BL motor strength and performance

[haydenlake 0]

It seems to me like BL (typically measured in Tesla meters) would be a good indicator of motor performance and overall subwoofer ability.

With that said, I am supprised that numerous woofers that people commonly call, good strong subs have a fairly low BL in comp to other subs.

Subjectively strong subs (by overall consumer babble)

Image Dynamics IDMax Bl about 10

Jl Audio W7 series BL about 10

MTX 9500 series BL about 10-20 depending on model

SS RL-P BL over 20

So is the SS motor twice as strong as a motor rated at 10TM, or am I completely off base.

How do BL and RE interact, it seems that BL would be a constant bc its a measurement of the magnet strength (which shouldn't change) and RE is what can manipulate BL (is this exagerated by a DVC since you can manipulate the RE)

any help would be appreciated.

[Nikuk 1]

Short version:

BL = B x L

B = force produced by flux.

L = length of wire.

BL is a measure of motor strength and is equivilant to the result of: length of wire in the magnet field multiplied by the field density.

BL = 9.8(Ma) / i. Whereas "i" = current applied & "Ma" is known mass.

Basically, there is no way for a motor to have a constant BL. If You've ever seen FEA software for magnetic flux & gaussation, then You could see how the field moves as the coil moves. There is ALOT of science in the motor as far as pole design, shorted turns, faraday rings, top & bottom plate material and thickness

There are MANY more factors that go into a drivers design that affect overall accuracy, tonality, sensitivity, output, etc...

That's actually about as short as I can make this post. Sorry.

I would HIGHLY suggest that You pick up a book called "LoudSpeaker Design Cookbook" by Vance Dickason. You'll learn more then You'll ever need to know about the electromagnetic transducer and it's design variations.

-Nick

[Steven Kephart 0]

High BL is not necessarily a good thing. You want just as much BL as needed, and that's it. To understand why, you must understand what happens to the parameters when you change one thing like BL. Here's a quote Dan Wiggins once posted that explains how they interact with eachother:

"Vb is proportional to Vas (double Vas, double Vb). Vb is proportional to Qes squared (halve Qes, quarter Vb). This is for the same Qtc/alignment... And since Qes is inversely proportional to BL squared, if you double BL you cut Qes by a factor of 4, meaning the box volume for a given alignment is cut by a factor of 8.

Now, that is all well and good, but consider what Fb does in relation to Qts/Qes... Fb is inversely proportional to Qts, thus as you raise the BL, and lower the Qts, you raise the Fb accordingly. Double BL, cut Qts by a factor of 4, meaning Fb just jumped up 2 octaves!"

As for BL and Re, when you double Re you double BL. So two subs with the same BL and different Re's have different motor forces. That's how they interact. I'll try to get more information to explain why.

I hope that helps.

Steven Kephart

Adire Audio

[mrray13 30]

yeah..after reading that..i gots a headache..time for a beer...lol

but for real, some serious info in these last two posts...

wheeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeee

[Jmac 24]

Makes perfect sense to me ... and I dislike beer, so I'll have a coke

Motor Force = BL^2/Re

So 7.071 Tm @ 1 ohm = 10 Tm @ 2 ohms = 14.142 Tm @ 4 ohms = 20 Tm @ 8 ohms

[Steven Kephart 0]

Ok, BL is made up of the B field; the magnetic flux effected by the coil, and L which is the number of turns (length of wire) effected by the magnetic flux. When you change the number of turns (changing BL), you change Re.

Now on to dual voice coils. Think of each voice coil as seperate. Each voice coil has it's own Re that will multiply by two when seriesed and divide by two when paralleled. Well BL is effected because when in series there is twice the length of wire, therefore higher BL. In Parallel you in essence double the thickness of the wire.

Steven Kephart

Adire Audio

[JimJ 11]

This actually makes sense to me...good job, guys.

[mrray13 30]

yep...it's getting pretty informational in here...lol...

wheeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeee

[req 0]

wow that was pretty good info by some pretty intelegent members.

its late. and iprobably cant spell ;p

[dave_Edwards 1]

Steven---glad to have you here.