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I purchased a 10" kenwood marine, free air sub(Kenwood P-WD250MRW), which I would like to mount in the box underneath the seat. I've done some research on free air subs and still can't figure out if they need to be in a sealed enclosure, so the front sound doesn't mix with the rear sound. The box I want to mount it in has an opening on the side for storage and is open on the top. My question is, do I need to seal off the openings, so the front & rear sound waves don't cancel each other out? I've attached a pic. Thanks
 

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For free air (IB) to work, you much isolate the front and rear wave. This works really well if you fire subs into the seat backs of a sedan and seal the trunk off from the front. If you can't isolate the front from the rear, then you need to seal off the box.
 

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"Free Air" typical means it has a lowish fs, high vas, and high compliance. All of those things contribute to being a good candidate for an infinite baffle install like gi is saying. It also means they will work well in sealed enclosures. Seeing as this is for a boat, you probably don't want a perfectly flat "SQ" style flat response, so putting it in a medium or even smallish sealed enclosure should give a nice big bump in the middle of the passband which lends itself well to that sort of PA sound you need to overcome environmental noise.
 

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Good luck with that one. Unless you can find the true specs for it, you're shooting in the dark. It is designed for infinite baffle in a marine application where most just install it in the cabin walls and leave it be, thus Kenwood probably hasn't provided specs. Unless you find someone that can draw the T/S parameters from it so you can install it into an actual box, expect the performance to be poor.

You might want to look at other options before putting in the time & money to install it. That place under the seat is not IB.
 

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Discussion Starter #5
Thanks for the input guys. Bayboy, why is the box under the seat not IB? Is it because of the openings? I've built plenty of car audio boxes but never messed with free air subs. I found this sub with an amp as a package deal, which had pretty good reviews. As boats can be money pits, I opted to go the cheap route on the bass. I'm not looking to compete, just looking for a little bass instead of nothing. I'll wait till the sub arrives and check out the specs, but probably go with sealing off that whole box under the seat.
 

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Thanks for the input guys. Bayboy, why is the box under the seat not IB? Is it because of the openings? I've built plenty of car audio boxes but never messed with free air subs. I found this sub with an amp as a package deal, which had pretty good reviews. As boats can be money pits, I opted to go the cheap route on the bass. I'm not looking to compete, just looking for a little bass instead of nothing. I'll wait till the sub arrives and check out the specs, but probably go with sealing off that whole box under the seat.
From your explanation, if I'm picturing it right, it is because of where the hole exits which is back out to where the front wave can meet and cancel it out. As mentioned earlier by others, the front & back wave must be completely separated for IB to work optimally. Now if the sub's rear fired into the hull while the front fired out onto the deck, it would be better.

As far as sealing up the sides, it could work, but without knowing just how small of a box is too small it becomes a guessing game. You could luck out and get a response that's better than expected, or you could get one that sounds like poop. Also depends on the music you listen to. Got to remember, most of those marine subs are installed into cuddy cabin boats where there is an actual room gain to take advantage of for low end, not on decks playing out into the open. Maybe a small box with a high Q will give you something if you luck out.
 

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Most IB subs want a large box, I mean large like 2 sq ft for a 10". If you sealed off the sides and top and mounted it in the front with the rear open, well it might work but its far from optimal. If you had a large storage under the rear seat and put it in that it would work, unless you had the seat up. It does not have to be air tight, just way smaller than a port would be. Or put it in the bulkhead cuddy to outside somehow, it really depends on the boat design. I've also done sonotubes for boats, the tubes are much lighter than a wood box. I ported most of them.

The idea with IB subs is roughly...you could use that 2 sq ft box for it, or your 15 sq ft trunk, or your 6 sq ft storage under seat, or the whole cuddy, or.....whatever for a box and it will work fairly well.
 
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