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jl audio subs for free air?


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astro88 
Copper - Posts: 195
Copper spacespace
Joined: December 01, 2003
Location: Canada
Posted: February 18, 2007 at 1:21 PM / IP Logged  
I have 2 JL audio 12w0's and i was thinking of using them for a free air situation in my 96 sunfire 2 door coupe.
I will be giving them 100 wrms each. I wanna put them behind the back seat on a .75 mdf board. trunk totally sealed from the cabin.
Im thinking that since the trunk is on the small side (which is why i wanna do this) that two 12s should be good to go with this airspace.
would the jls sound decent with this set up, are they IB useable at all,it says nothing in the manual for this type of usage at all.
TIA
bellsracer 
Silver - Posts: 703
Silver spacespace
Joined: January 14, 2006
Location: United States
Posted: February 19, 2007 at 1:06 AM / IP Logged  

I'm not too familiar with the coupe's interior, but if you measure it out to be less than 7 cubic feet inside it, then it should help it (albeit not exactly quality sound). If you measure out 10 cubic feet or more, then don't expect them to sound any good. (Might as well just be sitting on the back seat) Even at 7 cubic feet, the sound quality will really suffer because the back of the subwoofer's sound waves are not isolated very well from the front of the subwoofer.

All in all, you could do it, but don't expect it to hit hard or sound very good. Your key here to help maintain sound quality is to isolate the sound in the trunk from the rest of the car.

Good Luck.

Never send your ducks to eagle school.
The difference between ordinary and extraordinary is that little extra.
The 3Ls of life: Learn from the Past, Live for the Present, Look to the Future.
stevdart 
Platinum - Posts: 5,816
Platinum spaceThis member has made a donation to the12volt.com. Click here for more info.spaceThis member has been recognized as an authority in Mobile Audio and Video. Click here for more info.spaceThis member consistently provides reliable informationspace
Joined: January 24, 2004
Location: Pennsylvania, United States
Posted: February 19, 2007 at 6:49 PM / IP Logged  

I would be concerned with the excursion of these subs.  With 8.6 mm Xmax and 100 watts, these subs will surpass the voice coil length at just about 40 Hz.  See the file:

12w0-4excursion.jpg

You could make sure there's a highpass filter on the amp you power them with.  It would need at least a 12 db/octave filter at 30 Hz, or a 24 db/oct filter at 25 Hz to keep the subs healthy.  But you might also consider a sealed 3.5 cu ft enclosure on the trunk side of those subs rather than using the full trunk cavity.  No filter would be necessary that way.  You'd get slightly more SPL.

For more info, read what kfr01 wrote in Guide: Using WinISD

"Determining Excursion Based Limits - i.e., How to Use Your Brain and Not Kill Speakers

Alrighty.  All too often I hear, "What the heck, my subwoofer is rated for 500w but I broke it using a 200w amplifier!" 

Manufacturers often list THERMAL power limits.  These limits are NOT mechanical limits depending on your enclosure.  Ported enclosures REDUCE power handling capability.  When you use a ported enclosure, EXCURSION, not thermal capabilities, will be the limiting factor in your SPL.  This varies based on enclosure and system design.  I'll show you this.  I'll also show you how important subsonic filters are. 

To learn about excursion limits click the arrow next to "SPL" and select "Cone Excursion."  Funky graph, right?  As frequency drops, cone excursion will increase.  For ported enclosures excursion will then decrease when approaching the tuning frequency.  Below the tuning frequency excursion will rise rapidly.  This should be what you are seeing.  As you should see, the 15" driver has plenty of excursion headroom above 20hz when given 200w of power.  However, let's increase the power a bit.  Try 700 watts.  See how the curve now crosses the red line?  Your subwoofer would have gone past xmax and would likely be damaged.

Please note the way the curve is FAR above the red line around 15hz or so.  This shows the need for subsonic filters on ported enclosures when you have a huge amplifier.  If a 15hz note happens to fly through the amplifier when the volume is up, damage is likely.  You may even simulate filters with WinISD. Go to the "EQ/Filter" tab of the driver information box and add a highpass butterworth at 20hz.  See how the driver is now protected?  No more over excursion at 15hz.  Flip back to the SPL view.  This filter has affected frequency response above 30hz, but only slightly. 

Play around with these tools.  Try comparing the excursion limited power handling and SPL capabilities of the two drivers.  You will quickly see that the 10" driver will meet excursion limits at only 350 watts compared to the 700 of the 15" in our ported enclosures. 

This is also useful in making amplifier buying decisions!  It would be unwise to buy an amplifier above 300w for this 10" driver in a ported enclosure if you like to push your drivers.  You'd very likely push the driver past xmax on a frequent basis and break the driver."

Use this program to help you find the best way to use those JL subs.

Build the box so that it performs well in the worst case scenario and, in return, it will reward you at all times.
tcss 
Silver - Posts: 1,623
Silver spacespace
Joined: June 07, 2004
Location: United States
Posted: March 03, 2007 at 12:33 PM / IP Logged  
JL's marine subs are designed to run free air.
There is no such thing as free installation!

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