it permanently, if you still insists. Always try to have a possible recourse for unknown or unchartered territory. Once the preference is established, you can do whatever you want peermanently. |
Hi Matchstikman,
The physical and electrical parameters of a driver used in a vented system are significantly different than those for a driver used in a sealed system. It is highly unlikely that you will improve your subwoofer by closing the vent. If you do experiemnt make sure you can remove the closure and taht you make no other permanent changes so that you can return the sub to its present state.
Best Regards,
Barry Kohan |
I agree with the above. If you really want to tinker, buy a sealed enclosure raw driver and build an enclosure yourself. That would be the best way to go. |
I thought about just plugging the vent, but doesn't the actual, physical tube inside the subwoofer affect the souhd in some way? Won't completely removing the tube and not just plugging the tube do something? Will plugging the vent or removing the port tube completely provide the same results? |
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. |
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. |
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 bungs? Great name for a band. |
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. |
|
Well designed ported subs (most), and I am talking about true subs here...work as well as sealed IMO.
Ported bookshelf and mini speakers are a different story, most try to do a little to much, with very few well designed examples that I have listened to...thump, thump, thump...chug, chug, chug.
Small room and low volume is the best many can do before the port noise sets in.
Dave |
For any given driver (raw speaker), the box size for a good sealed box is usually biogger than the proper size ported box. When you put a driver in a sealed box that is too small, you get amplification at resonance - in other words a peak in the response. This will make the sub sound boomy. How much will depend on the very specifics of the driver and box. I would say go ahead with a temporary blockage. Maybe a block of wood held over the port with bungee cords around the sub. I agree that sealed boxes are almost always better than ported, so it's a worthwhile experiment. Also, make sure you get a good seal. Any leaks will become small ports with there own small wavelength resonances, which will sound like whistlilng. Kind of like the sound a pigeon makes when it flies if you are close to it. |
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 |
I certainly dont agree that a sealed accustic suspension is easier to pull off the a port, if so then why is every damn speaker at Best Buy and Wall-Mart, Sears and so on ported? re-think it man! |
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 |
>I certainly dont agree that a sealed accustic suspension is easier to pull off the a port, if so then why is every damn speaker at Best Buy and Wall-Mart, Sears and so on ported? re-think it man!
A ported speaker gets you lower bass without using a larger (more expensive to build, ship, and stock) enclosure (that is also less domestically acceptable and limits the market) or increased (more expensive) power. It gives you the low bass output you want without requiring the sealed sub woofer's higher (more expensive) excursion.
Band-pass and ported designs are ideal when the primary design requirements are cheap and small. |
|
Drew, please contact the engineering departments at B&W, Avalon, Monitor Audio, Proac, Spendor, Eggleston, Wilson, Sonus Faber, VMPS, Harbeth, et. al. and let them know about the problems with their "cheap" and "small" designs. |
A properly designed sealed enclosure or acoustic suspension subwoofer will exhibit superior transient capabilities or known as faster group delay response. There are always trade offs and sealed enclosure type subwoofer requires more robust drivers and a whole lot more amplification power. Read latest Absolute Sound article on subwoofer and review on JL Audio Fathom F113 subs and also do google search on Adire Group Delay. For music systems, I like sealed subwoofer designs. For non critical low frequency effect of home theater one note type sub sound there are pleny of ported or passive radiator subs available - Velodyne, Earthquake, Hsu, Sunfire, to name a few. But if you want quality sealed enclosure type subwoofer that can keep up with transient capabilities of your main speakers check out subs like JM Labs Sub Utopia, Focal BE 1000 sub, JL Audio Fathom F113, Zu Method, Martin Logan Descent for starters. |
