Changing port dimensions?

[Creelman]

Greetings all, this is my first time posting here on your fourms, seems that you have quite a nice establishment here! I tried searching for a topic on changing port sizes but alas, i couldn't seem to find what i was looking for.

I was wondering if it is opssible to change the tune of an enclosure by modifying the entrance of the port to a smaller size? I made a labyrinth type slot port, and realized i made it an inch higher than i wanted throughout. If i put a piece of MDF in the size i need to "cut off" the extra port space, would this essentially make the tune as if it was 2" the whole way?

i know its usually never this simple, but i figured id try! conciseness

 

[Creelman]

I suppose it would also be feasible to put another piece of 1 inch thick MDF on the bottom slot, making it 2 inches, however there is another slot above it, that i do not have access too (though i suppose i could if opened it up at the back...), i guess it is an option to just lay another piece of MDF on both layers of the slot port, making it a 2"x16"x48", exactly what i need. I was just hoping that if i placed a smaller "trap" infront of the port that it would solve my problems.

Would this just be best?

 

[TLS Guy]

I suppose it would also be feasible to put another piece of 1 inch thick MDF on the bottom slot, making it 2 inches, however there is another slot above it, that i do not have access too (though i suppose i could if opened it up at the back...), i guess it is an option to just lay another piece of MDF on both layers of the slot port, making it a 2"x16"x48", exactly what i need. I was just hoping that if i placed a smaller "trap" infront of the port that it would solve my problems.

Would this just be best?

No, it will not solve your problem. The cabinet is tuned by the mass of air in the port and the port resistance. All you will do by closing the end is create turbulence. You need to revise the whole port cross section through out its length.

 

[Creelman]

Ok, so i understand i cant cut off access to the port and make a smaller starting point on the port, but can i add layers of MDF, gluing them ontop of what i already have? that would bring down the area of the port, and i would still have tonnes of room in terms of port chuffing (around 12ms max with modded 2 inch slot port).

If that holds true, would it be possible to just add to the bottom layer, as it is 1\2 of the area of the port in itself?. Say, if i modded just the bottom to be 1.5 inches, while the port above would be 3". Would this just make an average of 2.25" port, but have the vent velocity of a 1.5" port? I emagine it like a giant double flared slot port, bringing down the tuning freq and bring up the vent velocity.

If that doesnt work, would it just add more turbulence coming out the port? I dont want to take my box apart, i clamped and glued and screwed that thing tight.

 

[TLS Guy]

Ok, so i understand i cant cut off access to the port and make a smaller starting point on the port, but can i add layers of MDF, gluing them ontop of what i already have? that would bring down the area of the port, and i would still have tonnes of room in terms of port chuffing (around 12ms max with modded 2 inch slot port).

If that holds true, would it be possible to just add to the bottom layer, as it is 1\2 of the area of the port in itself?. Say, if i modded just the bottom to be 1.5 inches, while the port above would be 3". Would this just make an average of 2.25" port, but have the vent velocity of a 1.5" port? I emagine it like a giant double flared slot port, bringing down the tuning freq and bring up the vent velocity.

If that doesnt work, would it just add more turbulence coming out the port? I dont want to take my box apart, i clamped and glued and screwed that thing tight.

Your port needs to have uniform cross sectional area through out, except at the flares, at entrance and exit. You will need to make the port smaller through out its length, not just part of it.

Can you close the port and place tube vents?

 

[Creelman]

Not really, i've tried to draw a diagram of what the layout of the subwoofer is.

I need to change the 3" parts to 2" for best results. I do have access through the back, and it might be possible to take off that last 6 inch piece of the port, and be able to put a bunch of small pieces of mdf in through it to fill up the top layer , but thats about all i can think of.

It really sucks, i wish i could just throw some foam in there or something to bring the tuning freq down.

hmm, i see i cant post attachments. Hopefully this will work need to spam some posts

 

[Creelman]

spam ``````````````````````````

 

[Creelman]

alright here goes

 

[TLS Guy]

alright here goes

Where did you get the plans for that enclosure? That is NOT a ported reflex enclosure, and you can not use standard modeling programs to model it.

It seems it is a labyrinth enclosure, which is a form of TL. To design a labyrinth, you need to calculate Fp, Lp and Vp. The pipe needs about a 3 to 1 taper. The widest part of the pipe will be near the speaker, and gradually taper at each turn so that the outlet has about one third the cross sectional area of the beginning.

The line then needs carefully stuffing with Polyfill, through out its length, so there is one impedance peak, and not two as in the reflex ported enclosure.

The line does not look big enough, or long enough to be a sub as a true labyrinth design.

If you tell me what driver you have, then I will calculate your Fp, Lp and Vp. It will take a while as you have to do the calculations with long hand math.

From the look of it, your enclosure does not look right at all.

 

[Creelman]

Whoa, guess im in over my head. I was told that if id id something like that i would treat it similar to a slot port, but i would have to average the corners, it is similar to a build that i saw in here.

I was following both the the sticky thread and aversi (?) when he started to make his enclosure for the 12.1 mid, but instead of running the port along the bottom and back, i just folded it on the bottom (like aversi did?).

Dont tell me i completely messed everything up. Using the 12.1 VQ in low mode.

Im sorry to have to make you break out a pencil and paper to help me out, i thought i would calculate it like the a normal slot port!

 

[TLS Guy]

Whoa, guess im in over my head. I was told that if id id something like that i would treat it similar to a slot port, but i would have to average the corners, it is similar to a build that i saw in here.

I was following both the the sticky thread and aversi (?) when he started to make his enclosure for the 12.1 mid, but instead of running the port along the bottom and back, i just folded it on the bottom (like aversi did?).

Dont tell me i completely messed everything up. Using the 12.1 VQ in low mode.

Im sorry to have to make you break out a pencil and paper to help me out, i thought i would calculate it like the a normal slot port!

You seem to have an error in your port calculations. If the cross section is 2 X 16 inches, then the length needs to be 60 inches. For that driver, with the Low Q insert the optimal enclosure volume Vb is 3.024 cubic feet. To allow for driver volume bracing etc, the final volume Vt would be 3.3 cubic feet. Now if you can take the back of your enclosure, it seems to me the best thing to do is to narrow the port to a width of 8.75 inches. This will optimally tune your box, if you have made it the correct volume. Your vent air velocity will be 13m/sec.

That seems your best bet. If you want to make the vent narrower, then it need to be 1.5 inches high, not 2 inches.

A true labyrinth would require two drivers with mid Q insert. The total volume of the pipe Vp would need to be 6.5 cubic feet, with a line length of 11 3/4 feet. There would need to be a 3:1 reverse taper, with the closed end having the highest cross sectional area. The point where the two driver chassis abut should be three to four feet from the closed end. The polyfill packing density should be around 0.8 lb/cu.ft.

That would give an F3 of 25 Hz. However the system is non resonant and rolls off 12 db/octave rather than 24db per octave. The cones would be supported up 75 Hz by the port rather than the narrow range of the port of a reflex enclosure.