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Old 18th June 2009, 09:29 AM   #11
stuman is offline stuman  United Kingdom
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ok, so what small driver would you guys recomend for a small sealed enclosure? or what specs should i be looking for?
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Old 19th June 2009, 05:33 AM   #12
GM is online now GM  United States
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Well, to be a true HT sub, it needs to be able to handle +10 dB peaks referenced to the mains, so whatever you set them to, the sub needs to have enough Xmax/low thermal power compression to do this down to 20 Hz in-room. That's a tall order in a small box, especially a sealed one. Actually, if the mains are set to 'small', then the sub has to handle their LF output also, which is stretching the laws of physics to do it with a single point source speaker.

Bottom line, the larger the driver, Xmax, power handling with the lowest Vas and a Qts around 0.4 is what you want. Not being current on what's available in this type of driver, the only ones that comes to mind were the 15" Adire Audio Tumult and the later 18" 18D2 variant with a LRT filter to EQ them flat down low in a relatively small cab. Note that you'll need power measured in kW and I imagine that you've figured out that such drivers don't come cheap.

As you drop down in driver size, Xmax and power handling has to increase to offset the loss of acoustic efficiency (Sd) for a given peak SPL, so at this point you either accept a lower peak SPL and/or put the sub near/at the seating position. Since you're in AV sales, seems like you should already know all this though.........

Anyway, don't know of any small, high SQ drivers with all these other requirements, much less with a low Fs, so not sure you can get what you want.

GM
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Old 19th June 2009, 07:29 AM   #13
stuman is offline stuman  United Kingdom
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yeah that all makes sense,

ok, so with a little sub, ie 8 - 10" i would need to strike a balance between the LF extension and total SPL output.

so what im now thinking is a linkwitz transform circuit with adjustable low pass filter so depending on output i could adjust the roll off at the absolute bottom end to a level where a compromise is found.

are there any schematics for subwoofer controllers like this?

also the sector of AV sales i'm in doesn't really call for knowledge of the physics of loudspeaker design more the implementation of multi-room ect. however my unquenchable thirst for knowing everything about everything and loving design in general has got me very interested in this side of the industry as well and being able to design a product to MY specs (learning that the laws of physics can get in the way) is very appealing. i've only been in the industry 18 months but still get called on as the go to guy for answers, so this is all a great resource for my steep learning curve. and one day i hope to be a useful contributer to the forum instead of a knowledge leach.
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Old 19th June 2009, 02:55 PM   #14
GM is online now GM  United States
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Correct.

OK.

Don't know, all I have is the LRT design SS with the basic schematic and never built/used one AFAIK unless the little Creative 2.1 system's ~6" 'sub' someone gave me for my computer has one, which I doubt based on its high roll off/boom-box mid-bass 'thump'. Anyway, it's been around long enough that if it can be done by DIYers it probably has, so a WWW search seems in order.

Yeah, I was really tired when I responded and more 'thinking out loud' from frustration WRT the sales philosophy that not only is it not necessary to know the technical side of the market beyond peddling the marketing dept.'s (mis) information, it's a hindrance.

Cool, we need more folks with your attitude to replace the cognoscenti that fade away/leave for whatever reason.

GM
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Old 7th July 2009, 02:48 PM   #15
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Stuman, I have used a few of these 8" W7 drivers. From several batches, I found that the VAS published on the factory spec is off considerably. I have verified my VAS is proper by modeling the response and then measuring near field response in a sealed cabinet of specific volume and the results matched up; so there was nothing wrong with my VAS measurements. It may be a revision(?) in later models of the 8W7, and the factory specs never changed to reflect this change(?).

I would ignore JL's suggestions to be honest. You can use WinIsd and figure out the safe conditions of use for yourself.

Here is an average T/S reading from recently purchased 8W7 driver:
Qms: 7.765
Qes: 0.734
VAS: 9.23 Liters
Fs: 38 Hz
Re: 2.78 Ohms
Le: 1.34 mH
BL: 10.39
Power(1dB compression, constant): *500 watts

*Power compression is very low with constant 500 watts input; this is based on power input vs. theoretical level increase, as measured by a car audio magazine testing round. Only 1dB of compression was present at 500 watts, as they incrementally increased the power to the driver and monitored calculated SPL increase vs. actual/measured. At 250 watts, only about 1 - 2 tenths of a dB compression was present. I have no idea how much power this driver can take thermally, continuously, but it certainly appears that 500 watts is not at all dangerous from a thermal standpoint, which also is the safe range listed by JL Audio for the driver.

Another interesting fact is that in-car SPL tests demonstrated this 8" driver, in real world use, to produce as much SPL as most conventional 12" subwoofers. I can only guess this has to do with the lack of thermal compression of this driver vs. most 'ordinary' drivers, as well as it's very unusual linear excursion(mechanical limit is far more than listed linear xmax; xmax appears to very conservative).
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Old 7th July 2009, 04:46 PM   #16
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Greets!

Good to know, thanks!

GM
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Old 7th July 2009, 06:14 PM   #17
stuman is offline stuman  United Kingdom
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fantastic thank you for the information.

do you have any suggestions for box designs that have worked well for you?

have you used any of these drivers in a home theatre orientated sub build?

do you think i could then get away with a 750W plate amp?

stu
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Old 10th July 2009, 08:57 PM   #18
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Default TH with JL-Audio ???

Click the image to open in full size.

This tapped horn will get you a decent response from 100 liter big "box".
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Old 10th July 2009, 11:27 PM   #19
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I don't have any particular recommendations. But the driver, with it's very low thermal compression and extended linear excursion, should perform very closely to modeled response at and near full rated power, unlike most woofers. It has extraordinary performance for an 8" driver. If you need a compact subwoofer, this is an excellent driver to utilize in your application. Note: this really must be used as a subwoofer. I measured harmonic distortion vs. frequency, and as you leave the subwoofer band, harmonic distortion rises very quickly. The absolute limit one could use this driver is to 150-160Hz, and that is with some compromise.
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Old 13th July 2009, 05:24 PM   #20
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This is a little OT, but where did you get one of those amps?

I've looked all over the place, but, from what I've seen, you can only buy them in America...

Chris
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