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+1 to that
i find that winging it by using a long port then cutting it down til i find a sweet spot works pretty well. ive started using sketchup to design basic ideas i dont even touch winisd or bassbox anymore. i got a copy of LEAP Enclosure Shop that ive been trying to use but so far the best results ive had are while messing around with the box and drinking some beers.

theres this too
Lv=8466.4*r2/vb*(fb2) - 1.463r
2=squared as in r*r
Lv= port length
Vb= volume of enclosure in cubic ft
Fb= desired tuning frequency
R = radius of port being used

Experience. As far as I'm concerned there isn't a box modeling program that's even a starting point for car audio. The response of the cabin changes the entire enclosure. It also changes your reactive load which changes inductance which changes the ts parameters of the speaker. So what some people consider a starting point is really a failed attempt. The best thing to do is get an RTA and build and test multiple enclosures. Im also a firm believer that port length calculators are about %50 accurate. Testing and tuning far outweighs any computer programs to date.

So win ISD sucks? Would the box I designed for my own car be worthless then? Damn it Bill. I hate bad news. lol. So what's a worthwhile way to model sub enclosures if not WinISD?

First WinISD wont do shit and those spikes are completely useless for anything SPL related. If you want to go package deal just get some prefabs. There cheaper and they will be loud enough to satisfy the chillins all while maximizing your profits and time. If someone is smart enough to want a custom box then that changes things.

Edit the reason those spikes don't matter is because the resonance of the car will far outweigh anything that Winisd shows as a boost in overall volume.

Bill ... or anyone who want's to chime in ...

I'm always doing things in a sound quality sort of way. So I don't have a lot of experience with SPL. If/when designing a box to appeal to the young kids ... what are decent characteristics to build into the design? If you make the box big and port it, you can get a huge spike at a given frequency depending on what you tune it to. How shitty does a 9 db spike around 45 hz sound? lol. How much gain is reasonable? At what point do even the kids think it sound like dung? What's a good tuned frequency? How high is too high for a subsonic filter? I know it's usually in relation to the tuned frequency, so one answer will probably change the other.

I want to build some package deals to sell, but don't want to spend a ton of time experimenting and wasting wood to get to a good starting point. That's why we have this forum right? :) So once I nail down a good starting point, I'll experiment a bit from there.

All input is appreciated of course. I know Bill is the SPL god, so ... :)

I use WinISD to model enclosures. Not that that's going to change responses or anything. But that's what I'm working with for designing.