Single 12" ported box for an MK7 Golf GTI

[3vil]

You really shouldn't think of the relationship in just terms of box volume and port size. The power you'll running is a big factor in optimizing port size, and that's where the X cubic ft of volume = Y sq inches of port rule fails.

[Triticum Agricolam]

Thanks for the response!

.. I think it 19sq in of port per foot is a bit extreme for my application....

19 sq per foot may sound extreme, but you are going to have a bit of an extreme situation due to the relationship between how much power you are running and the size of the box. Even with nearly 40 sq in of port area your port velocities will still be peaking to nearly 32 m/sec. At that point you will already be losing a fair amount of output to port compression, once port velocities gets over 33 m/sec or so it REALLY starts to hurt output, which is what will happen quite quickly with any less port area. It's your box and your money, so do what you think is best, it doesn't matter to me. For your own benefit, please keep in mind that there really is no disadvantage to having port area error on the large side, where as having not enough port area has significant consequences.

[Kinser]

The power to box volume ratio is something that I have never taken to mind when designing an enclosure.

I do try follow general rules such as the 12-16sq or port per cube, more box volume for less power and vice versa, but never took into account watts per cubic feet.

I will be taking this to mind when I design the next enclosure.

For more SPL oriented builds, I'll aim for much more port area.

Since this setup is daily and I'll not be competing, I think I am willing to sacrifice output for bandwidth and design limitations.

My plan is as follows:

Building a test box that is 14.1" high, 14.1" deep and 27.5" long. Port will be 4" aero firing to the side. Sub will be mounted up/back (a matter of flipping the box).

I'll measure output and response using SPL-LAB meter and an RTA in both positions.

Hopefully it will sound better firing to the back - this will allow me to build a deeper box, mount a 6" aero port on the side and maybe even mount the subwoofer inverted to gain a little more volume and plenty of room for the port inside the box.

[Triticum Agricolam]

Smaller ports aren't a good way increase bandwidth, that's a common misconception.

Since you have access to a SPL meter you can measure how much output you are losing to port compression, which you might find interesting. To calculate how much output is being lost you need to take four measurements. First measure your SPL right at your tuning frequency at fairly low volume, low enough that you don't get any port noise. Write that number down. Then measure and record the result at one octave above that leaving your volume level the same. For example lets say you get 112 dB at 32 hz and 110 dB at 62 hz. Now repeat those measurements at max volume. This time let's say you got 134 dB at 32 Hz and 138 dB at 64 Hz. Your 32 Hz output increased by 22 dB (134 - 112 = 22) and your 64 Hz output increased by 28 dB (138 - 110 = 28). The difference in how much output increased at those two frequencies is how much output you lost to port compression, so in this case its 6 dB (28 - 22 = 6).

[DLHgn]

Smaller ports aren't a good way increase bandwidth, that's a common misconception.

Since you have access to a SPL meter you can measure how much output you are losing to port compression, which you might find interesting. To calculate how much output is being lost you need to take four measurements. First measure your SPL right at your tuning frequency at fairly low volume, low enough that you don't get any port noise. Write that number down. Then measure and record the result at one octave above that leaving your volume level the same. For example lets say you get 112 dB at 32 hz and 110 dB at 62 hz. Now repeat those measurements at max volume. This time let's say you got 134 dB at 32 Hz and 138 dB at 64 Hz. Your 32 Hz output increased by 22 dB (134 - 112 = 22) and your 64 Hz output increased by 28 dB (138 - 110 = 28). The difference in how much output increased at those two frequencies is how much output you lost to port compression, so in this case its 6 dB (28 - 22 = 6).

I will have to try this out sometime. I'm afraid that my port is too small for the power i'm running to my sub. I can hear port noise at certain volume levels and pitches. I'm thinking of either rebuilding with same specs (obviously larger port area) or using a different type of enclosure. We'll see. I'll try this method and see what's up.

[Kinser]

Smaller ports aren't a good way increase bandwidth, that's a common misconception.

Since you have access to a SPL meter you can measure how much output you are losing to port compression, which you might find interesting. To calculate how much output is being lost you need to take four measurements. First measure your SPL right at your tuning frequency at fairly low volume, low enough that you don't get any port noise. Write that number down. Then measure and record the result at one octave above that leaving your volume level the same. For example lets say you get 112 dB at 32 hz and 110 dB at 62 hz. Now repeat those measurements at max volume. This time let's say you got 134 dB at 32 Hz and 138 dB at 64 Hz. Your 32 Hz output increased by 22 dB (134 - 112 = 22) and your 64 Hz output increased by 28 dB (138 - 110 = 28). The difference in how much output increased at those two frequencies is how much output you lost to port compression, so in this case its 6 dB (28 - 22 = 6).

Wow, thanks for the advice! I'll try that out.

As to my comment regarding less output for more bandwidth, I was referring to the fact that with a smaller port I will be able to play lower but not as loud whereas with a large port my tuning will be fairly high and I'll not be able to reach the low 30's thus, limiting my bandwidth.

[3vil]

If you're more interested in a flatruck response instead of major output, why not just run it sealed?

[Kinser]

I wouldn't say I want a flat response. I want extension to the low end and a sealed box will not accomplish that, not with the output I'm after.

I'm looking towards a box that will peak around 35hz with the ability to play from the upper 20's to the upper 50's