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 ?
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 ?
You mean I won't get wall-shaking impacts on drums etc ?
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 ?
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.
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.
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.
Blocking sounds like indicating Xmech. Xmax is ussually quite some lower as Xmech (like a factor 2).
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
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 !
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 !
"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.
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.
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 ?
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 ?
Push pull always seemed like a waste after being closely examined.
With Push-pull Distortion is lower because of some harmonics canceling, BUT
Distortion will also be lower with two drivers seperate because Sd doubles and Xmax does not have to be as great. Double drivers also gives double displacement and double efficiency(mutual coupling) plus double the power handling(mechanically and thermally).
The only real advantage is enclosure size....
You gain everything back from decreasing the box size with a linkwitz transform tho, so I'd go with that.
With Push-pull Distortion is lower because of some harmonics canceling, BUT
Distortion will also be lower with two drivers seperate because Sd doubles and Xmax does not have to be as great. Double drivers also gives double displacement and double efficiency(mutual coupling) plus double the power handling(mechanically and thermally).
The only real advantage is enclosure size....
You gain everything back from decreasing the box size with a linkwitz transform tho, so I'd go with that.
BassAwdyO's comments are on the mark.
The apparent extension increase comes with the expense of half the efficiency, so rather than gain in the deepest bass, you've really just thrown away output at the upper end.
And the half box volume isn'trealized either because the extra driver has to go somewhere.
The apparent extension increase comes with the expense of half the efficiency, so rather than gain in the deepest bass, you've really just thrown away output at the upper end.
And the half box volume isn'trealized either because the extra driver has to go somewhere.
OK thanks very much for the advice !
I'll order another driver and build the two solutions to compare. Both have pros and cons, so I'll give it a try during holidays
It will only be a few components to change on the linkwitz transform circuit, and I won't need hundreds m² of wood.
I'll post my experiments when I begin them
Greets !
I'll order another driver and build the two solutions to compare. Both have pros and cons, so I'll give it a try during holidays
It will only be a few components to change on the linkwitz transform circuit, and I won't need hundreds m² of wood.
I'll post my experiments when I begin them
Greets !
If you want the even-order distortion cancelling of the clamshell isobaric you can even do this with two seperate boxes while still having the 6dB bonus. You would just have to mount one of them with the magnet outside and wire it "out of phase". In order to work you'll need to feed them the same signal (i.e. a mono sub or you order more drivers and have 2 per side, the latter maybe just in case of friendly neighbours !!!)
Regards
Charles
Regards
Charles
Yes, it was of course what I planned to do : the drivers face each other and are wired out of phase. One is in the sealed enclosure and the other has the magnet outside.
It cancels the suspension non linearities of the drivers, with one going forward when the other goes back.
Otherwise, I don't see the advantage of using two drivers going in the same direction. Their non linearities won't really cancel.
What do you mean with the two separated boxes ? You mean one volume for the back of the first driver, and the other between the two membranes ?
It cancels the suspension non linearities of the drivers, with one going forward when the other goes back.
Otherwise, I don't see the advantage of using two drivers going in the same direction. Their non linearities won't really cancel.
What do you mean with the two separated boxes ? You mean one volume for the back of the first driver, and the other between the two membranes ?
No, he means, you build a big box with two holes.
One driver the normal way, one driver the magnet outside.
It's not isobaric, but it's push-pull.
You need an enclosure 4 times bigger than the isobaric enclosure, but you get much more gain.
You get all the advantages of isobaric, without the disadvantages (except the size of course).
One driver the normal way, one driver the magnet outside.
It's not isobaric, but it's push-pull.
You need an enclosure 4 times bigger than the isobaric enclosure, but you get much more gain.
You get all the advantages of isobaric, without the disadvantages (except the size of course).
Thanks for the suggestions ! The push-pull with two drivers outside sounds like an excellent solution, ideally the best.
I have ordered two other drivers, the last available. I will resell one to a friend and keep the two drivers which will have the closest parameters.
I think I will keep the 130L push/pull with the drivers facing each other because my bedroom is smallish, and though I rarely make compromises on sound quality, I can hardly put a 600L box under or next to my desk.
Anyway, I don't think I will ever be near the driver's limits, mechanically and electrically in a 14m² room ! I have parents, a brother and a little sister, so I can rarely listen loud, and don't like this anyway. And the cheap drivers probably don't deserve such a big enclosure, I may even spend more on wood and cables !
Maybe when I will have my own house with dozens rooms and a giant home theater, I will build a big 1'000L sub with thousand watts amplifiers, or even a bass horn, but it won't be before a few years I think
I'll keep you up-to-date of my experiments !
Greets !
I have ordered two other drivers, the last available. I will resell one to a friend and keep the two drivers which will have the closest parameters.
I think I will keep the 130L push/pull with the drivers facing each other because my bedroom is smallish, and though I rarely make compromises on sound quality, I can hardly put a 600L box under or next to my desk
.Anyway, I don't think I will ever be near the driver's limits, mechanically and electrically in a 14m² room ! I have parents, a brother and a little sister, so I can rarely listen loud, and don't like this anyway. And the cheap drivers probably don't deserve such a big enclosure, I may even spend more on wood and cables !
Maybe when I will have my own house with dozens rooms and a giant home theater, I will build a big 1'000L sub with thousand watts amplifiers, or even a bass horn, but it won't be before a few years I think
I'll keep you up-to-date of my experiments !
Greets !
Lol !
what a nice idea You should patent it : "dream-shaking subwoofer with integrated bed" The little problem is that my bed is up in the sky : my desk is under it, and my computer and audio stuff etc...
It saves me much place, and I have two places left for it : under the desk, or on the right of the desk. In both cases, the place is not such a problem for a sub up to ~ 150L (volume of my future sub counting the wood and the speaker and the room arround it).
Moreover, I will leave home next year and have no clue what my future student room will look like
, so I make it not too big to be sure it can fit any bedroom easily.600L was the "ideal" volume for an enclosure with two drivers inside, not isobaric anymore, though. And not push-pull also, because both the drivers communicate with the outside.
Hey, you all have great ideas, but don't forget these drivers were really cheapy, so they probably don't have the requirements for an "ultimate" sub, so building huge boxes for them is maybe overkill IMHO, even if it is always worth building the best thing possible with what you have !
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