Porting a currently sealed box

[wb131988]

Hi,
I have recently come across the idea of making my enclosure into being ported, and provided Sandness in the 'What is Clipping?' topic with information on the dimensions of my box in respect to the diagram provided there. So then, how do I proceed? Thankz

[Google]

[sandness]

Here are your box dimensions from the other thread.

D1- 11 inches
D2- 15.5 inches
H1- 9 inches
H2- 14 inches
Width = 37.5 inches

It is possible to port you existing box, but performance increase won't be the same as if you were to able to build a new, bigger ported box. Anyway, this is what I came up with: Your net volume will be 1.8 cubes, tuned to 31Hz via 2 4" diameter 36.5" long ports. In all of the following grahps, the yellow line is your current ported box and the blue line is the said ported box.

This is the simulated response in car. Basically, it is showing roughly 2dB increase over the useable passband. Since my simulation is based on the size of my van, your smaller car should see a bit more increase. As a reference, a 3dB increase is normally found when doubling the power to your speakers (assuming they can handle it).


Here is the troublesome part- the air velocity of the port. When going ported, you want to keep the port's air velocity at or below 5% of the speed of air. This is about 17m/s normally. So, as you can see in the graph below, it stays below this down to about 33Hz. Luckily, most music doesn't dip below the 30's, so you should be alright. If you do listen to something with notes down into the 20's, you'll hear port noise- a chuffing sound. I simulated this with 800w from your amp. If you have more power, then you'll get the noise at higher frequencies too.


Also, assuming 800w, you'll need to turn your subsonic filter up on your amp. That filter is simply a high pass filter that blocks out the lows you can't hear or shouldn't play. So, since your subs have an X-max of nearly 14mm, you can see in the plot that your subsonic filter should be set a about 22Hz or so to prevent the sub from 'unloading' below the tuning frequency of the box. Unloading can damage the speaker as it can mechanically rip itself apart if you play too low of a note without the subsonic filter set.


Finally, you would want to consider the group delay of the sub. Higher delay times lead to muddy bass that lacks the tightness and impact. With the given box, the delay remains pretty low, not much different than your sealed box. It starts to rise below 35Hz or so, but this shouldn't affect the impact of most music.


Now you may be wondering, 'How do I fit 2 36.5" long 4" diameter ports into my box? I'll make a pictoral and get it posted.

[sandness]

There's my crappy drawing. Basically, you need to snake the 4" PVC pipe inside the box using angled pieces. The final length should be about 36.5". This should be measured down the center of the pipe. So, you'd have to measure the center of each angled joint, add that to the length of all the straight pieces to get the desired figure. Keep in mind both ports will need to be the same lenght.

Snaking the port inside the box may be a pain. That's why porting after the fact can also be a pain. Also, smaller boxes equate to longer port requirements. This is another reason building a new, bigger box is often easier- shorter ports to work with.

I'll leave it up to you if you wish to give this a shot. I assume the easiest place to exit the ports would be in the old 6x9 hole between the subs.

Just for consideration, you can also run ports outside of the box. Doing so means that the ports don't take up extra volume inside the box and thus can be shorter. However, most people don't want pipes running outside the box as it looks goofy as hell.

[sandness]

Here's a front view of the ports running in the box in case you were having trouble imagining it. Keep in mind it is merely a idea, not drawn to scale or measured, as I will leave measuring the port up to you.

[wb131988]

OK thanks alot, so these ports will make the lower frequencies down to about 33Hz more deep bass then, right?

[wb131988]

is there any specific distance between the two ports when they come out the box, and will they just be held in place by super glue or summat?

[ToggleHead]

personally wb.... with those Rtype subs.... i would leave them sealed..... i personally like how they sounded sealed a lot better than ported.... and you cant UNDO a port easily.....=]
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[JayDubya]

Definately sealed for the clean tight bass. Plus, if you are gonna port a box, u will want to start from scratch.
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JayDubya aka JW Jay JD ^> ﺵ