[musiclear]

I've heard of a couple ways to lower FS.  To increase mass which really lowers efficiency and takes a lot of the benefit out of it, and raising CMS by reducing the compliance of the suspension which seems to work pretty well in theory.....I think.

My purpose is to lower fs and lower tuning frequency which increased efficiency of my system in lowest octave.  Some modeling I did suggests that in an application I am looking at, lowering the fs of my driver from 25hz to 20hz and tuning appropriately, I would gain about 3db at 24hz which is equivalent to doubling the power needed to produce that frequency. 
It seems like a pretty good trade off.  Reduce the total power the driver can take by reaching xmax with less power, but producing more of the frequency I want with less power.

Has anyone any experience with lowering the fs of their drivers or like to add to my question? 

[Davey]

How do you plan on doing that?  Change the surround and/or spider?

Dave.

[musiclear]

Well that is really the question.  There have been some suggestions that the spider doesn't add much tension in a lot of drivers and by changing that, you don't change much.  It wold take a complete rebuild also.  The surround of the drivers I am thinking look to be rather tough and changing those may make much more of a difference.

Honestly, I have some theory, but very little actual practical info at this point.  I'm hoping to find someone who has some experience to guide me with what I can expect.

I would really love to lower the FS on these drivers from 25hz-20 hz or so.  It looks like with a lower resonant frequency and retuning the box, I could pick up a lot of efficinecy around 20hz.  I really don't know how procticle that would be in reality though. 

I was also looking at potencially adding a little mass, but it looks like the reduction in efficiency eats up any gain in low end extension. 

Any suggestions?

[Russell Dawkins]

I've heard of a couple ways to lower FS.  To increase mass which really lowers efficiency and takes a lot of the benefit out of it, and raising CMS by reducing the compliance of the suspension which seems to work pretty well in theory.....I think.

A technical detail - you are talking about reducing the compliance; you need to increase the compliance to lower the Fs. More compliant is freer-moving, less compliant is stiffer.

[JohnR]

What driver is it?

[musiclear]

Thanks for that correction Russel.  I knew what I meant and didn't mean what I said.  You are correct and what I meant was to increase compliance by making the suspension more forgiving or less tense.

I understand this will reduce the power capacity or the drivers, but the drivers I am thinking about and at the SPL I am looking for, this shouldn't be an issue.

I've got a pair of Dayton RSS265Hf's in a 1.75cf box tuned to 25 Hz and these do sound very nice. 

My room is large though and I could use additional SD to really fill the room with that 1st octave.  I do have a couple 12in Lambda Acoustics in 3cf boxes which really are spectacular drivers, but I am hoping to simplify and reduce the foot print of the system by adding a second 10" driver to the main speaker box making it larger to accommodate the second driver and give more efficiency at 20Hz.

Down size and still get that 1st octave.  Not to much to ask? 

All that seems to mean I need reduce the FS and tune the box appropriately.  Otherwise, the tuning frequency is just to high and I won't get the efficiency at 20Hz I want. 

Also, it looks like as I tune lower and lower, the delay gets better and better and I can imagine that might be audibly tighter bass.  Just a guess though.

[nickd]

I think you may be under estimating the the resistance to movement the spider contributes. In my opinion it will be much easier finding a replacement driver than changing existing driver suspension.

Is I understand it as compliance increases the driver is more likely to favor a sealed box. There is still tuning to be done, but not by adjusting the vent parameters.

[musiclear]

You may be right about that.  I am rather new to thinking along these lines.  From what I understand, some speakers have rather heavy resistant spiders and others have rather flimsy spiders. 

I would imagine that since the Dayton driver can be pushed with so much power, it may have a pretty stiff spider. 

If so, then from what you seem to be saying, by reducing the stiffness of the spider, the compliance would go up and the FS would go down.  Also, I think you mentioned the Q would go up and that might make it less desirable for the ported box.

Does anybody have a formula that could be used to model what these changes would do to driver parameters?

It was mentioned in other posts that I read that some have cut slices in the spider to relieve stiffness and increased CMS.  I am interested in potentially trying it, but am hoping to be able to some what predict whether the out come would be beneficial or not.

Again, I am hoping to lower FS enough to tune the box lower and pick up more efficiency around 20hz with out giving up system efficiency.  Is this even a potential reality?

[*Scotty*]

As it is right now the woofer responds in a predictable symmetrical manner with low distortion. It will be almost impossible to alter the spider in such a way as to raise the woofers compliance without introducing non-linearity into its motion. Any differences in compliance around the circumference of the spider will cause the voice-coil to become off-centered in the course of its travel raising the distortion.
 The due to constant flexure the spider may deteriorate near the slits or holes and fail completely necessitating an expensive replacement.
I would try lengthening the the port which would lower the system tuning frequency. You aren't really trying to lower the the 3dB down point that much. This experiment could get you where you want to be and it's a reversible procedure.
Scotty

[musiclear]

Well Scotty,  I imagine you are probably right.  A disappointment, but I understand there is a lot of torque and tension keeping all that energy moving in a straight line and I guess the spider is an important alignment tool.

What do you say about the potential of using a more compliant surround?  There is more mechanical advantage to changing the surround?

My mind suggests to me that part of the reason these drivers have such a stiff suspension is to be able to increase power ratings.  Maybe not so. 

Maybe they went out to design the driver they did and they happened to get a driver with lots of power tolerance.

Still, I am not sure if I were to make the driver have more compliance and have a lower FS, just how much lower the FS would go and if other changes in driver parameters would negate the advantage of the lowered FS.

From some very preliminary modeling that lowering the fs from 25hz to 20 hz and tuning at 20 hz gives me an increase in efficiency at 20hz of 3db.  I can certainly add more power and E/Q, but I was hoping for a fun and moderately easy way of improving the driver for my desires even if I were to give up some power handling.

[*Scotty*]

You can still try tuning the enclosure to 20Hz and leaving the driver alone. This might get you where you want to be.
Scotty

[musiclear]

I hear you.  I took a look at building a bigger cabinet and tuning lower.  That will also get me the efficiency I am looking for.  I'll only be able to use one driver rather than two, and it just isn't as much fun as toying with drivers.  But what works works.

So I guess, unless someone has some great idea they want to share, I'm going back to basics and working with cabinets.  Thanks for all the input.