how hard is it to build them correctly and what kind of range do they have?
dawg its as easy as any other box, but your cuts have to be exact size and angle for it to sound best. Depends on the horn but usually the idea behind them is to produce increased output at the fullest range possible. Some designs have a little trouble hitting the low low lows say down to 25 and below hz. The design I like the best is Decware's wicked one, because it's kept smaller yet has really good increased output at almost every frequency. Its just basically 2 sealed chambers and then 2 tappered horns with a shared mouth. I will say this though that design is made for smaller subs if you put a couple bigger higher powered subs in there you use up a good amount of the sealed chamber so you might want to look into moving the baffle to allow for more airspace in the sealed chambers.
Um I hope you have a bigger SUV for a full sized horn for 4 12s....yeesh I'm thinking at least 15ft cu especially with that much of a horn length thats pretty nuts man looking like a radiator. Sure would sound good though.
If you wanna cut use up less space maybe think about having the horns flare into the same mouth.
Maybe start with the subs at the outsides of the box and tapper the flares toward the middle and have them intersect at the mouth.
I say spend a good amount of time thinking about how much room you have and how much seal chamber volume you want and then what you have left see what kind of flares you can fit. That will start to tell you how many subs you should run..3 12s in a horn would be nuts man very very loud. But I think 3 12s is approx same surface area as 2 15s, and some horns that are made for 2 15s are just straight large man. like the size of a refrigerator.
Right a true horn is massive man, but there have been many designs scaled down to something that will fit in a vehicle, but like I said even scaled down if you want a horn for 3 12s its gonna be big man. Like I said you can save space by sharing the mouth instead of having 2 or 3 separate ones.
well i dont have that much space i guess ill move on to the next idea lol maybe ill just do a sealed enclosure then all i have to worry about is volume. then again do have these little 220 rms sharp 6'' mid bass drivers i could still play with for home audio are tapped horns horrible for music or home theater?
A couple different sets of plans for free there. You could always scale the wicked one down and play with it. It's a mean monster
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