Port volume - excessive if.....?

[speedracerx 10+ year member]

Over the last year and a half I've built a lot of "test enclosures" for various drivers to learn what "the numbers" actually sound like. I might build 3 boxes for the same sub for three different Q, etc. etc. so I can logically build boxes to peoples various different tastes in music, and comprehend reviews in fine mags like Car Audio. I feel I have a good hold on sealed boxes, the formulas and what specs to look for when shopping for a specific application build. What I haven't successfully backed up is the increased spl I'm suppose to get when I build very large, multi ported boxes. The tuning for frequency is simple enough, but the couple of "overkill- screw sq for big spl" boxes I've built always seem dead in all but an extremely narrow frequency range and not much louder even in its sweet spot. I have had great success with reasonably sized ported (usually rear) home speakers, so I'm wondering if I have been trying TO MUCH port volume ( large diameter, flared multi ports) even though tuned for correct freq. I have never messed much with "air speed" numbers figuring that stuff is just to keep air noise down. ......What happens when ports are to large .........(but not for freq), where am I messed up? Even the two 4 in dia, 11 in. long ports in a 4.5 + net cu ft box (rough numbers) seemed to make a Mojo flat. My 2 12 in Brahmas in half the size sealed box (as the single 15 in Mojo's ) with same amp at similar impedance, played much louder and the sq was no comparison.

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[Volenti 10+ year member]

When the ports are very large in area they tend to need to be very long as well, this length can introduce quarter wave effects that affect the response, also large ports react to the loading in the car and the real tuning frequency can be lower than predicted.

As for how much port area is too much, I typically stick to under 1/2 the combined drivers SD for vented and bandpass. Other enclosure types, like transmision lines, need at least 1/2 SD to work properly at all and are optimum at 1-1.8 SD.

 

[speedracerx 10+ year member]

I usually end up trying 2 round ports, so they are not terribly long. Often I never listen to (compare) them in a car but I know I should. I think the Mojo may have been hurt before I played with it, and admittedly most of the others where really sq woofers that needed sealed boxes to sound best, but I figured they would still be clearly louder. Thanks for the info.