Placing the port(s) behind the drivers - diyAudio
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Old 17th June 2003, 12:34 PM   #1
cm961 is offline cm961  Canada
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Default Placing the port(s) behind the drivers

Hi,

I have a new subwoofer box, about 2.7 to 3 cubic feet, which is about optimal for my two 10" JL Audio 10W0 subwoofers. I can fine tune the volume. However, it has two 3 inch ports which are about 11 inches long. This gives a tuning frequency of about 35 Hz, and optimally I'd like about 26 Hz. Anyway, I'm going to have to put a bend in the ports as they are too close to the walls as it is. This will leave the opening of the ports behind the drivers, but with the opening perpendicular. Is this acceptable or will it throw things off?

Pete
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Old 17th June 2003, 02:32 PM   #2
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Pete it will be fine.....did you think about going to a larger port diameter and then you can shorten the port length>???

DIRT®
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Old 17th June 2003, 08:08 PM   #3
cm961 is offline cm961  Canada
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Hey Joe,

I thought a larger diameter meant a longer port? Anyway it has two 3" ports in there already and I don't think there's room for more.

My main concern is that its way too boomy and lacks low bass. According to simulation software (linearteam) two 10W0s in a box this size should have f3 at 26Hz, which I don't think its doing.

I'll try making the ports longer to lower the tuning frequency. I have to bend them though as they are already far too close to the back wall, actually the lower one is pushing against the fiberglass insulation. I'm sure that can't be good, there's definitly more air coming out of the top port versus the bottom, and that can't be good. Also its in a car, so there's got to be a lot of resonances and that can't be good. Also, the amp I'm using is questionable and the head unit is definitly questionable, and that can't be good...

Thanks for the reply!

Pete
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Old 17th June 2003, 08:19 PM   #4
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You can't use a larger diameter, it will raise the tuning frequency. The tuning frequency is determined by the length/diameter ratio of the port. The larger the port, the longer the port must be.
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Old 17th June 2003, 08:54 PM   #5
cm961 is offline cm961  Canada
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That's what I thought, and if I use a smaller port I risk getting port noise. So I think I just need to make them longer using bends and such.

Firstly I want to pull the box out of the car and run a response curve on it. Even if its close to flat, which I'm sure it isn't, I've still got to EQ it to make it flat inside the car.

Pete
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Old 18th June 2003, 01:26 AM   #6
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Just make a prototype port for temporary and let it stick out the front and see if this solves the "Boomy" problem. That way you can fine tune the length with test tones and get it right where is sounds best without too much trouble. Just make it extra long and trim to length (about 1" per Hz).

Then worry about how to fit that length into the box, there is always a way.
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Old 18th June 2003, 01:58 AM   #7
cm961 is offline cm961  Canada
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Thanks King, I'll give that a try. Do I have to worry about getting a good seal around the test tubes while I try them?

Pete
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Old 18th June 2003, 02:46 AM   #8
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Yes they must be sealed very well, however it's very easy to do, just use some rope-caulk or electrical tape. Rap several times around to fatten the port then just press it into the hole firmly to squeeze the tape or gasket or caulk into the gap, you may need to ad a little on the outside to complete the seal, I use aviation f-4 tape which is a thin sheet of rubber on a roll just like tape that will weld its self together when it makes contact with its self. It’s kinda like instant thin rubber gasket.
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Old 18th June 2003, 02:54 AM   #9
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Here is a pic, notice the redish/brown tape (F-4) also notice I have 2 different lines of tape. Only about 12" of the port was inside the enclosure.

I was experimenting by flipping the port around which left either more or less of the port inside the enclosure, just wanted to hear the difference if any. Use test tones to tune the Fb, when the cone barely moves relative to just 1 Hz above or below a cretin frequency then that is the tuned freq.
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File Type: jpg tube seal.jpg (77.8 KB, 161 views)
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Old 18th June 2003, 04:54 AM   #10
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are we talking about kingdaddy`s sub here>??...because then I retract my statements...I think that a larger port with a shorter length will produce the original design which i have no clue... a small variance in port diameter and then adjusting the length in a subwoofer is no big deal...were talking 100Hz and below here
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