Bass driver Vas mods? - diyAudio
Go Back   Home > Forums > Loudspeakers > Subwoofers

Please consider donating to help us continue to serve you.

Ads on/off / Custom Title / More PMs / More album space / Advanced printing & mass image saving
Reply
 
Thread Tools Search this Thread
Old 15th October 2006, 03:31 PM   #1
diyAudio Member
 
Join Date: Aug 2003
Location: Calgary on the Bow
Default Bass driver Vas mods?

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.
__________________
moray james
  Reply With Quote
Old 15th October 2006, 03:53 PM   #2
AndrewT is offline AndrewT  Scotland
diyAudio Member
 
Join Date: Jul 2004
Location: Scottish Borders
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.
__________________
regards Andrew T.
  Reply With Quote
Old 15th October 2006, 04:27 PM   #3
diyAudio Member
 
Join Date: Apr 2004
Location: Costa Rica
Send a message via AIM to johninCR Send a message via MSN to johninCR Send a message via Yahoo to 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).
__________________
Everyone has a photographic memory. It's just that most are out of film.
  Reply With Quote
Old 15th October 2006, 04:41 PM   #4
Frode is offline Frode  Norway
diyAudio Member
 
Join Date: May 2003
Location: Bergen
I guess he is speaking of this driver . Buy two and use them in an isobaric setup.

Frode
  Reply With Quote
Old 15th October 2006, 04:49 PM   #5
diyAudio Moderator
 
pinkmouse's Avatar
 
Join Date: Apr 2002
Location: Chatham, England
Quote:
Originally posted by Frode
Buy two and use them in an isobaric setup.
Yup, that's what I'd do as well.
__________________
Rick: Oh Cliff / Sometimes it must be difficult not to feel as if / You really are a cliff / when fascists keep trying to push you over it! / Are they the lemmings / Or are you, Cliff? / Or are you Cliff?
  Reply With Quote
Old 15th October 2006, 06:11 PM   #6
Ron E is offline Ron E  United States
diyAudio Member
 
Ron E's Avatar
 
Join Date: Jun 2002
Location: USA, MN
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.
__________________
Our species needs, and deserves, a citizenry with minds wide awake and a basic understanding of how the world works. --Carl Sagan
Armaments, universal debt, and planned obsolescence--those are the three pillars of Western prosperity. Aldous Huxley
  Reply With Quote
Old 15th October 2006, 06:19 PM   #7
Frode is offline Frode  Norway
diyAudio Member
 
Join Date: May 2003
Location: Bergen
On the forum at "Cult of the Infinitely Baffled" a 4ohm version is mentioned:

Quote:
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
  Reply With Quote
Old 15th October 2006, 06:33 PM   #8
Joules is offline Joules  United States
diyAudio Member
 
Joules's Avatar
 
Join Date: Nov 2004
Location: Manassas virginia usa
fifty six bucks each - coool

Stereo isobaric line arrays !!!
  Reply With Quote
Old 15th October 2006, 07:17 PM   #9
diyAudio Member
 
Join Date: Aug 2003
Location: Calgary on the Bow
Default Ok I had it the wrong way around...

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.
__________________
moray james
  Reply With Quote
Old 15th October 2006, 07:56 PM   #10
AndrewT is offline AndrewT  Scotland
diyAudio Member
 
Join Date: Jul 2004
Location: Scottish Borders
Why are bargains like these never available in the UK?
__________________
regards Andrew T.
  Reply With Quote

Reply


Hide this!Advertise here!
Thread Tools Search this Thread
Search this Thread:

Advanced Search

Posting Rules
You may not post new threads
You may not post replies
You may not post attachments
You may not edit your posts

BB code is On
Smilies are On
[IMG] code is On
HTML code is Off
Trackbacks are Off
Pingbacks are Off
Refbacks are Off


Similar Threads
Thread Thread Starter Forum Replies Last Post
PA bass driver a007udio Multi-Way 6 31st May 2006 11:06 AM
Burgera Bass Driver LightwaveDude Instruments and Amps 17 15th March 2006 02:29 AM
bass amp and driver leander Solid State 4 3rd November 2005 12:53 PM
Getting the most bass out of a driver BAM Multi-Way 7 5th September 2002 04:43 AM
Looking for very good bass driver slicemaster101 Subwoofers 31 2nd September 2002 06:52 AM


New To Site? Need Help?

All times are GMT. The time now is 05:19 PM.


vBulletin Optimisation provided by vB Optimise (Pro) - vBulletin Mods & Addons Copyright © 2014 DragonByte Technologies Ltd.
Copyright 1999-2014 diyAudio

Content Relevant URLs by vBSEO 3.3.2