[Arlo]

I want to try something insane.  I want to stick this 10" driver in an open baffle:

http://www.faitalpro.com/products/schede/hps.php?id=201040110

Yes I know it has a Q of .2 and a high 60hz fs.  But...it has the highest force factor for a 10" that I've ever seen.  The mms is also reasonably low.  That powerful motor suggests tight and superquick sound. 

If I hook it up to a subwoofer amplifier, I should be OK because of the 9mm xmax.  And since the xmax is so high I'll bet the driver would stand EQ below resonance as well.

What do y'all think?  What are my potential pitfalls? 

[mcgsxr]

Welcome to the OB Circle here on AC, glad you found us, and glad you asked the question.

I think you called it with the 60Hz FS.  On OB, it won't play below that... so it would cut off rather high for my liking.

9mm XMAX is not that big either - have a peek at the DIY Cable and GR Research OB specific woofers if you are looking to find a good OB woofer, in my estimation.

Other options include pro audio choices such as the Eminence Alpha 15.

[Graham Maynard]

Hi Arlo,

What I notice is - Cms = 0.096 mm/N and the power rating is 700W.

This is a stiff driver and is going to sound not just tight but thin on an OB.

Probably will be fine and powerful with a sub though, so do let us know how you get on.

Cheers ......... Graham.

[John_E_Janowitz]

Yes I know it has a Q of .2 and a high 60hz fs.  But...it has the highest force factor for a 10" that I've ever seen.  The mms is also reasonably low.  That powerful motor suggests tight and superquick sound. 

To compare motor strength you need to look at Bl^2/Re. This puts this driver at around 120, which is fairly high, although not nearly the highest out there.  We have done drivers in the range of close to 200.  Motor strength alone also doesn't tell you a whole lot about a driver.  There are quite a few things to take into account for "tight and superquick sound" other than just the motor strength.  I have posted this link a few times but it is a good explanation of what the true "force factor" of a driver is and how inductance is the main issue in how "fast" a driver is:

http://web.archive.org/web/20010810141852/lambdacoustics.com/library/whitepapers/bl_mms.htm

If you want tight, accurate, low distortion sound you need to look at low inductance as one big issue. Other issues you need to look at are linear inductance, linearity in the BL curve and fixing the flux in the gap.   Our paper on the Lambda motor explains these issues:
http://www.aespeakers.com/Lambda001-1.php

If I hook it up to a subwoofer amplifier, I should be OK because of the 9mm xmax.  And since the xmax is so high I'll bet the driver would stand EQ below resonance as well.

I wouldn't consider 9mm Xmax to be extremely high.  Also, the way they rate it at 9mm is a little misleading.  The physical overhang is only 5mm.  They rate "Xmax = [(winding depth - magnetic gap depth) / 2] + (magnetic gap depth / 3)".  We could get 2-3mm more Xmax out of every driver we do that way, although it would not change the performance to rate them that way.  Looking at the Bl curve and point where BL falls to 70% of the rest value is about the most accurate way to go.  The 1w/1m sensitivity is also rated at 96dB but calculates out to only 93.6dB. 

That all said, if you plan to use it for a midrange you may be ok.  As a woofer you won't get very good levels.  For example look at 30hz.  You are displacement limited to about 100dB in an infinite baffle.  The driver's own response is -19dB at 30hz though, requiring that much EQ to go flat to that point.  That is in an infinite baffle though.  In an OB you are going to have the baffle to deal with as well.  From 100hz to 30hz your response will drop another 10dB.  This means your max excursion limited output is only about 90dB at 30hz.  It will also require over 300W to get that level and need 39dB of boost to compensate for the baffle and driver rolloff.

In general any kind of OB for woofer use will require large drivers(typically multiples) with soft suspension that require much lower power at the lowest frequencies.  For OB use, you actually do not want high motor strength in most cases.

John 

[Arlo]

Well ya put some time into that post.  I probably should have mentioned what I'm after.  Having experimented with some drivers I found that the high Q small magnet woofers were too sloppy for my tastes.  As I moved down to a Q approaching .5 I got bigger magnets and a tighter sound but lost output in OB.  So now I want to say teh heck with Q; I want the tightest quickest sound I can get in OB and I'll amplify it separately.  I've been looking at the Beyma SM115k.

Based on your info, the SM115k with an LE of 1.2 would be alot better than the parts express woofer here:  (with an LE of greater than 3)

http://www.parts-express.com/pe/showdetl.cfm?Partnumber=295-455

Heh I keep adding to this.  I looked at AE speakers.  Nice stuff.  I noticed the AV15 has an LE of only .2.  This might be a good choice.  I gotta say I don't like the look of the IB15.  It has some very OB-friendly specs but that magnet is tiny (cringe). 

[John_E_Janowitz]

Well ya put some time into that post.  I probably should have mentioned what I'm after.  Having experimented with some drivers I found that the high Q small magnet woofers were too sloppy for my tastes.  As I moved down to a Q approaching .5 I got bigger magnets and a tighter sound but lost output in OB.  So now I want to say teh heck with Q; I want the tightest quickest sound I can get in OB and I'll amplify it separately.  I've been looking at the Beyma SM115k.

Couple things to note here.  Most importantly Q changes with excursion.  The farther you go from center, the less flux is reaching the coil.  A poorly done driver could have a Q of .7 at it's rest position, but by 10mm of travel could have a Q of 3.0.  Then add in the huge variation in inductance and you have some serious problems.  A lot of this is what you will experience in the drivers like the Alpha15 with very low Xmax and very high Q to begin with.  Compare that to our Dipole15 which has a Q of .94 at rest value with nearly perfectly flat Bl curve out to 10mm.  At 10mm the Q is still right around .94 and due to the copper sleeve, inductance is also the same as at rest.  The other huge factor in making something sound "tight" is to keep the flux from moving. 

The best way I can describe a magnetic circuit with flux moving is by an illustration.  Look at a propeller on a boat.  If you are going backwards and shift gears to forward, the propeller instantly changes direction.   The same goes for the current flow in the VC of a driver.  However, the boat doesn't instantly change direction until the water flow does.  This is much the same as the flux movement due to the current in the coil.  The copper sleeve keeps the flux from moving, drastically helping with how quickly the coil reacts.  We just replaced a bunch of B&C 18TBX100's for Elite Audio with our new TD18H's.  While motor strength is nearly identical as well as the driver Q, there is a significant difference in how they sound.  After the New Years Eve show that Elite did it was mentioned that the TD18H were "too accurate to be subwoofers."

Heh I keep adding to this.  I looked at AE speakers.  Nice stuff.  I noticed the AV15 has an LE of only .2.  This might be a good choice.  I gotta say I don't like the look of the IB15.  It has some very OB-friendly specs but that magnet is tiny (cringe). 

The issue isn't only the magnet size, or even the Q as mentioned before.  For one, I can take the same size magnet on the IB15's and end up with a Qts of .2 if I wanted.  It's all about balancing the parameters, and again keeping those parameters linear over their intended range.  How tight the gap is around the coil is critical also.  With 18.5mm Xmax, this means that you have a lot of travel before the Q can go to an unreasonably high level.  It also has the full copper sleeve on the pole as mentioned above to keep the flux fixed where it is supposed to be. 

John