Beginner Help: Car subwoofer for home use?

Hello All

A little background: I purchased a JBL GT1000 series subwoofer in about 1999 and built a ported box according to the spec sheet. The original intention was to get a very efficient speaker so that I could use it with a tri-way crossover on the home stereo amp I had at the time, and later put it in my car if I got the urge.

Well, almost 10 years later the speaker has seen very light duty and is in excellent shape as it never made it to the trunk of a car. I've gotten back into DIY lately and am wondering if this speaker would make a good home theater subwoofer design. I've got the speaker, MDF is cheap, and I'm willing to spend the money on a plate or rackmount subwoofer amp if you guys think its worth while.

So basically:
-Do you think this driver is worth fooling with?
-How does this speaker’s parameters (see below) vary from a speaker that is designed to be a dedicated home theater subwoofer?
-Are there specs in a speaker that would be more optimal than this?
-What would you change about their recommended box designs?
-What software (preferably freeware) would you recommend for getting started in a box design?

Thanks in advance for the input!
Matt



http://manuals.harman.com/JBL/CAR/Boxes and Parameters/GT1000.pdf
THIELE-SMALL PARAMETERS
VOICE COIL DC RESISTANCE: REVC (OHMS) . . . . . . 3.23
VOICE COIL INDUCTANCE @ 1 KHZ: LEVC (MH). . . . . . . . 2.13
DRIVER RADIATING AREA: SD (IN2) . . . . . . . . 54.78
SD (CM2). . . . . . . 355.00
MOTOR FORCE FACTOR: BL (TM) . . . . . . . . 11.48
COMPLIANCE VOLUME: VAS (FT3). . . . . . . . . 2.62
VAS (LITERS). . . . . . 61.73
SUSPENSION COMPLIANCE: CMS (ìM/N) . . . . 344.94
MOVING MASS, AIR LOAD: MMS (GRAMS) . . . . . 89.53
MOVING MASS, DIAPHRAGM: MMD (GRAMS). . . . . 85.68
FREE-AIR RESONANCE: FS (HZ) . . . . . . . . 28.64
MECHANICAL Q: QMS . . . . . . . . . . . 10.20
ELECTRICAL Q: QES . . . . . . . . . . . . 0.40
TOTAL Q: QTS . . . . . . . . . . . . 0.38
MAGNETIC-GAP HEIGHT: HAG (IN) . . . . . . . . . 0.31
VOICE-COIL HEIGHT: HVC (IN) . . . . . . . . . 0.88
MAXIMUM EXCURSION: XMAX (IN) . . . . . . . . 0.28
 

nunayafb

Member
2005-08-17 11:05 pm
I wouldn't use this driver. It might perform decently in a car, but with those specs you wont get low or loud in a room.
Main problems:
1. too small (Sd)
2. Fs should be ideally lower, especially with:
3. Qts is too low, with Fs you are not going to get deep bass w/o eq, but:
4. Not enough Xmax

If you look at their excursion plots you can see it is exceeding 7mm over way too much of the drivers intended bandwidth, this will produce adverse affects, depending on suspension and motor specifics.

But as you said MDF is cheap, so if you are curious, dl Winisd Pro or Unibox and model the sub, build it and listen. Then you will know first hand what a small imperfectly spec'd sub sounds like.

My ideal sub:
at least one 15" or more for larger rooms, ideally 18's
Fs < ~20Hz, preferably ~15Hz
Qts ~ .5 for IB and .7-1 for OB/U-frame
Vas, don't care as I run U-frames, this should be small if you want a reasonably sized box- but the combination of Qts, Vas and Fs are needed to determine box size and alignment.
Xmax ~ 10-15mm for 15's, 7-10mm for 18's should suffice, (dependent on quantity) Everyone has differing opinions on this one though, some people love really high xmax subs, I do! I just don't use them where I want the highest sound quality as I like the sound of more drivers that move less.
I've run 4 10inch drivers- clean and musical, which means weak on the bottom end.
I've also run dual 12's, better lows, but muddier sounding (these particular drivers)
I've run various 15's, most were not suitable for sub duty but they all pounded hard. I am currently running a 15". in a U-frame that sounds phenomenal and plays loud and deep (with a little eq:D). I was measuring 110 dB of drumbeats from a test disc the other night and you could feel it in you guts. I've run that test disc on every speaker set (including my car speakers) and this setup is by far the best
The only thing that compared to it is the 18" that I have from back in '98, it was when car subs spec'd out like home subs.

-J
 
Greets!

I'm going to have to disagree with J's assertions. Car audio subwoofers tend to have what it takes to make a sub-horn, so assuming the room's construction can handle the pressure, it looks like it can do DD reference on just a few watts without exceeding Xmax down to 15 Hz in a corner loaded pipe horn. Factor in a bit of room/corner loading and it ought to please all but the most hard core HT aficionados. The trade-off of course is size at 10+ ft^3:

GM
 

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It's a simple conic expansion/build as you can see from folk's pics in the tapped horn thread, so just some staggered straight boards and FWIW I've noticed over the years that 'newbies' are just as likely to tackle such 'brutes' as a seasoned DIYer. Yeah, 10 ft^3 is big if you live in a trailer or small efficiency apt., but a pipe horn can fold up vertically to reduce its footprint considerably.

GM
 
Hi

I'm using a 12" SPLX in my lounge, a Linkwitz Transform doing some madness with it. Its retuned @20hz. Its was also £10 cheaper than the JBL. I'm running it off a car amp driven by a PC power supply.

MDF is cheap as are car drivers. Spend some money on an amp and get some MDF. Download http://www.linearteam.dk/
WinIsd, is some good software and so you could look into box design.
Search around the forums to see what others have built. Eventualy you can find a box you are happy with, and sounds good. Then, say you like a sealed box, go and hunt for a suitable driver with better spec and you'll enjoy your box even more. Seriously delve into diyAudio Forums using the search engine, because even your box has more to it than six panels and a hole. Look into cross bracing, gluing screwing cutting and finishing. Its all here.

iUSERTLO72p:D
 

thejoneses

Member
2007-11-02 8:41 pm
Hi all

First of all, thanks for the responses.

I guess I should begin by clarifying my goal here…I’m looking for something that will reasonably fill the bottom end of my hometheater/music listening setup. I don’t really expect earth shaking db, but I would like a solid performer that goes to adequately low frequencies. I have the capabilities and resources for the woodworking aspect of the project; what intimidates me the most is having a product that turns out like the software says it should. Btw, thanks for the recommendations on the freeware programs. I’ll try a few designs and post back here.

On the topic of the horn design. Before I posted my initial questions, I found where a member had replicated a design from William Cowan using two of the speakers I have. The original post is here: http://www.diyaudio.com/forums/showthread.php?s=&threadid=78741 . What do you guys think about this with just one driver? Its size certainly seems reasonable.

I’m off to do some simulations and learning.
-Matt
 

txsmoke

Member
2007-08-24 1:30 am
Since you have it, it's worth trying. MDF is cheap. I modeled it in WinISD and came up with a pretty nice response with a 3 ft^3 box tuned to 25hz and crossed over at 80 hz with a 2nd order lowpass (typical setting in a AV receiver). Just exceeded Xmax with 100 watts at 35 hz. And reached 105db at 25hz. That's a 4" x 15" port. Group delay is a little high, but for HT, it's not that critical. -3db points were about 22hz and 100 hz.

[IMGDEAD]http://www.geocities.com/smokeoverfire/jblgt1000.jpg[/IMGDEAD]
 
Eureka!
This WinISD is fantastic and I assume I’ve done something right because I’m seeing curves and power limits a lot like txsmoke above; both in shape (when modeled with similar box and port size) and power limits at the xmax.

It appears that the speaker likes a lot of volume (relative to the JBL spec sheet designs), and I think I’m going to limit my design to about 60L for size constraints and a tuning frequency of around 30hz. This makes the graph look like a higher than 0.707 “Q”, but I’m assuming that’s ok since the frequency response hump is quite small like in txsmoke’s plot above (to a controls person it looks like overshoot, don’t know what you’d correctly call it).

Thanks all for helping me get started.
 
FYI, WinISD has some issues, though the 'pro' version is somewhat better. Other than Akabak, of the freeware I've tried, Unibox is the best/most comprehensive along with the other software listed at the FRD Consortium: http://www.pvconsultants.com/audio/frdgroup.htm

'Overshoot' = 'ringing' = under-damped (what we normally call it) response, so you're apparently interpreting the plot correctly.

GM
 

txsmoke

Member
2007-08-24 1:30 am
Boxes for cars are typically smaller since the need for low end extension isn't as great because the car's cabin adds gain the lower the frequency goes. Rooms do too, but the contribution typically doesn't start until much lower- and may not come into play in very large rooms.

So the reasoning for smaller car boxes is that vehicles have around 8-12db/octave gain starting around 60hz or so (varies car-to-car), so a smaller box rolls the sub off at the same rate giving a (summed) flat response and increases power handling due to the stiffer 'air spring' a smaller box exerts on a driver.

Mike