I need Bl/Bl Curve Explained

[sundownz]

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

[sundownz]

Of course feel free to make constructive additions to the information!

[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.

[sundownz]

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.

[sundownz]

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.

[NDMstang65]

It isn't just "BL" it's B..and L..

"B" is what it is,it is the magnetic strength of the structure.

"L" is the length of the wire on the coil.

A speaker is no different then an electric motor..if you do not have wire or winds there you do not have force for the motor to exert energy to. You can't have an alternator spin and make power if it was just a magnet spinning in there. You have to have wire (a pre-determined amount for a given application of course) in order to have that. If you look at the graph posted above you should see "BL vs. Position" which means this is what is going on (in theory) inside of the motor...but this is only good for reference as there are MANY more things going on inside of these magnets and coil assemblies then many people can even grasp. Flux modulation, saturation, flux compression and shifting of where "center" is or isn't...then you have other things like back EMF. All bets are off once an electron is generated with the wire going up or down in the gap with a signal on it...but it does give you a reference point.

Anyhow...you've got other cool stuff like split coil and multiple gap designs that don't suffer from the anemic dead/dry/lifeless fish sound as other topologies do as you are not getting rid of steel and messing with flux flow...you are just taking turns out of the gap for it to act on. Personally, I can't stand anemic sounding subwoofers...it's great for mids and tweeters as it makes it ultra compact and gets rid of peaks in response.

[sundownz]

Good information Nick -- flux saturation, modulation, and compression are not typically understood or talked about at all.

Do you have a BL chart for split coil ? Would be nice to have one for most of the designs in the thread for reference purposes.

[NDMstang65]

Not on this machine no....I'd have to look around for some.

Poke around in the JBL Differential Drive patent or on their website, there should be a decent explanation in there of what split coil is and does...granted their stuff is different, but it'll get the point across.

[sundownz]

Since Nick brought up Flux Compression here is a bit on that... this is a quote from Dan Wiggins :

"One thing most people overlook is the impact of inductance on the actual,

dynamic B field of the motor. The total B field is modulated by the field

set up by the voice coil; this is called flux compression and is a real

issue, especially with VERY high powered voice coils. The way to reduce the

field of the voice coil is to lower its inductance! Inductance is

essentially a measure of the magnetic field strength of a coil, so if you

can lower the inductance you can lower flux compression, and up the dynamic

B field in the motor.

One way to do this is to make a shorter, fewer turn voice coil. The other,

of course, is with shorting rings. Thick copper rings above and below the

gap will help - a lot!"

---

We have integrated large shorting rings into our Z v.2 and NS v.2 drivers to help combat this effect -- especially important with the NS v.2 as it has a huge voice coil.

FI also now offers shorting rings as well in their drivers called the "Heat Ring" -- as they also act as a heat-sink. Combined with the axial cooling vents on their drivers it is also quite effective as a heat-sink since all of the hot air in the motor will be pushed over the ring.

[Rogers]

wow this is great info... i could definitely read about the technical shit all day... but i might add that its good to have someone there to draw it out in Crayons for me...

Ive always looked at woofer specs and have been baffled on what each thing means, what it does, and what its related to as far as the other parameters... all i know is that when i was ready to buy new woofers i compared a few that were in the same power category and Found that the Sa-12 looked like it was king... so i bought a couple...lol