Originally posted over on SMD to answer a question - but I figured some folks over here may enjoy it as well :

I need Bl/Bl Curve Explained - SMD Forum

You will typically need a Klippel or DUMAX to measure the BL curve.

You can assumme, though, that most overhung (long coils) at quite parabolic in nature... most under-hung have a flat spot in the center and then become parabolic.

There are ways around this... Alpine flattens it out nicely in the new Type-R with some clever pole machine work:

They machined in a bell shape inside the pole that flattens out the BL curve in the center -- here is an image from their patent:

----------

Some other examples below.

First we have an overhung - the below is an approximation of the NS v.2 motor... very long gap / long coil design but still parabolic as it's overhung. As far as overhung goes the BL distortion is quite low as with the 71mm coil and 40mm top plate we achieve ~27mm linear one-way throw.

* You see some of the coil sits outside the flux field and the flux is centered in the gap at highest level.

-----

Next we have underhung - where you can immerse the coil in the peak flux field and it won't leave that area for some amount of stroke therefore giving you a small flat area before becoming parabolic.

* One issue is the tall top plate and short coil tends to make the motors weak as there is poor flux utilization unless you use a giant magnet.

-----

Now we have something really fancy called XBL^2 -- you have two gaps and the flux is steered around to immerse more coil than underhung and it also forces the flux out farther to extend the flat area of the BL curve.

* Not the greatest example for SQ purposes as it's a big Neo SPL woofer... but you can see how the flux is broken up into two peak concentrations (dark blue areas) with some of the coil in each of those areas. The middle is no longer the strongest spot. As the bottom of the coil leaves the lower flux concentration it equally enters the top concentration so BL does not change for quite some time. These gaps can be stacked if necessary to achieve TONS of flat BL.

-----

For the technical folks out there... I'm not attempting to fully explain any and all topologies -- just giving people a general idea of how they work. So lets keep the nit picking to yourselves. Feel free to add information if you like, though !

Edited January 27, 201213 yr by sundownz

Most car audio woofers utilize over-hung for the simple fact that the coils are longer and typically handle much more abuse for that reason. A good overhung driver can also sound quite good despite have a higher THD than a flatter BL design.

Many even prefer the sound generated by a driver with a parabolic BL curve as the parameter shift over excursion makes the sound "punchier" as excursion increases and the BL drops off, Qts rises, and your box gets a bigger peak in response which is often interpreted as "punch"

Objectively, though, a well designed under-hung or XBL woofer (or any woofer with BL flattening tricks) will have lower distortion and is more accurate to the source... a drop off of 30% of the BL typically equates to 10% THD so we at Sundown and many other companies list x-max as the 70% BL point.

I don't have the tools to show the graphs but you can also do "split coil" which results in a graph alot like XBL^2 but with alot bigger coil and more thermal power handling.

You actually have two separate coil windings on the former.

This has been utilized in several car audio drivers as well.

All designs have their advantages and draw-backs which include but are not limited to the depth of the driver.

Over-hung and split-coil designs tend to be the deepest designs -- so aren't as well suited for super shallow drivers.

Under-hung can be quite shallow as you don't need alot of clearance behind the gap for the coil as BL drops off so fast afterward and the coils are short.

XBL^2 tends to have the highest excursion to depth ratio possible -- I am not aware of a way to create more x-max with less depth.

Wow, very informative. Thank you Jacob! I thought I understood all that before, I really do now. and without feeling after reading it.

Very informative! Thanks for all the information.

This is a great link to add to the discussion of BL and Bl curves.

http://www.soundsolutionsaudio.com/forum/topic/9421-an-unbiased-comparison-of-linear-bl-technologies/

He actually posted that question on this forum aswell

Link here:

What program do you use Jake?

What program do you use Jake?

Femm to generate the flux concentration.

I'm going how stuff works stage. I'm researching everything I can at the moment.

Maybe after CES, I can ride up and hang with you for a day, and you can show me some stuff.