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Old 10th December 2005, 09:32 PM   #1
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Exclamation Awesome driver or hidden ****???

I've bought yesterday a 300mm/12inch aluminium woofer for a ridiculous 32 euro.

It is a Rockwood obsolete model that was sold for cheap.

It looks not so bad built, very stiff aluminium membrane, correct magnet size, decompressing vent on the back and rubber surround.

But what I was affraid of when I bought it was that I didn't know its T/S parameters at all(no one could give them to me, not even the manufacturer !)

So, I measured it with LspLab 3, using the constant current method, and the delta mass.

I used 60g of BluTak, carefully measured.

And I got surprising parameters !!

It models down to 27Hz in an infinite sealed box ! I modelled it with LspLab, CAAD 4.1, and MJK's closed box worksheet.

I did the measurements with different masses, different sweep configuration but all give me more or less the same.

All of this looks very strange...

I have no experience with big drivers, so could anyone tell me if this driver is worth designing a box for it and buying a few more units ?
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Old 11th December 2005, 02:49 AM   #2
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The Qts is quite high, meaning not a very tight response, and xmax is pretty small.

If it's not for critical or loud listening it's probably fine.
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Old 11th December 2005, 08:03 AM   #3
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Quote:
meaning not a very tight response
You mean I won't get wall-shaking impacts on drums etc ?

Quote:
xmax is pretty small
I assmued Xmax, as I have nothing to measure it. It is probably higher, the suspension allows more than +-5mm before blocking.


Will the burn-in reveal good or bad surprises ? I have run 16 to 32Hz test tones on it @ 25W during 4 hours yesterday, is it enough ?

Also, will it really go down to 27Hz in an infinite sealed box ?? I think an aperiodic design could help simulating an infinite baffle won't it ?

Concerning the parameters, I find two of them shocking :

- The Bl looks a bit weak for such a big driver : it must be a consequence of the low price

- The Mms is only about 60gr, it looks like the membrane is very light and is is quite astonishing : all the 12" drivers I've seen before are arround 150gr Mms ! Is it a problem with my measurement setup ?
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Old 11th December 2005, 08:14 AM   #4
AndrewT is online now AndrewT  Scotland
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Hi,
low BL could be due to a single layer voice coil or low Tesla (gauss) magnet or both. Keeps it cheap.
My 15inch drivers have a mass of 97gms and these are not exceptional. 150gms for a 12 is enormous and would indicate a low efficiency design specifically aimed at deep bass. The opposite to a PA driver where efficency and instrument bass is more important. Your 60gms fits into the PA category but 27Hz is lower than I would expect.
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Old 11th December 2005, 09:13 AM   #5
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Quote:
The Bl looks a bit weak for such a big driver : it must be a consequence of the low price
The lower Re, the lower the BL can be. It's an 4 ohm driver, the BL would be about 2^0.5 (1,4...) higher if it was 8 ohm. Still not that high but that's indeed normal for low budget drivers.

Quote:
It is probably higher, the suspension allows more than +-5mm before blocking.
Blocking sounds like indicating Xmech. Xmax is ussually quite some lower as Xmech (like a factor 2).

Quote:
It models down to 27Hz in an infinite sealed box ! I modelled it with LspLab, CAAD 4.1, and MJK's closed box worksheet.
It's not difficult to get a driver that models that low. But with a short Xmax it won't give low output on decent SPL-levels.

It's not a PA driver by a longshot, the Qts is way to high for that and n0 way to low. If it's not efficient it's probably good for bass

Wkr Johan
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Old 11th December 2005, 09:29 AM   #6
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I listen at moderate levels in a small room, so Xmax may not be a problem. Moreover, I plan to buy a second unit to have two subs, one per channel. (or maybe an isobaric enclosure, got to think about ot)

My problem comes rather from the size of the box : it would need more than 1000L to be flat down to 27Hz, otherwise, it would show a peak and fall like a cliff after.

I found no software on the net that models aperiodic boxes. Is it true that a driver in an aperiodic enclosure behaves virtually like on an open baffle ?

If so, it would solve both my box size and type problem !
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Old 11th December 2005, 10:54 AM   #7
Volenti is offline Volenti  Australia
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Quote:
Originally posted by youyoung21147


My problem comes rather from the size of the box : it would need more than 1000L to be flat down to 27Hz, otherwise, it would show a peak and fall like a cliff after.

In my modeling (using Bassbox 6) I found ~300L to be the sweet spot (which of course is still very large) with liberal use of fibreglass or polyester fill you could get away with a slightly smaller enclosure (~200L) and still mantain an acceptable response.
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Old 11th December 2005, 12:08 PM   #8
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Thanks Volenti !

I think I will build a ~ 100L box which leads to a reasonably small size and try to build it aperiodic. I cannot find any quantitative thing about this enclosure, quite boring

Otherwise, I have the option of a Linkwitz transform circuit, it models very well this way too.
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Old 11th December 2005, 07:51 PM   #9
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"Is it true that a driver in an aperiodic enclosure behaves virtually like on an open baffle?"

Absolutely not.

Aperiodic is another way of saying nonresonant.

This is achieved with damping, which dissipates the stored resonant energy by forcing air through a restriction, either small holes or a heavily stuffed box.

It lowers efficiency even further, since it absorbs energy at all freq, not just the resonant freq.

Aperiodiv also won't give Fb = Fs like an IB.
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Old 11th December 2005, 08:55 PM   #10
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I have talked with a sound engineer friend this afternoon, he really encouraged me to build a sealed push-pull subwoofer, seen the drivers parameters.

I did some search, and what appears is that push-pull allows the same response with half volume !

So, the calculation is simple :

a 130L push-pull subwoofer behaves virtually like a 260L simple woofer, and since heavy damping increases by 20 to 25% the virtual volume, it leads me to a ~ 330L virtual enclosure !

I simulated this and it looks very pretty, not far from the infinite baffle response, like Volenti pointed it.

Add it a Linkwitz transform circuit, and the sub goes down to 24Hz with only 6dB maximum boost at DC !


I think I have my design, just have to build it !

Does everything look right ?
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