Just curious: is “l” in a voice coil’s Bl the unwound length of wire or the solenoid length as shown here?
The "L" in the BL product of a voice coil in a loudspeaker, stands for the total length, in meters, of the voice coil where the magnetic field of the permanent magnet is working on.
So, a voice coil may be e.g. 10 meters long, but in an overhung voice coil, not all of that is in the air gap where the field is strong, so the effective Length is e.g. maybe half of the total winding length.
Does this answer your question?
So, a voice coil may be e.g. 10 meters long, but in an overhung voice coil, not all of that is in the air gap where the field is strong, so the effective Length is e.g. maybe half of the total winding length.
Does this answer your question?
I think it's better if the OP asks either a question about solenoids, or about voice coils, because now both answers look oke;-)
Thank you. Follow up question:
Does that mean number of windings (N), wire gauge and number of layers do not affect the Motor force factor, since none of those affect B or L?
Does that mean number of windings (N), wire gauge and number of layers do not affect the Motor force factor, since none of those affect B or L?
Thanks for the answer but I’m still trying to understand this.
B is the magnetic flux density. Webers/m^2. That is the magnet’s contribution to BL. That makes sense.
What is L?
Let’s assume an underhung voice coil.
Is L like getting out a tape measure and seeing how long the (wound) voice coil is? Then wouldn’t a 1-inch long 25-awg voice coil have the same L as a 1-inch long 15-awg voice coil? (both measure 1 inch)
And wouldn’t a 1-inch long single-layer VC have the same L as a four-layer VC? (Both measure 1 inch on the tape measure).
Or is L the (tape measure length * # of layers)? Or something else?
Is there a formula for L or a diagram I could see?
B is the magnetic flux density. Webers/m^2. That is the magnet’s contribution to BL. That makes sense.
What is L?
Let’s assume an underhung voice coil.
Is L like getting out a tape measure and seeing how long the (wound) voice coil is? Then wouldn’t a 1-inch long 25-awg voice coil have the same L as a 1-inch long 15-awg voice coil? (both measure 1 inch)
And wouldn’t a 1-inch long single-layer VC have the same L as a four-layer VC? (Both measure 1 inch on the tape measure).
Or is L the (tape measure length * # of layers)? Or something else?
Is there a formula for L or a diagram I could see?
Not sure some of the above explanations are correct, "L" here is the unwound wire length.
The voice coil + magnet system is an electric motor, which does a mechanical job pushing/pulling a load: the cone + air.
And B is the magnetic field strength.
That said, if coil is longer than the gap, and now I am speaking about the winding length, not the wire one, wire which is outside it will still contribute to BL but in a much reduced way, because B drops like a rock outside.
Yet some is still present.
The true answer needs to integrate BL value along the full winding length,and in fact that is what you get, in an indirect way, when measuring TS parameters.
The voice coil + magnet system is an electric motor, which does a mechanical job pushing/pulling a load: the cone + air.
And B is the magnetic field strength.
That said, if coil is longer than the gap, and now I am speaking about the winding length, not the wire one, wire which is outside it will still contribute to BL but in a much reduced way, because B drops like a rock outside.
Yet some is still present.
The true answer needs to integrate BL value along the full winding length,and in fact that is what you get, in an indirect way, when measuring TS parameters.
To include latest questions:
Again, what matters is wire length, call it unwound wire length if you wish for clarity.
The parameters you mention will have influence as much as they affect wire length.
Available space inside a gap is VERY limited (and that´s an understatement) so you can´t randomly change one or the other at will, also remember you must meet an impedance spec.
Practical example: my typical high power Guitar speaker voice coil is:
- 2 layer (99.99% of all VCs are so)
- nominal 0.20mm wire diameter, so actual 0.22 mm occupied, because of enamel and adhesives,plus windding is not that tight.
- 10mm winding length so 2 layers of 45 turns each
- 2" former/bobbin diameter so 52 mm
- so average turn length is Pi*52mm=163 mm , round it up to 16.5 cm
- 90 turns of 16.5 cm each=1485 cm, round it up to 15 meters wire length.
As you see, winding length and wire length are absolutely different.
This recipe gives me a nominal 8 ohm VC; now IF as you mention,you:
*change wire diameter: small changes have a HUGE influence on impedance 😱 , an apparently disproportionate difference, reason is impedance varies with the CUBE of wire diameter, go figure.
I´ll let you figure the Geometric reasons for that 😉
*change VC former diameter: you´ll need to recalculate everything to keep impedance about the same.
* change winding length: assuming you keep gap (top plate thickness) the same, you will get either a "maximum efficiency" speaker (hint: Guitar or some PA) with a "same length" VC, "waste" some expensive to get "B" with an underhung one, "waste" a little with a long VC, simply because some wire will be outside.
As you see, speaker motor design is not difficult, BUT you must choose between tons of contradicting options,there is always something to sacrifice.
Pick your poison 😉
Oh, about 4 layers: apparently you get "something for nothing" ?
Not so, by far.
You nominally duplicate "L", which is "good" ... BUT:
- gap must be almost twice as wide, dramatically dropping B for the same magnet.
- Unless you are happy with duplicating speaker Z , you must use thicker wire, which means less turns but "almost" twice heavier coil.
Imagine same truck with twice the load in the back, and a weaker engine.
That´s why this is only "acceptable" in Subwoofers, specially in those clumsy Car type ones (ugh!!!)
Not sure its clear yet after all the posts, but speaker force factor is related to the amount of coil covered by the gap height only, so you need to consider the number of turns in there times the diameter of the coil. That's the important length for calculating BL. There will of course be some "stray" additional flux acting on the coil outside the gap, especially for extended pole types, but for paper calculations it's all about the gap height.