Sub Question
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Joined: Dec 2008
Posts: 6,928
axiomite
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OP
axiomite
Joined: Dec 2008
Posts: 6,928 |
Given a sub with *more* than one port, how is it that plugging one of the ports will make the sub reach lower? this seems counter intuitive to me. Anyone?
Half of communication is listening. You can't listen with your mouth.
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Re: Sub Question
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Joined: Mar 2008
Posts: 1,378
connoisseur
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connoisseur
Joined: Mar 2008
Posts: 1,378 |
Not a designer of the sub, but... Plugging a port will cause higher pressure inside of the cabinet which should cause greater driver excursion (at the expense of losing clarity of the higher frequencies).
Edited... I take that back. Plugging a port would cause the driver to work harder to "work". It's a good question, I'd like to hear the answer.
Last edited by SRoode; 04/29/10 11:23 PM. Reason: I'm a dumb-ass
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Re: Sub Question
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Joined: Jan 2004
Posts: 13,840 Likes: 13
shareholder in the making
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shareholder in the making
Joined: Jan 2004
Posts: 13,840 Likes: 13 |
I think the SVS cylinders offers this, maybe there website explains the purpose of plugging 1 or more ports.
M80s VP180 4xM22ow 4xM3ic EP600 2xEP350 AnthemAVM60 Outlaw7700 EmoA500 Epson5040UB FluanceRT85
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Re: Sub Question
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Joined: Dec 2008
Posts: 6,928
axiomite
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OP
axiomite
Joined: Dec 2008
Posts: 6,928 |
On Outlaw's website, they show the measurements with both ports open and one port plugged, with the plugged one going noticeably lower. I checked SVS site and Outlaw's site, they didn't really give any info on it other than the numbers.
Half of communication is listening. You can't listen with your mouth.
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Re: Sub Question
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Joined: Apr 2003
Posts: 16,441
shareholder in the making
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shareholder in the making
Joined: Apr 2003
Posts: 16,441 |
Let's do some math! I found the following port-tuning formula on this page. where Lv is the length of your port in inches, R is the inside radius of your vent tube, Fb is the desired tuning frequency of your enclosure in Hertz, Vb is the internal volume of your enclosure in cubic inches. Solving the formula for Fb gives us If the original sub design has two ports, plugging one port will only change one variable, R, effectively halving it. In this formula, you have an R^2 in the numerator and an R in the denominator. Without actually running the numbers, this relationship between the Rs should be enough to show you that halving the value of R will reduce the value of Fb, the desired tuning frequency.
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Re: Sub Question
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Joined: May 2002
Posts: 10,654
shareholder in the making
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shareholder in the making
Joined: May 2002
Posts: 10,654 |
Adrian, a ported sub has a lower tuned frequency than a sealed sub(other factors being equal)but rolls off at a rate of 24dB/octave below the tuned frequency while the sealed sub rolls off at 12dB/octave. Plugging one of two ports has the effect of moving the tuning part of the way toward being a sealed sub. The frequency where the sub begins to roll off is then actually higher, but because the roll-off is slower, the usable low frequency limit is extended. So, more output higher up with both ports open, but more extension with one plugged.
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Enjoy the music, not the equipment.
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Re: Sub Question
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Joined: Dec 2007
Posts: 7,786
axiomite
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axiomite
Joined: Dec 2007
Posts: 7,786 |
And thats exactly what happens. Same driver, same tuning same port lenght. The pink trace is a 1" smaller port.
Fred
------- Blujays1: Spending Fred's money one bottle at a time, no two... Oh crap!
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Re: Sub Question
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Joined: Dec 2008
Posts: 6,928
axiomite
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OP
axiomite
Joined: Dec 2008
Posts: 6,928 |
Thanks for the input, Peter, John and Fred. That's interesting how much you can change the dynamics of the sub(or any ported speaker) by adjusting the port. So it would appear then, to generalize with all other parameters remaining the same...an "aggressively ported" box will have more bass support but drop off more quickly(not as low) and a more conservatively ported box would be a compromise between the two. Would it be safe to deduce that a sealed sub would go lower but on an even more gentle roll-off?
Half of communication is listening. You can't listen with your mouth.
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Re: Sub Question
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Joined: Dec 2007
Posts: 7,786
axiomite
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axiomite
Joined: Dec 2007
Posts: 7,786 |
Well, that depends on the driver. In general, a ported design will go lower than a sealed design with the same driver. However, some drivers will do very very well in a sealed design and with a lot of linear excursion can handle some very agressive eq to boost the low end. It also depends what you believe about the audibility of distortion at low frequencies. Sealed subs tend to show more distortion the lower they go. EQing would further push up distortion. Here is the same driver with two sealed models added. The one with the highest roll off is sealed with no boost. The blue line is with 6db of boost added at 25Hz. You can see that the ported designs outperfom sealed all around. This driver does best in a ported alignment though.
Fred
------- Blujays1: Spending Fred's money one bottle at a time, no two... Oh crap!
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Re: Sub Question
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Joined: Dec 2007
Posts: 7,786
axiomite
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axiomite
Joined: Dec 2007
Posts: 7,786 |
To follow on, a ported design is more efficient because the driver is not: a) fighting a sealed air mass to achieve its excursion, b) more efficiently coupling with the air in your room. For a given level of power you will get more loudness and/or extension out of a ported design.
On the other hand, the sealed mass acts as a natural damper and you don't have to worry about a rumble filter to keep the driver from unloading at tuning.
Fred
------- Blujays1: Spending Fred's money one bottle at a time, no two... Oh crap!
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