• Hello Guest, welcome to the initial stages of our new platform!
    You can find some additional information about where we are in the process of migrating the board and setting up our new software here

    Thank you for being a part of our community!

Porting a Sub Box

Leivon

Member
Joined
Aug 20, 2003
Location
Ladner BC, Canada
Ok... i got a 15 inch sub and i am unhappy with it, i think i need to port my box.. why cus a couple of my friends suggested it, now i want to ask anyone who knows... how the hell do i port a box??? is it a matter of cutting a hole and sticking a pipe in there or something... please explain this...
i think DEM will be able to help me the most, but if anyone else knows whats up then please inform me...

Thanks. Leivon
 
Levion, the task isn't so simple because you need to calculate the hole size and port tube length to throw the acoustic wavelengths that you want to hear through the speaker. If the cone your using is a rolled edge woofer that is intended to throw the low frequency wavelength (meaning create the acoustic wave in front of the box), then you can't take advantage of porting the box. The cone for a ported speaker is rear firing, meaning the acoustic wave is created inside the box and the wave exits through the tuned port.
Determine which wavelength you want to exit the box then measure the depth, multiply by 2X (it reflects off the back of the box) and make the port tube length 1/4 of that distance. Optimum path length is a quarter wave (where you get harmonic occillation of the first order) and the diameter should be chosen to be a diameter that is divisible by the quarter wavelength.
Lower frequencies (longer wavelenghts) need a longer path, either a bigger box or more reflections inside the box. The diameter of the port governs the cut-on frequency of the long wavelenghts. A lot of passive low frequency woofers actually create the 1/4 wave occillation by a rear facing driver that occilates in unison with the front active driver. This design doesn't need a long path. Path length can be shortened by going to 1/8 wave, 1/16 etc, but this limits the efficiency that the wave amplitude has when exiting the box.
Take some measurements of the box and compare to the active driver's frequency response. If the box dimensions are not a 1/2 wave dimension (depth) of the average frequency (or lowest freq, if that's what you want) of the woofer, you will loose the low frequencies of the driver due to internal acoustic reflections and interference. You may be better off getting a box of the right dimensions than porting the box you have.
Math:
Wavelenght = speed of sound/frequency
Speed of sound = 344 meters/sec
for a 60 hz signal, the wavelenght is 5.46 meters (about 17 ft)
1/4 wave is 1.36 meters or about 52 inches.
 
man thats some einstein calculations. port lenght has alot to do with the range of the sub. your best bet is to visit the website of your speaker and search for specifications of an enclosure. they will have vented, sealed, bandpass, and sometimes isobaric specs. even the proper port lengths. from the pics of your car your box is humungus for a sealed enclosure. porting it will help.
get back to me with measurements of your box and the type of speaker and i will tell you what i think.
 
Well, the important info that is missing is whether the speaker was made to be a an acoustic suspenssion forward firing speaker, or free resonating bass reflex, rearward firing speaker.
If it is an acoustic suspension speaker, a closed, dampened and sealed box of the right dimensions should be used.
If it is a bass reflex speaker, a ported box with the right port tube length and diameter will get you the bass response. Note, a bass reflex speaker that is in a sealed box will not emmit much low frequencies.
Keep in mind the numbers of speaker resonance (23 hz) and bandwidth (40 hz). These frequencies should be used to calculate the port length and diameter. This speaker you have goes to 250hz, but with a ebp of 40, anything over 65 hz will be produced below the efficiency of the speaker design, and should be filtered out of the incoming signal by a low pass crossover. If you have such a crossover on your amp, set it at 60 hz and listen for improvement. This speaker should give you some thundering bass with about 50w rms @ 40hz, which is a lot of power in a short bandwidth.
 
i'm pretty sure its a free resonance speaker, cause the flat piston style are high end. his is only 400watt max
not much info on that speaker. the website lists entry level 15's with the same power handling as yours with ported enclosures @ 2.25 cubic feet. no port length though. you can buy ports from a stereo shop 3in or 4in. i would use the 3 in. they are all the same length though (6in i think) you can also use abs or pvc plumbing
pipe whitch you can make to any length. i agree with peter on the freq don't set your amp below 60hz on the crossover. when u put the port in, put it on the same surface as the sub. make it at least 3 in away from any wall of the box. for the length i would email rockford, but if you don't want to wait put a 3in port by 8in long and use your ears. if its not hitting right keep shortening it untul u like it then glue it in place.
 
ok cool thats what i shall do. im thiking on getting some Pvc Pipe so the 3 inches that is circumfrence right, not diameter... i measured my box and it has 2.4 cubic feet
thanks for all your guys help
Leivon
 
thanks cool, i got 3 pieces of pvc cut, i dont knwo why we had some, and they are ready to go in one is 6 inches one is 7 and one is 8. the hole i cut should be on the same side as my sub you say, yet 3 inches off of the wall... my box is only 7 inches deep on the side that the sub is on...

thanks, Leivon
 
i dont actaully plan on using all three pieces it will jsut be to test which one is best, you said 3 inches in diameter, thats quite a bit bigger than the pvc i had, maybe two ports? or will that work?
 
its too small. give it a try since you dont have specs.
i would try the short one. if its no good you can always make the hole larger for the 3 in.
 
did it i used the shortest one which is 6 inches long... it sounds better, deeper. i think i will try putting the 3 inch pipe in it for sure tho, but i will give it a couple days to test it out and see.
Leivon
 
Correct me if I'm wrong but. Ported box = lowder Sealed box= better sound quality. I personally am not a huge fan of ported boxes. All the ported boxes I've heard haven't impressed me. Even with 4 12's in a custom built box. The subs are JL w-0's. I liked my sealed box with my Infinity Kappa Perfect 12.1

Jack
 
Don't use PVC. It's going to have resonance that can cause unwanted reflections inside the box. Note that any plastic elements have un-natural reflections because ther're not a dampening material (sound absorbing).
Go to the Container Store and get a thick walled cardboard tube, and 3"dia x 6 inches is a good start. They are cheap and you can experiment easily cut from a 3 foot tube. When you put it in, glue it in very firmly, Duco cement. The energy of bass frequencies will rattle anything out of the hole and get unwanted vibrations.
 
Oh btw, looking at your box, the depth is too small to reproduce long wavelength frequencies. Looks to be about 12 inches, would be better if it was 14 inches or more, which is the harmonic resonant path lenght for a 60 hz signal traveleing 1/8 wave, front-to-back, then out your port. Any lower frequency response will need a deeper box.
Note that length and width are pretty much not important (to a point) in a bass reflex design, since the wave is generated in reflection and out the tube. Path length is the more important. In a sealed box it is very important.
I would say the optimum size for that 15 inch woofer would be 15" deep, about 42" long so the port can be placed 1/16 wave away from the side of the speaker cone and equal distance from the side of the box, and 18 wide, or just wide enough to fit the speaker. You would want the speaker all the way to one side of the box, and the port 1/2 way from the speaker edge to the other side of the box.
 
Back
Top