Monitor Audio Sub Driver With 50Hz Fs

[Depth Charge]

I recently purchased a 12" subwoofer driver on eBay. It's from a Monitor Audio RS W12 sub (http://www.monitoraudio.co.uk/produc...duct=17&area=2).

It seems to be a very stiff driver. Having done some brief tests, it would seem that the drivers Fs is at about 50Hz! I want to run some more definitive tests, but in the mean time it would seem that Qts is about 0.52 and Vas about 10.1 L (I'm less sure of that one though, need to get my hands on some scales).

How could a 12" woofer with specs like this be made to give good low end? My thoughts are that maybe Monitor Audio use a Linkwitz transform circuit in their sub. On the other hand maybe I have a dodgy driver.

What are your thoughts and comments?

Cheers,
Steve.

[Depth Charge]

Right, this driver definitely has and Fs in the region of 50Hz. I'm wondering if it is possible to lower Fs by making the suspension less stiff (it's REALLY stiff at the moment). Could this be done with solvents? How would it be done? I think there are two spiders, one could be taken off, not so keen on that though.

If it is possible to make the suspension less stiff, would there be any other changes other than Fs and Vas? Would the Qms or Qes change at all?

If you think this is a crazy idea, please let me know. I don't want a ruined driver on my hands that I could have sold and gotten my money back for.

[FullThrottleRic]

You don't really need to lower the Fs too much with qts that high.

I'd say you are right about it origonally having a Linkwitz Transform, 3" long VC and 1kw amp make it seem very likely.

[MJL21193]

Hi.
This woofer has good potential for Linkwitz transform. The high Fs is a good thing, as you want to drive it below Fs and the high Qts is also good for the sealed box.
How did you measure it? Speaker workshop? Your mention of a scale says that you have no reliable weights. A double A (AA) cell is around 24 grams, that's pretty much what I use (one or two).
You will want to make a temporary sealed box around 30 litres to test it for suitability. Build the box, install the driver and drive it with 100 watts at 20 Hz. If it makes no racket, it should be ok.

[Depth Charge]

Guys, thank you so much for responding!

I added 16 x 2 pence coins plus Bluetac and measured using a Wallin jig and Speaker Workshop. Apparently that's about 128g, but I'll have to borrow some scales to get a proper Vas reading. Assuming that it is 128g, Vas=11.85 Litres or 0.418 cu ft. Qts is 0.5 and Fs is 49.4 Hz.

As the driver is at the moment (without out a Linkwitz Transform) it models as having an F3 of 70Hz and a Qts of 0.6 in a 30 Litre box (in WinISD).

Having seen Zaph's Dayton RSS315HF woofer project (http://www.zaphaudio.com/archives.html), I was quite tempted to sell the Monitor Audio driver and see if I could get hold of one of those RSS315HF drivers. Don't know where I would get one in the UK though.

Is the driver I have worth hanging on to and worth the extra money of a powerful amp and Linkwitz Transform circuit? Is the power/size ratio a good trade off? Won't thermal compression be an issue?

Cheers.

[FullThrottleRic]

Have you got any detailed pictures of the drive unit?

[Depth Charge]

Yes. Going to see if I can get my web hosting working and I'll put some photos up. I can't host photos from DIY Audio can I?

[Depth Charge]

Here we go: -



The magnet is 18cm in diameter and 4cm tall. The vent in the pole piece is 3.5cm in diameter. The voice coil former would appear to be about 7.5/8cm (or about 3") in diameter.

The top of the coil (which I can see through those holes in the side of the basket) would appear to overhang the top of the top plate by about 9mm. The top plate is 5mm. Assuming that the coil overhangs the bottom of the top plate by the same amount, this would give an xmax of 9mm in each direction would it not?

When I had it hooked up to my Nad C320BEE for testing/running in, as I increased the power it seemed to get to a point where the excursion stopped increasing. Increasing the power much further than the point at which this happened just caused my amp to cut out, but the cone seemed to be moving by about that amount (9mm), though I can't say for certain.

What sort of amp would you recommend to power something like this (assuming the use of a Linkwitz Transform)? Are there any power amp kits or modules out there that aren't hugely expensive?

I'm sure it's not entirely impossible that I could build an amp myself, would this be any cheaper?

Cheers.

[pjpoes]

Depending on options available I would recomend a 300-500 watt amp. I mean in reality the linkwitz transform is going to require in the multi thousand watt below say 30-35hz, but thats based on matching peak output above that level, which isn't necassary. If you assume a listening level of 100 decibles or less, then you could get away with less power. It will always be limited, and always wont handle the power it needs at 20hz with that much equalization, so I would still stay within the power limits of the driver, which is probably less than 500 watts. Obviously 500 watts was used originally and they probably used either a LT cuircit or some type of equalization, and output was probably limited to between 85-100 decibles. In room will be better of course. It's hard to say without knowing the xmax and speaker power handeling to know how far you can stretch that though. If you are going to build yourself, 500 watts might be a bit more difficult. However you can buy 300 watt amps so cheap anymore it seems like a worthwhile investment over rolling your own.

[mcgsxr]

I wonder if this little baby could be used to create that LT?

http://www.reckhorn.com/index.php?ln=en&prod=b1