help building a box for a hdc4 18

[jboy2015]

Never said you were wrong lol. I assumed that first post was just a typo meant for a 15 too. I got similar numbers too with that one so yes BBpro is accurate. However your port area is on the low side of things and i'd try to account in 2ndary baffles, bracing and sub displacement.
The owner of the black nasty, Andrew McCullough has done a lot of testing with soundqubed subs as well and here are his results for a nice workable range with the HDC series

HDC3 & HDC4 Optimum Enclosure Size - Subwoofers / Enclosures - SMD Forum

I had my hdc3 18s at 12.5 cubes and 16.5 port area per cubic feet at 205 ish sq inches of port and they did well up top and down low to 25hz. Output could be gained with a higher tuning but I my vehicle peaked a bit higher so i tuned a bit lower at 35hz to compensate.

I always thought soundqubeds needed mega huge boxes. 2.25 for an HDC312 is much smaller than i expected considering he said its for both sound AND performance

 

[wew lad 5,000+ posts]

I always thought soundqubeds needed mega huge boxes. 2.25 for an HDC312 is much smaller than i expected considering he said its for both sound AND performance

depends how much power you plan on running to it, but 2.25 net sounds right to me for a 12.

 

[Jeffdachef 5,000+ posts]

I always thought soundqubeds needed mega huge boxes. 2.25 for an HDC312 is much smaller than i expected considering he said its for both sound AND performance

Its related to power. Big boxes tend to give better efficiency but actually is not always optimal for sound quality imo. Properly sized boxes (in relation to the woofer and vehicle) with lots of power sounds best frequency response wise along with better group delay (accuracy) vs overly big boxes trying to get the most low end bloated output with less input power and sacrificing lots of group delay.

 

[tc300 10+ year member]

my buddy has 2 18" hdc4 in a huge box and lots of port area.... plays music great and gets low.

 

[jboy2015]

Its related to power. Big boxes tend to give better efficiency but actually is not always optimal for sound quality imo. Properly sized boxes (in relation to the woofer and vehicle) with lots of power sounds best frequency response wise along with better group delay (accuracy) vs overly big boxes trying to get the most low end bloated output with less input power and sacrificing lots of group delay.

Thanks Jeff, I'm gonna talk to stang later today about my next box. Gonna order it up!

 

[Papermaker85 5,000+ posts]

Its related to power. Big boxes tend to give better efficiency but actually is not always optimal for sound quality imo. Properly sized boxes (in relation to the woofer and vehicle) with lots of power sounds best frequency response wise along with better group delay (accuracy) vs overly big boxes trying to get the most low end bloated output with less input power and sacrificing lots of group delay.

I agree larger box the higher you group delay.

Once you get to the point to where you have the sma port area as sd its acting more and more like two drivers. Phasing between the sub and the port becomes more reveliant. The ports compliance changes and will have more affect in the box size than the driver..