Enclosure & Port Selection, Definitive Guide
Should I go ported or sealed?
This gets asked a lot, and here's how you find out! You need to calculate your woofers Efficiency Bandwidth Product (EBP).
Get your subwoofers T/S specs and look up its Qes & Fs, and then perform the following calculation:
EBP = 1/Qes * Fs
Now that you have you EBP, you know what your speaker is suitable for!
If the EBP is 100 or more, it’s best for ported boxes. (And 6th order bandpass)
If the EBP is 50 or less, it’s best for sealed boxes. (And 4th order bandpass)
If the EBP is between 50 & 100, it will work in either style boxes. (And either style bandpass)
What’s Qes? Qes is the speaker's Q at resonance, due to electrical losses.
What’s Fs? Fs is the drivers free air resonance, in Hz. This is the point where driver impedance is highest.
How is tuning achieved?
Tuning is a function of box size, port size & port length.
Bigger box = lower tuning (smaller = higher)
Bigger port area = higher tuning (smaller = lower)
Longer port = lower tuning (shorter = higher)
That’s pretty general, because it’s all relational too. IE. A small box needs longer ports to tune to the same frequency as a large box. You also need to take into account port size in relation to woofer size; as if it’s too small it will make a hissing "choof" noise.
How do I calculate if my vents to small?
Visit this page http://www.carstereo.com/help/Articles.cfm?id=4
They have a handy little vent mach calculator. As they recommend, try to keep the Mach Percentage under 10% to eliminate any port noise. (If you click File -> Save As you can save it to your computer to access it offline)
What frequency should I tune to?
Depends on what you’re doing.
I like to tune enclosures to around 40-45hz for daily driving & playing music LOUD.
For SPL you may want to tune higher, say 50-55hz (depending on your car, get it on a meter & test).
For SQ and bowel loosening lows, tune low, say 30-35hz.
How do I know how big a box to use?
As a general rule of thumb, the following sized woofers work best in these sized enclosures for most situations:
8" = 0.7 to 1.0 cubes
10" = 1.2 to 1.5 cubes
12" = 1.7 to 2.5 cubes
15" = 3.0 to 4.5 cubes
What are Cubes? This is the enclosure volume, measured in Cubic Feet.
What if I have more than one woofer? Multiply ! If you have 2 12"s, use 3.4 to 5 cubes. If you've got three, 5.1 to 7.5, etc.
But I've got, for example, 2 12"s and can only fit 2.5 cubes, does that matter? As I said, those recommendations work in most situations but sometimes you need to adapt.
You can try them in 2.5 cubes, but you might not have enough room to fit a big enough port, or long enough port to tune the way you want! In this case, you should consider using less woofers, or smaller ones.
Putting it all together
Here's an example for a daily driving box I designed to suit a Commodore sedan.
A 40hz tune, in a 3.5 cube box to suit a single 12" can be achieved with:
Single 6" tube, 9" long
Two 4" tubes, 9" long
Single 6" X 6" X 12" (Width x Height x Length) square port
The two circular options give you the approximate same port area (25 sq.inch), hence are the same length.
The box port gives you more port area (36 sq.inch); hence it is also longer to achieve the same tuning as the smaller area circulars.
Hey, isn’t that box a bit big for a single 12”? Again, those are only recommendations. A big car like a Commodore has a pretty low resonant frequency and boot space to burn, so the bigger box brings the tuning down a bit. The increased size of the box also adds to the efficiency of the system, so it'll be a bit louder. Big cars need big bass ;)
Why would you use the larger area box port over the circulars? Lower vent mach for better sound quality, and the larger port helps increase output too.
What’s the difference between a circular & square port, aside from the obvious?
Easier (just cut some PVC)
More efficient air flow (especially if you use Aero flared ends)
Less efficient use of space
Large sizes can be hard to find or expensive
More efficient use of space
Stronger (depending on how you build them)
Fiddly to make
Less efficient air flow (sharp corners etc)
What’s a slot port? A square port which uses a wall of the enclosure as part of port. This makes the enclosure stronger (it acts as a brace) and also makes the port shorter. Check in my enclosure building tutorial for a link to a slot port calculator.
What type of port is better? Neither is "better", as each kind has its pro's and con's. Choose which evers pro's are most applicable to your situation, and cons are negligible. Eg. if space is at a premium and you need a large port, slots are hard to beat. If you're after efficiency and only need a small port, use circulars.