I recently came across this formula: if X is over 100, use a ported box; under 50, use a sealed box; and anything in between, either box works. I get the formula, but could someone explain how it works? Also, what exactly is ‘X’ in this context? I’ve installed subs before and know some basics like RMS, watts, ohms, amperes, etc., but please explain it as if I’m new to car audio. I just learned this and want to understand it more so I can improve my knowledge in car audio.
Let’s say a subwoofer has a resonant frequency (Fs) of 30Hz and an electrical quality factor (Qes) of 0.5.
30 / 0.5 = 60
EBP < 50 - use only for a sealed box
EBP 50 - 100 - can be used in either
EBP > 100 - ported box only
Fs is the free-air resonance of the driver
QTs is the total Q of the driver
Vas is the volume of air equal to driver compliance
Xmax is the amount of voice coil overhang in millimeters
EBP (efficiency bandwidth product) helps you figure out what box works best for your driver.
Three other things to think about: reference efficiency, displacement-limited acoustic power output, and electrical input power required to produce that output.
Reference efficiency shows how well the driver can produce SPL. It’s mainly useful for mids and highs.
Displacement-limited acoustic power output is how much output the driver can give without distortion.
Electrical input power shows the watts needed to produce that output.
When building a box, it’s better to go a little larger than expected to make up for anything that could affect the volume.
Always take resistance into account when doing calculations. For example, if your box is 4 ohms and you plan to run it in parallel (2 ohms) or series (8 ohms), don’t just assume 4 ohms.
@Kumi
Great explanation! It was a lot to take in, so I’ll need to read it over a few times to fully get it. Thanks for the help, but I’m still learning.
Hayden said:
@Kumi
Great explanation! It was a lot to take in, so I’ll need to read it over a few times to fully get it. Thanks for the help, but I’m still learning.
No problem! This stuff takes time to learn. I’ve been reading and studying it myself, so I totally get why you’re asking. Just keep at it, and you’ll get it soon enough. If you like reading, you’ll do great in the audio world.
@Kumi
I do this for fun. I watched my uncle work on his subs and speakers once, and I was just handing him tools. But when I did it for my car, I got it right the first time. I installed the amp and subs without touching the stereo, using a toggle switch. I even charged someone $300 to do theirs later.
I tell people I know how to do it, but I live in Hawaii, and the drives here are short, so people don’t really get to listen to the audio for long.
@Hayden
Just give them a good reason to hang out in the car a little longer, or outside with some good music bumping! 
Have a good one, and feel free to text me if you have more questions.
Kumi said:
@Hayden
Just give them a good reason to hang out in the car a little longer, or outside with some good music bumping!
Most people here just turn the volume up and like having space in their trunk for beach or fishing stuff. But for those who are into audio, they come to me with questions. 
X is called EBP, and it’s more of a rough guideline than a strict rule.