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Old 3rd April 2010, 11:42   #16
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Originally Posted by low_bass_makker View Post
Size I can help you design the enclosure but what thing you are missing. How is the sub performing now ? It there two much low end or to much of high end (I mean in term of bass low means sub bass...30-40 hz and high end in terms of midbass 60-80 hz) ?
LBM bhai the sub is missing out a lot of low end freq. in the size of the enclosure. It will do well in the 30's but misses out on the low end below that.
In the enclosure Frank is having has a peak of 1-2db from 60 to all the way upto 120hz. In a way not giving out a smooth response. The Qtc or the total Q of the enclosure is high at the volume, above 0.92


Frank@ The Fs value of the sub is high about 31hz, overall looking at the specs ported will sound the best and really pound. Don't worry about it getting boomy. That depends how well build the enclosure is and what freq. is the port set to.
As per the manual it say ideal enclosure size of 2.25cu.ft with port tuned to 30hz. however if you need a little smaller size enclosure the port will need to be tuned above 30hz. which means some low end response will be sacrificed. You need to make a pic.

Last edited by Invinsible : 3rd April 2010 at 11:48.
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Old 3rd April 2010, 12:20   #17
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Originally Posted by frankmehta View Post
JBL sub documentation says that the enclosure must be 1 cu ft. in size. They haven't given the exact dimensions
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Originally Posted by abhinav.gupta88 View Post
non cubic enclosure of around 1.08-1.1 cu ft.
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Originally Posted by frankmehta View Post
How does one tune a port??? I am confused.
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Originally Posted by frankmehta View Post
even my Luccents outperformed it in terms of sheer quantity of bass.
And when the volume is pumped up, it sounds like a ONE NOTE sub.Can someone help me design a 'ported' enclosure (PURELY FOR SQ) please?
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Originally Posted by frankmehta View Post
ported enclosures can sometimes give better results than sealed ones.
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Originally Posted by low_bass_makker View Post
Size I can help you design the enclosure but what thing you are missing. How is the sub performing now ?
Sorry for jumping in late. From the comments Frank has made I assume his expereince is as follows:
On some music the sub does not play loud enough (atleast when compared to his Illusions), on other music it sounds boomy and very tubby (one note).
Frank am I right in assuming this?

1. Remember a subwoofer system is 1/3 sub and amp, 1/3 subwoofer enclosure (design and execution), and 1/3 room.

You can determine real world tuning frequency by measuring the current with an A.C. ammeter hooked in series between the power amplifier and the loudspeaker; the maximum current reading will coincide with tuning frequency. Once you have the tuning frequency you can use the calulations on the websites (mentioned by above) to get the port length. see links below
PORT Size Calculations and Formulas for WOOFER and Subwoofer BOXES
The Subwoofer DIY Page - Port Calculations

2. I have not seen your FG enlclosure but many who make FG enclosures forget that an enclosure must not only be stiff (which most FG enclusures are) but also non-resonant and well damped (which most FG enclosures are not). For many years I have been propogating the idea of using layers of MDF or marine grade ply along with FG to make the enclosures not only stiff but also damped as well as easy to build (using other more difficult to work with materials like marble/stone, carbon fiber, etc...).

3. From the symptoms Frank has described he is PROBABLY facing 3 seperate issues. A resonant FG enclosure, a subwoofer whose frequency response is not in accordance with the transfer fucntion of the car, and a box that might be a triffle too small.

4. Also in the old days we used to usually add a Infra-bass filter (a filter that rolled off bass below bass tuning frequency) to ensure that the woofer does not decouple below port resonant frequency. Below the Port frequency there is little control over the cone and the the subwoofer acts like it's gone nuts and flaps around like a loose sail on a sail boat. This can permanantly damage the suspension of the subwoofer. In the old days when we could not make big voice coils the added issue we used to face was that above resonant frequency since there was less cone movement required for any particular SPL there was also less air being pushed through the voice coil keeping it cool and subwoofers would burnout. Today with aluminum formers and vented pole pieces this is not much of an issue but something to keep in mind with a subwoofer as massive as the Gti.

Frank please remember that the published specs for most drivers deviate by as much as +/-10%. Now if the Fs is -10% and the Qts is +10% (a rare but possible condition) you might end up with a box that is not nessacarily correct for your given woofer. You need to measure your woofer's T/S specs.

Sealed boxes are more tolerant of deviation from published specs. However you might be able to control your subwoofer using resisant damping see link below
Strassacker: Speaker, Do-it-yourself

Some amps have this built in
http://icondealerservice.com/catalog...&productId=169

In principle this works similar to aperoidic loading of sealed boxes. The diffference being that one is electrical and one is mechanical. Both are resistive and that means there will be some loss of overall efficiency - there is NO free lunch.

Another option might be to tune the subwoofer to where your car transfer function is falling. This way the subwoofer's resonance peaks do not add with the car's transfer function peaks. The emperical way to do this is to make athe port a bit longer than required (abtou 30% longer) and cut and test many many times. In my 800 and Zen for example we found that tuning a 8" sub to about 45Hz worked best. Yes I lost out the last octave or so but atleast there was no boom that distracted me from the music. For me audio is about music, not about Fs, Qts, Vas, Watts, Amps, FFT curves, or any specifications.

Hope this helps.

Last edited by navin : 3rd April 2010 at 12:28.
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Old 3rd April 2010, 12:48   #18
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3. From the symptoms Frank has described he is PROBABLY facing 3 seperate issues. A resonant FG enclosure, a subwoofer whose frequency response is not in accordance with the transfer fucntion of the car, and a box that might be a triffle too small.
I have been thinking on the same line. FG enclosure do face lot of resonance issue. In which case many believe not to dampen it since the weight is as much as the mdf enclosure. Along with this I believe the other issue is facing the sub which would require in playing around with phase to get the best result. A FG enclosure needs a lot of work even after building the enclosure.
Sealed enclosure can result a smoother response but its not always to the taste of one. Frank's enclosure is not a tad bit small but almost half the required size which is not giving out the desired output. Aperiodic vent could help but again it has it's limitations too.

The best I suggest Frank get the sealed enclosure build in MDF and make use of little EQ to get the best result.
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Old 3rd April 2010, 16:34   #19
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Let me simplify Frank's query for all you guys.

Firstly, Frank wants a sealed enclosure. He does not have anyone who can make an accurate ported enclosure. So let's stop with the suggestions for a ported box (Also since Andy confirms that it will sound nearly just as good with a sealed enclosure).

Secondly, he wants to mount the subwoofer in a conventional manner, i.e. not inverted with the motor showing.

Now, the manual says that the ideal sealed enclosure is 1 cu. ft. and the driver displacement is 0.12 cu. ft. Can anyone calculate and provide some box dimensions please?
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Old 3rd April 2010, 17:05   #20
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Hmm...No ported enclosure :( Anyways the woofer EBP is 57.7 so it is more suited for a vented one..My take is..

Sealed box -> for a flattest response and for .7 QTC it will need a 1.869 CUFT box. With a 1 cuft box it will be have a QTC of .819 which is very high but still aceptable so I would go for a box in between. Like 1.3 cuft which is quite widely accepted.

If we go with a ported box it might play better but since our Dr Sahib is going for sealed then let it be...

Last edited by low_bass_makker : 3rd April 2010 at 17:19.
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Old 3rd April 2010, 17:17   #21
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Now, the manual says that the ideal sealed enclosure is 1 cu. ft. and the driver displacement is 0.12 cu. ft. Can anyone calculate and provide some box dimensions please?
Ajay, As I mentioned I had modeled the response in a software. Given the parameters and power he has the best possible response will get in 1.25-1.4 cu.ft that's including the sub displacement with the Total Q of 0.74.
Also, had confirmed the same with Andy.

LBM@ EBP of 50-75 is suitable for both sealed and ported. Anything below 50 is more suitable for sealed anything above 75 is suitable for ported.

Last edited by Invinsible : 3rd April 2010 at 17:23.
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