Bass driver Vas mods?

[moray james]

Am wondering about a drive with the following T/S data
T/S parameters:
Fs = 30.4 Hz
Re = 7.2 Ohms[dc]
Qt = 0.35
Qes = 0.39
Qms = 3.41
Mms = 219.3 grams
Rms = 12.3 kg/s
Cms = 0.000100 m/N
Vas = 243.8 liters
Sd= 1029.22 cm2
Bl = 27.625095 Tm
ETA = 1.07 %
Xmax = 12mm
Diameter= 36.20 c
This is an interesting driver (18 inch) and only $59.00 Cnd. Problem is that the Vas is high. I am wondering if the Vas could be lowered? My thought was to cut out a series of round disks from the cone and then plug them with a much lighter closed cell foam plugs. Question is the impact of driver Fs and of course Vas? Also if one was going to do this where on the cone would such holes need to be cut to keep the cone stiffness intact? I am just making a guess but I would think that one would need to remove about 60 - 75 grams of mass to get the cone where I would want it to be this would take into account the mass added back to the cone from the plugs and glue. Any thoughts or comments? anybody ever try to modify a drivers Vas? Thanks and regards Moray James.

[AndrewT]

Hi,
your Vas is fairly low for an 18inch driver.

The mass is a bit high.
The surprise is with all that mass, the Fs is also high.
That tells me the surround is quite stiff.

The Vas is the equivalent volume of air that has the same stiffness as the cone surround. So my final comment that the surround is quite stiff corresponds to the low Vas. A low air volume is stiffer than a bigger air volume.

Do not try to lighten the cone. You will end up with a ruined driver.
It may be possible to mass load the cone and lower Fs but this will not alter the Vas.

I have repaired a couple of drivers that had the wrong (too stiff) surround fitted and the resultant Fs after fitting a softer surround was much lower.

[johninCR]

Moray,

That Vas looks fine to me. In fact, the specs look great for $59can. Do you have a link. A cone mod like that isn't something I would dream of trying, and it seems to have a high risk of catastophic failure. If you want a lighter cone, then recone it. Lightening the cone isn't going to change Vas anyway. Vas is a component of compliance. Less mass will raise the Fs though, and probably lower Qts (less mass with same motor to control it).

[Frode]

I guess he is speaking of this driver . Buy two and use them in an isobaric setup.

Frode

[pinkmouse]

Originally posted by Frode
Buy two and use them in an isobaric setup.

Yup, that's what I'd do as well.

[Ron E]

Removing mass won't do anything but raise Fs and lower Qts and raise sensitivity.

Vas=Cms*Sd^2*rho*c^2

Cms is the compliance of the suspension - the only way to reduce Vas is then to reduce compliance (increase suspension stiffness) or reduce cone area, since the density of air and the speed of sound are pretty much not changeable

This woofer actually looks very nice for the price - it has a big resonance at 400Hz, but with 12mm of xmax (if true) it might make a very nice Linkwitz transformed (LT) subwoofer.

The largest box I could tolerate would be ~140L. One of these in 100L sealed box would be Qts=0.35*sqrt(244/104+1)=0.58 and Fb would be 50 with an F3 of 65Hz or so. Vented in 140L, these would get down to ~37Hz - which is deep enough, but it would be difficult to prevent vent chuffing.. 2 x 6" ports ~19" long would be recommended.

These things have awesome sensitivity. I think I may buy a pair. $56 has to be close to OEM cost for these woofers unless they are chinese junk.

You could also always do a 4th order bandpass.... Those have a lot of design flexibility.

[Frode]

On the forum at "Cult of the Infinitely Baffled" a 4ohm version is mentioned:

Hi Everyone,

If this is out of line for the board, then please remove it with my apologies...

Due to peer pressure from the car audio world, a 4 Ohm version of the Mach 5 MJ-18 is currently being designed. Here are the prelimiary specs (please note that these may vary a bit):

Fs = 24.3Hz
Re = 3.2 Ohm
Qts = 0.33
Qes = 0.35
Qms = 4.99
Mms = 310.8g
Cms = 0.14
VAS = 265l
Bl = 20.9
SPL = 92.2db

This may provide further design options to those who are thinking about using the MJ-18 in an IB config.

Best regards,
Mark

I have some 120L boxes that I would like to try these drivers in. They seems to work nice in this box size, both sealed and vented(Fb@20-25Hz).


Frode

[Joules]

fifty six bucks each - coool

Stereo isobaric line arrays !!!

[moray james]

rather than making the suspension stiffer (hence the idea to lighten the cone) I would need to make the diaphragm heavier. Which would also lower the Fs. but eat up some efficiency.
From all that I can gather on these drivers they are built like tanks and only suffer minimal air noise on the back side (am told this is low enough not to be heard when playing music). Yes these ought to rock in a dual "W"dipole. Dont know what the Le is and if too high might cause some concern but not if you did not want to run them up too high in frequency. Here is the link http://www.mach5audio.com/index.php?...4eeb788dd9d5b3

In the dictionary under loss leader there is a link to this company. Suggest you buy some before they get tired of loosing money on these, they are bringing in a 15 inch unit as well. Take a look at the little 6.5 inch for $13.00 wow.
regards Moray James.

[AndrewT]

Why are bargains like these never available in the UK?