Help with port size 2 x JBL-GT5 15"s

[sxc_sound]

Hey guys, iv got 2 x JBL-GT5 15"s, i used to have a 4cb ft partitioned sealed box, which i pulled a 137.8db i then ported the box stupidly with 2 x 4.25" diameter ports and lost 11db lol....




ANYWAY.. I have a new box made, built slightly under spec for these ported as i couldnt fit anything bigger in my boot. The car is a Holden VT commodore sedan.

So the box is made, and i have the subs, all i need is help on what size port.

If anyone is willing to help me i have all the internal measurements of the box and the specs of the subs etc. I am leaning towards slot port.

The box is around 6.5Cb foot and is a single chamber.

HERE IS THE DIMENTIONS OF THE BOX, IT IS APROX 6.5 cubic foot

http://i396.photobucket.com/albums/p...dimentions.jpg

and here is the specs of the subs

http://www.jbl.com/resources/Brands/...Tech051908.pdf

Thanks in advance

[HCVP]

You need to tune your port to a specific frequency.If you after maximuim SPL, then the tuning on the port would be far too high for day to day listening.

Personally i would move the subs closer inwards, and i would use 2 ports, rather than one.I'd also use a round port, as it's much easy to alter the tuning if your not happy with it.I'd go with a 4 inch port on either side,and experiment with differnt lengths until you are happy.Flare you ports if you can, and if possible use a bend in each port, so that there is plenty of space between each port, and also each driver.

[holdenboy]

Hmm, are you after maximum SPL or for music mainly?

I honestly dont see how you lost 11db by adding two 4.25" ports to a 4 cu.ft. box for those subs. More than likely the peak frequency would have changed, were you playing music or a test tone when you got it metered?

Im with HCVP on the port/s, round ones are easier to install and tune, but are a little expensive (for the pvc pipe). In saying that a round port will be louder than a slot, if both are tuned properly and the correct size (port area). Its also going to be harder to slot port it as the box is wedged shaped, not impossible but a little more difficult.

If youre wanting it for SPL, 2 x 6" ports is ideal, the best way to tune it is to make them as long as possible (leaving atleast 4" of room at the end of the pipe and box wall), then get on the SPL meter, find your peak frequency then cut the ports down, 1" at a time untill the score drops. Then you'll know the best length to use.

If its more for music, probably go with 1 x 6" port or 2 x 4" / 4.25" ports, once again as long as possible and then do the same as above to get your loudest peak frequency. I can almost guarantee this option will give you a lower tuning freq. which is good for music, but sacrifices some SPL.

Definitely flare the port ends too, this gives more output (higher SPL) and less port noise (chuffing). I think i tested this on a Termlab meter and gained over 2db by flaring the port!
Heres a good tutorial on how to flare the pvc pipe, dosent require much skill / experience. Have a good look through the other tutorials and articles on there too, its a site my good mate created and he's partly responsible for me getting 150db+ in my car! (if anyone frequents MEA its Steve AKA BlackIce )

Build Your Own Aeroport's (Port Flares)

[HCVP]

Holdenboy gets a woody when you talk to him about ports!!

[sxc_sound]

Yeah im only wanting SPL, i dont care how laggy it is or how shit it sounds. The aim is 140db minimum.

I had 4.25" x 6" ports (2) in the 4cb foot box, which was partitioned, so 2cb foot per port, and it went from 139db to i couldnt even make the term lab recognize the stereo (meaning it was under 120db) i was playing all difffernt tones and i got it to a peak of 132.9db, i was using test tones the whole time.


So as you can see im a bit hesitant to go with round ports with these subs again. iv been told, and worked out myself that 6" wide by 12" tall by 13" deep in a slot port would tune the box at 40Hz which would sit right in the middle of the box facing towards the tail lights.


So with the box now 6.5cb foot, take away the volume of the subs, which is 7.11 litres. so making the total internal volume of the sub box 6cb foot. the above slot port should work... im hoping lol


How do you think i would go with a slot port in the wedge shape? maybe just make the port slightly wider so it doesnt need the height or depth? or doesnt it work like that lol

[holdenboy]

LOL @ HCVP

Round ports are almost always louder than slot ports. With your box having angle in it, that means the port area decreases towards the back of the box. Kinda hard to tune on paper but in saying that tuning on paper is a "stab in the dark" approach to it, youre much better off tuning with a termlab like i described above. An easy way to find your peak frequency is to play a sweep, the termlab will have a bar graph at the bottom of the screen, showing which frequency is the loudest. From then on just play that frequency. In my experience ive found you dont change the in-car peak frequency by changing for depths, the only thing that varies is the score. So once you find that peak then chop the ports down 1" at a time, leave a good 5 min in between tests to let the battery recharge (leave the engine running or hook a charger up to the battery). Then retest, make sure to write down the results too, its easy to forget the last score

Those 4.25" ports you used should have been WAY deeper (more like 12-14", maybe longer), i believe thats the main reason why youre score dropped so much.

For SPL, go with 2 x 6" ports like i described in my first post. How much power are you running into them, what sized power cabling and battery?

If its half decent, 140db should be no problem at all......even on music

[sxc_sound]

Wiring - 1/0 gauge from battery to boot, split int 4ga which is only around 50cm long. 10" 1/0 gauge earth which again is split into 4 ga about 50cm long, and upgraded "big 3" with 1/0 gauge cable

Battery - Optima yellow top D34

Power - OA1502 (1500wrms @ 1 ohm mono block, im running it at 2 Ohm 950wrms


OK well i think il go with the 2 x 6" ports, Should i be fine to put them in the centre of the box on top of each other? if you get what i mean.

[holdenboy]

Yep that wiring sounds fine. So the JBLs arent 4 ohm DVC? pity.

I would definitely go with 2 x 6" ports. Much easier to tune (cut / lengthen) and louder. Sounds sweet to mount them in between the subs.

Theres a few things you can do to improve your score, not sure if youve done them or not with old setup but here you go:
* Play around with the loading distance (the distance between the box and the tail lights), somewhere around the 20-30cm mark will be ideal, might be closer though. So just start testing with as much distance as possible, then move the box 2cm back, retest, and so on.
* Dont play the system before a comp or testing, give it ample time to cool the amp and subs. Cool gear = higher SPL.
* If you havent got 6x9s fitted in the parcel shelf remove the metal sections covering the holes, this allows for more bass to get into the cabin.

[sxc_sound]

The jbl's are single 4 ohm

yeah i had 6x9's in the shelf but removed them and put 6"s in the rear doors. my old box used to port up through those speaker holes lol.


The cooling etc iv been doing, but only for around 5 mins between runs, should probly leave it a bit longer.

Bass loading i haven't really tried, will have to give it a go and see what happens, Il be sure to post up my findings/results when i know

I came 3rd at MEA event 1 and 2nd at MEA event 2, see if i can get a 1st :P

[holdenboy]

Ah, you had it ported through the parcel shelf.....good in theory but in the end the old original style works better, as a general rule of thumb its best to keep the port/s on the same side as the subs. Ports kind of act like additional subs, so it helps to keep them firing in the same direction.

Good luck

[sxc_sound]

Ive now chucked in 2 x 6" diameter ports with the flared end as per the link you posted and they are both 32.5cm long (13") and it is REALLY and this will sound retarted but REALLY punchy and perfect on beat. Here i was thinking it should be reallly laggy and sound like crap with music but have a higher spl.

and it isnt as loud as i was thinking. So would this be the port to long or too short? i worked out 11.77" long would be tuned at 40Hz.

[sxc_sound]

6 inch diameter
6 cubic foot box

11.48" in length each = 40Hz exactly (not the 11.77" i mentioned earlier)

currently they are 12.79" long. so il start by cutting the port shorter, will see how i go after that

[holdenboy]

Yeah it sounds like theyre too long. But i do stress not to cut them down until you get on a termlab and test. Otherwise you might find that the 13" depth is the best
Generally, 8-10" is best for SPL.

SPL is so hard to pick by ear, what sounds quiet can be very loud, on the other hand what sounds loud can be very quiet. My car peaked at 51hz and did 150db, but if you were to sit in the car during that 150db tone you'd swear that "joe blow's" car sounds heaps louder on a 45hz tone, which in fact only does 140db. Hope that makes sense

[sxc_sound]

Yeah i wont be cutting anything untill i get it tested. My local Stathfield just got Termlab in today which is very handy, so il head around there tomorrow and get it tested before and after i cut the ports

[holdenboy]

Did you flare the inside end of the port too? I hope not, as you'll have to cut this off . Ideally you flare just the outer part firstly until you find your depth. Then make up new ones with 2-3cm added to take into account the flare and the fact that it dosent add to the port length (well, only a fraction of it does, i think 1/3rd of the flare does).

Just run a sweep at near full volume (90%) and find your peak frequency. Then charge for 5 minutes and just play that tone at full volume, note the score and charge for another 5 min. During this time take the ports out and cut 2cm off each one. Put them back in the box and play that same frequency at the same volume. Make sure everything else stays the same (like car on / off, box in the same position etc) as that can give false results. If the score goes up, cut another 2cm off and so on. Once youre score drops, leave it there. Youre optimum port length is the current length pluc 2cm. So if you like you can make new ones up to that length.
Once this is done and youve got time, test the loading distance in between the box and the tailgate. Push it as far back (towards the seats) as it goes and do the same testing procedure, just move the box forward 2cm at a time and record the results. You should find a sweet spot here too
I'll be surprised if you dont crack 142-145db after youve found the right port length and loading distance. Just remember to keep everything the same for each run, something small like having the a/c on before a run can affect it.

[sxc_sound]

nah i didnt flare the inside end, only the outside end .

Il definitely work out the port length and loading position, just got a hold of a mate from strathfield and he told me the termlab didn't get setup yet il be patient though, i have a good feeling about this setup

[sxc_sound]

got the stereo db tested yesterday with the ports at the length of 12.78" and scored 130DB

Then i cut the ports to a length of 180mm scoring a 133db

(NOTE - This is with the gain, bass boost etc all OFF)

So i got the amp tuned so its putting out 24.4V to the speaker outputs, being the recommended for 300wrms 15"s. Now a stupid amount louder. So will be getting it tested again this weekend, going all out aiming for 140db this time