how to lower the Fs of a driver - 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 2nd November 2005, 01:04 PM   #1
Puggie is offline Puggie  United Kingdom
diyAudio Member
 
Join Date: Oct 2004
Default how to lower the Fs of a driver

I'm looking for subs to use in a aperiodic/infinite baffle setup in a car (see my other thread on here). I've found a couple of options all with Fs in the mid to high 30s and I'd really like to get it down to below 27Hz.

So could I take a PA driver with dual spiders and modify the top spider by cutting sections or the entire top spider away to reduce the suspension stiffness? would this have the desired effect of lowering Fs? I presume it would also reduce Qm and I have no idea how it would affect Vas etc.

any ideas guys and girls???

p.s. I'm looking at a sub with relatively affordable re-cones
  Reply With Quote
Old 2nd November 2005, 03:57 PM   #2
diyAudio Member
 
Join Date: Jan 2005
Location: Rotterdam, NL
Add weight to the cone.

Wkr Johan
  Reply With Quote
Old 2nd November 2005, 04:07 PM   #3
diyAudio Member
 
Join Date: Aug 2003
Location: Santa Cruz, California
Are you really sure you want to do this? I would highly recommend using a Linkwitz transform instead, since what you're really after is a lower F3.


Cheers,
Francois.
  Reply With Quote
Old 2nd November 2005, 04:49 PM   #4
Puggie is offline Puggie  United Kingdom
diyAudio Member
 
Join Date: Oct 2004
Ahhh nice thought on the linkwitz but currently I'm trying to do everything passively or mechanically. Agreed, more mass on the cone will lower Fs but won't that also slow the sub down and make its transient response a bit 'loose'?
  Reply With Quote
Old 3rd November 2005, 02:32 AM   #5
GM is offline GM  United States
diyAudio Member
 
GM's Avatar
 
Join Date: Jun 2003
Location: Chamblee, Ga.
Greets!

Only if it raises Qts too much:

mr = (Fs/Fs')^2

Qes' = Qes*mr^0.5

Qms' = Qms*mr^0.5

then: Qts' = Qes'*Qms'/(Qes'+Qms')

where:

mr = mass ratio
Fs' = new, lower Fs

GM
__________________
Loud is Beautiful if it's Clean! As always though, the usual disclaimers apply to this post's contents.
  Reply With Quote
Old 3rd November 2005, 04:32 AM   #6
diyAudio Member
 
Join Date: Aug 2003
Location: Santa Cruz, California
Quote:
Originally posted by Puggie
Ahhh nice thought on the linkwitz but currently I'm trying to do everything passively or mechanically. Agreed, more mass on the cone will lower Fs but won't that also slow the sub down and make its transient response a bit 'loose'?
I bring it up because you may not like the effects of modifying your driver, particularly since modern woofers tend to have heavy cones so cutting slits in the spider will likely cause it to fail in fairly short order. Driver compliance has little effect on F3 in most boxes designed to fit in cars, anyway, since Vab tends to be smaller than Vas and thus dominates the equation Vaeffective = (Vas*Vab)/(Vas+Vab). Vaeff is what forms a resonant circuit with the equivalent mass (can't remember the name right now), ergo Fb = 1/sqrt(Vaeff*M). With Vab = Vas(original) and increasing Vas by a factor of 2 by cutting a _lot_ of slits in the spider, you get Vaeff going from Vas * 0.5 to Vas * 0.67, but that reduces your box frequency by only sqrt(0.5/0.67), or about 0.86.

In other words, the probability of destroying the driver is increased by quite a lot to gain only a sixth of an octave in low-end cutoff. In actuality, you don't even get that much since increasing Vaeff by that much also decreases the cabinet Q, so it starts rolling off a bit earlier.

Adding mass onto the cone comes with its own set of problems, increased stress on the suspension being one, and reduced efficiency being the other. You could derive the loss from the math, I suppose, but it's simpler to think of the voice coil/magnet assembly as a motor of known strength which now has to push a greater mass - it simply won't move it as far, hence the cone will produce less sound.

Long story short: you could spend a lot of time messing around with razor blades and plasticine, or you could short-circuit a bunch of grief and use the Linkwitz Transform before, instead of after, killing a couple of drivers. It's your choice.


Francois.
  Reply With Quote
Old 3rd November 2005, 09:39 AM   #7
Puggie is offline Puggie  United Kingdom
diyAudio Member
 
Join Date: Oct 2004
Thanks for the input guys looks like I'm going to have to do some math!

Just to clear a few points up, the driver in question is a 15" PA driver with relatively low moving mass (so increasing mass even by only a small ammount could have a pretty profound effect)

you talk about 'most boxes designed to fit into car, this isn't your typical install, the drivers will be using the entire boot/trunk as the enclosure with the cabin and trunk entirely sealed off from each other. I will also be using aperiodic mats to restrict the airflow to limit cone excursion.

So looking at the Mass Ratio theory:

current Fs = 39Hz,
aimed Fs = 25Hz

(39/25)^2 =2.43

the current moving mass is 98g so I would need to increase it to 238g, now that sounds like a lot to me!

Doh, I'm going to have to go and do some maths, if anyone else feels like having a play, the two drivers I'm currently looking at are:
Volt R3825 (www.voltloudspeakers.co.uk)
and
Precision devices PD1550 (http://www.precision-devices.com/showdetails.asp?id=14)

Thanks guys.
  Reply With Quote
Old 3rd November 2005, 09:47 AM   #8
forr is offline forr  France
diyAudio Member
 
forr's Avatar
 
Join Date: Dec 2004
Location: Next door
Have a look to Stahl's works to see how you can electronically modify the mechanic parameters of a loudspeaker. More dependable than a Linkwitz transform because it is less an "a priori" correction.

~~~~~~ Forr

  Reply With Quote
Old 3rd November 2005, 10:00 AM   #9
diyAudio Member
 
Join Date: May 2002
Location: Switzerland
Increasing the mass of a driver will definitely lower its efficiency (which is a function of fs^3) over its whole passband.
Special tunings combined with the appropriate electronics only affect the efficiency at the lower end.

LTF is recommended for closed boxes.

The ACE method by Stahl is recommended for vented alignments but you should only attempt it if you have enough patience and experience.

Just my $0.02

Regards

Charles
  Reply With Quote
Old 3rd November 2005, 12:48 PM   #10
forr is offline forr  France
diyAudio Member
 
forr's Avatar
 
Join Date: Dec 2004
Location: Next door
Stahl's method changes the apparent parameters of the driver, so it can be applied to any kind of enclosures.
The trade for a lower frequency is of course a need for more power, however under 50 Hz, power distribution of the most demanding records falls at a 6 dB/o rate, so it is not as much as firstly thought.

~~~~~~~ Forr

  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
Best lower midrange ? dkxdn Multi-Way 14 5th February 2009 10:45 AM
Lower rails for lower impedance? 95Honda Solid State 7 19th December 2007 09:59 AM
What Driver Is Best From 800Hz & Lower? thetubeguy1954 Multi-Way 12 30th August 2007 07:09 PM
L-Lower? oliverh Pass Labs 12 23rd August 2006 05:25 PM
Modifying a driver for lower Fs DeonC Multi-Way 4 22nd April 2004 04:03 AM


New To Site? Need Help?

All times are GMT. The time now is 09:33 AM.


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