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wonderfulaudio 16th September 2020 06:28 AM

Does adding mass to a voice coil reduce its BL?

Does adding mass to a voice coil reduce its BL?

I was looking at T/S parameters of some of the competition sub-woofers and noted that though some of them have huge magnets, but they dont have particularly high BL, I mean they do have higher BL than non competition subs but its not that much high. In order to make the sub in a smaller box these competition subs have high moving mass and that raised a doubt: is the added mass somehow reducing the BL?

Thanks and Regards,

TBTL 16th September 2020 06:42 AM

No, force factor BL is not coupled to cone mass.
BL means: the product of the magnetic field flux density (B) inside the voice coil gap and the length (L) of the voice coil wire that is submerged in the gap.

One reason why car audio woofers can have a low BL is because they have a low impedance. A low impedance means thick voice coil wire, resulting in a low L. This does not mean that the motor is weak, as currents will be higher, resulting in the same force for a given nominal power. If you want to compare motor strenght, divide BL by sqrt(impedance).

bentoronto 16th September 2020 09:45 AM

Would it be correct to say that some force (just the sqrt) is gained by lower impedance but more is gained by longer (and lighter) wire in the gap?

The design of "competition" woofers (and extreme power handling speakers in general) results in designs that seem at odds with home audiophile purposes.


TBTL 16th September 2020 12:57 PM

Force is simply current multiplied by BL. It does not depend on wire mass or wire diameter.

Higher impedance woofers need a higher BL product for similar performance (same Thiele-Small parameters like efficiency and Qes). One approach is that a higher wire resistance results in less current at a given power, therefore a lower force and therefore a lower efficiency. Another approach is that the higher wire resistance acts as a series resistor, which reduces electrical damping and increases Qes.

bentoronto 16th September 2020 04:02 PM

I can't claim to know the driver model but I think you are talking about "holding some variables the same" and I am asking about alternative design concepts.

When you use conventional audiophile fine wire, you have small magnetic gaps, lots of turns, low mass for the cone assembly, and long lengths of wire in the gap.

That seems a whole lot better than gross wire (that can handle 2000 watts without melting), big gaps, heavy weight, and therefore need for giant magnets to get adequate force. Which might be OK for somebody's car audio "competition" but hardly suitable quality for home audio.*

* maybe true horns could use such a driver in a pinch

chris661 16th September 2020 04:49 PM

Ben, as always you show your prejudices.

Varying the wire thickness allows the manufacturer to alter the impedance of a coil. It's that simple.

Which is better: four layers of thin wire, or two layers of wire twice as thick? We can see the former will have higher BL, but also higher Re. That means it requires higher operating voltages (and less current) to produce the same amount of force as the latter motor.

The magnetic gap width would be equal, the moving mass would also be equal, and I dare say the long-term power handling would be very similar, too.

You've run with the false assumption that the only reason for thicker wires would be to increase power handling. As I've explained above, that simply isn't the case.

Further, I'd like to point out that there's nothing inherently wrong with using high-power drivers. At sensible levels, they'll be loafing along. I've shown elsewhere that drivers don't appear to be non-linear when operated at very low levels, so there really is no harm in that direction.


bentoronto 16th September 2020 05:20 PM

"Ben, as always you show your prejudices." = chriss661, that kind of free-floating nastiness has no place in this forum. I hope you will apologize.

You seem to be dismissive of the math. As TBTL pointed out (and I repeat based on his authority in post #3) the force increases at the square-root of current while the BL is linear proportional. Can somebody shed more light?

In general and as we see with portable power tools, cars, lawnmowers, power distribution networks, and maybe too audio amps, higher voltage tends to be better than lower voltage for the familiar Ohm's Law reasons. Perhaps stuck with 12vdc in gasoline cars, "competition" audio amps just love high-current designs - which seems pretty obvious when I finally think of it.

At basis is the old Lotus versus Corvette debate. As a previous Lotus owner, I know where I stand on light-and-nimble versus weight-added-to-weight. Perhaps if you drove a Lotus much you'll think the same. Is this just a matter of taste or something more?


LORDSANSUI 16th September 2020 06:43 PM

Thermal losses in electricity is proportional to current ^2, so probably, comparing two driver with the same power handling, the one with less current but with high voltage will have less thermal compression, and regarding the standard used to classify the long term power they probably will not be classified as the same power rating.

chris661 16th September 2020 06:49 PM

Ben, TBTL pointed out that force is proportional to current, which is correct. Having gone over the equations regarding magnetic motor force, resistivity, etc, I can't find any reason there ought to be a square-root in there.

Looking at the equations,

F=BIL. Force is equal to the product of the magnetic field strength, current, and the length of wire in the gap.

We know I=V/R, which is an arrangement of Ohm's Law.
From the resistivity equation, R=pL/A, where p = the resistivity of the coil winding material, L is the length of the wire, and A is the cross-sectional area of the wire.

We can assemble those to get F=BVA/p, which is interesting.

If we keep the volume of wire constant, then the mass of the wire also stays constant (I'm ignoring any extra enamel etc that might be needed).

We can see, then, that the force of the coil is proportional to the length of the coil, and also the area of the wire.
ie, if we double the length of the wire and then halve the area (keeps the mass of wire constant), then the force will be exactly the same. The DC resistance would change by a factor of four.

I didn't intend to be particularly nasty with my comment, but I must admit that I find your constant derision of certain types of drivers to be wearing, especially since those drivers often do their jobs very well.
If you want useful LF reproduction from a smallish box, then power handling is an important factor and should not be dismissed.

Here's a post I did a little while ago on the subject of driver speed, and touches on moving mass:

Hope that clears things up.


egellings 16th September 2020 11:37 PM

No. BL is determined by magnetic field strength in the gap and length of voice coil wire submerged in the gap. How much the voice coil weighs does not affect that.

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