DarrellGS4

I wanted to gather some thoughts on switching to a free air sub. I currently have a JL 10W6 in a sealed JL box with about 400 rms running to it right now, and it sounds pretty good. Back when I was in college I had an Audiobahn pushing 800 rms, and that thing use to hit really hard.

However lately I have been thinking about switching to a free air sub to clear a little room in the trunk.I just wanted to get some opinions on subs and or the sound difference in the two based on the setup I currently have. Feel free to vote on your opinion.

19psi

a well set up free air with enough power will definitely work well. they can sound good and tight with the right equipment. i cant' recommend anything as i havne't done a free air personally but the guys on www.realmofexcursion.com can.

Robert_J

Free-air subs give you very little bass because there is nothing separating the front and back waves created by the driver. Now and infinite baffle sub would work great.

IB subs are just sealed subs in no enclosure or an enclosure so large that it has no impact on the sound of the sub. Will switching from a JL in a sealed box to an IB be an improvement? Don't know. You need to determine the driver you are going to use. As I've posted dozens of times, you don't need one labeled "free air" or "IB". That's just marketing. I like my IB subs to have a Qts around .5 to .7, with an Fs of 30hz or lower and at least 16mm of linear excursion. The old TC-7 and its cousins (Eclipse had a model and the old ED O series) are perfect examples.

The downside of an IB install is reduced power handling. Since there is no box, there is no air to act as a sping to control cone moton. People have posted about using Kicker subs in an IB so I modeled them using Unibox. They usually handle about 35 watts of power before reaching their linear excursion limits.

-Robert

19psi

robert J, i thought free air and IB were the same thing, just different terms? either way, you'd have to seperate front and rear waves or it would be worthless. on some car audio forums i'm on, some guys like hte ib setups but say you need a lot of power to make it work. i can't see 35w being considered a lot of power by anyone, so if a kicker sub is reaching it's physical limits of excursion at 35 w, would one need a hugely tight and strong sub to be able to handle enough power to make it worthwhile?

DarrellGS4

Well from my understanding and experince with car audio, it doesnt always take a lot of power to have a sub sound good. I have seen some that use less power and sound better than others that use more. I have also seen the ones with less power can have a much cleaner sound than some with the more power. It is my understanding that there is a direct correlation with power and distortion, true that the distortion is not as much a factor as in the past however it is still there. I wonder if the switch from my 10W6 to a 10Ib4 would be a good ideal sound wise. Would it even be possible to do a mounted 12ib4 mod?

Side note I see that they both are around 100w rms much lower than a sealed box.

Robert_J

On most forums, they are both used the same way but that is incorrect.

I'm not sure where they got that. If you model any sub in WinISD, Unibox or your favorite speaker modeling program, you will see that it needs very little power to reach xmax. When modeling an IB for a car, I use 30hz as my limit. If you listen to rap, 25hz may be the cut-off and that will lower the power handling even more. For home theater IB's, we use 10hz as the limit where we should reach xmax.

You need a sub with a high Qms (mechanical Q) and a not so powerful motor structure. The high Qms leads to a higher than average Qts (see my earlier post) and the weaker motor doesn't easily overdrive the sub. For example, look at the IB specific subs that Scott Atwell from Fi Car Audio designed - link. Now compare them to the Q series at Fi Car Audio. The cone and the frame are basically the same driver. Scott added a stiffer suspension (spider) and used a smaller motor. These subs are selling like crazy for the home theater IB users.

You get two types of distortion. The first is power compression. As you double the power to a speaker, you should get 3db increases in sound. But in real life, it is not linear. The more power you add, the smaller the increase in output. Also, the closer you push a speaker to its mechanical limits, the higher the distortion. The average home theater IB user may have four 18" subs but they are not trying to shatter any SPL records. They want to limit the excursion of the sub and keep distortion down. To increase the output volume, they use multiples of the same sub. Going from 1 sub to 4 gives you 12db more output.

JL makes a very high quality sub but out of my price range. I also like to be able to talk to the guys that actually engineer the speaker. A 12" would be better. I've seen some guys use a piece of MDF as an adapter plate and mount the 12 from inside the trunk. Or you can increase the size of the hole in your rear deck.


Ignore the RMS rating when picking an IB sub. Modeling them in software is the most accurate prediction short of testing them after installation.

- Robert

19psi

ok, good info. thanks. so based on your concept of a free air sub, what's the point of doing it that way? if the IB is actually what most people think of as free air, what's a good application for a true free air?

DarrellGS4

And with a 12" how much power would I be looking to push it with?

Robert_J

There is no application for free air other than testing the driver. I use Woofer Tester 3 from Parts Express. The driver sits on my desk in a free air application and the software determines the Theile/Small parameters. I also use free air applications to break it in. High excursion to loosen the suspension but since the front and rear waves intermingle, there's not as much bass. Really good when an 18" sub is connected to a 500w amp. Otherwise the entire house would shake.

Driver size has nothing to do with required power. If a 10" and 12" sub have the same T/S parameters, then it would take the exact same amount of power to reach max excursion in an IB. The 12" sub would be louder since the larger cone area can move more air.

-Robert

Robert_J

As I've mentioned earlier, the TC-7 is almost a perfect IB sub. There's one for sale at Sound Domain - http://forum.sounddomain.com/ubbthre.../1#Post1362901

Here is the old TC-7 page from the Wayback Machine - http://web.archive.org/web/200707080...ds.com/tc7.htm

-Robert

19psi

aha, got it. thanks.

DarrellGS4

Thanks.

Some people know things and Some people know things that others don't.

(makes sense when I say it in my head)

VEGAS DRE

get a ported sub enclosure built for the jl 10" woofer you have already that will sit on the shelf right behind the set with the woofer and port firing away from the rear seat! get it made with the following dimensions and it'll do very well with 400 wrms! 31" wide x 12" high x 10" deep with a port that is 10.5" high and 1.25" wide and 14" long ! this enclosure works very very well in all gs models fits very snug and wont move around in the trunk and plays very accurate and low bass !

DarrellGS4

Thanks, I'm going to see how that sounds...do you have any pics with this setup?

VEGAS DRE

i dont have a picture but ive built sbout 6-8 of them and they play very well and the customers always very happy