Hi,
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,
WA
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,
WA
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).
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).
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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.
B.
The design of "competition" woofers (and extreme power handling speakers in general) results in designs that seem at odds with home audiophile purposes.
B.
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.
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.
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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.*
B.
* maybe true horns could use such a driver in a pinch
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.*
B.
* maybe true horns could use such a driver in a pinch
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.
Chris
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.
Chris
"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?
B.
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?
B.
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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.
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: https://www.diyaudio.com/forums/multi-way/358425-questions-baffle-speakers-2.html#post6302335
Hope that clears things up.
Chris
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: https://www.diyaudio.com/forums/multi-way/358425-questions-baffle-speakers-2.html#post6302335
Hope that clears things up.
Chris
Thanks for your comments.
So, BL does not depend on the mass, just depends on Re for the same given magnet and gap.
The takeaway from this discussion is that in order to compare BLs of various motors (with various Re) the BLs must be normalised to a fixed Re say 8 ohms and then compared, just like we normalize sensitivity to 2.83 volts.
The reason why I was looking at competition subs for home use is that they can be put in very small enclosures which reduces quite a bit of real estate and visual footprint especially in case of multi-subs, amplifier power is quite cheap (and compact) these days with plethora of class D amps so the higher power requirement is not a big issue.
So, BL does not depend on the mass, just depends on Re for the same given magnet and gap.
The takeaway from this discussion is that in order to compare BLs of various motors (with various Re) the BLs must be normalised to a fixed Re say 8 ohms and then compared, just like we normalize sensitivity to 2.83 volts.
The reason why I was looking at competition subs for home use is that they can be put in very small enclosures which reduces quite a bit of real estate and visual footprint especially in case of multi-subs, amplifier power is quite cheap (and compact) these days with plethora of class D amps so the higher power requirement is not a big issue.
The price of higher mass is lower sensitivity. It is not an issue at car audio, where one can pump several hundred watts into a high mass / low compliance (-> low box volume) inefficient loudspeaker. These are not meant for home audio. One reason is group delay is high, resulting in boomy bass.
Actually, efficiency scales with (BL)²/Re. That can be easily understood by looking at a dual coil woofer. The conversion efficiency does not depend on how you wire it up (voltage-based sensitivity does, of course). Series vs. parallel doubles BL but resistance quadruples. In the end the efficiency depends on the amount of VC material in the gap and the conductivity of the material. And it depends on total mass of the cone assembly. For compact subwoofers a high mass is beneficial because it makes resonant frequency of the box low enough and the system Q (Qtc) just right to obtain a good voltage-based sensitivity in small enclosures.
Just sprung to mind: car audio subs usually have a large excursion capability. This implies that coil overhang is large. All that extra coil outside of the gap does not contribute to BL, but raises Re. The desired impedance and therefore Re is fixed, so we must use thicker wire and less turns to adjust Re to a reasonably value. Less turns means less wire in the gap and a lower value of BL. Medium sized magnet, low (BL)²/Re.
Now BL can be increased back to 'normal' by using a thicker pole plate, exposing more of the wire to the magnetic field. The drawback of a thicker pole plate is that it requires more magnet material and results in a larger magnet structure. Large magnet, medium (BL)²/Re.
Now BL can be increased back to 'normal' by using a thicker pole plate, exposing more of the wire to the magnetic field. The drawback of a thicker pole plate is that it requires more magnet material and results in a larger magnet structure. Large magnet, medium (BL)²/Re.
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Yup. That's because they are being used for other purposes.
Car audio is not like home because the "room" volume is very tiny-especially in those cars where almost the whole thing is being take up by enclosures. And the SPL becomes nonlinear. And the "room" is becoming the same size or less as the enclosures. (Note: I'm talking about enclosed competition cars, not the open-up-and-blast kind popular in South America)
To get low resonance in a small enclosure requires high mass:
Fs=1/(2*pi*sqrt(Cms*Mms). However efficiency is proportional to 1/(Mms)^2. But but BUT this is a MID-BAND efficiency. I dimly recall that more mass can be used to get some more efficiency down low, at the expense of reduced efficiency in the midbass and above. I don't have a tool handy to recheck this.
I do recall clearly that adding BL (more motor force) would increase output above resonance. From the midrange, this looks like there is "less bass" but actually the efficiency around resonance was the same, there is just "less bass" since actually now there is more midbass and midrange.
Good clarifications. Thanks.
Might be mentioned that in shaping loudness, the clever designer uses the speaker resonance as part of the deal. So if you are canny about where that occurs, you do get bass loudness low down, but maybe not quite right to relate that to efficiency in the strict sense of the term.
B.
Might be mentioned that in shaping loudness, the clever designer uses the speaker resonance as part of the deal. So if you are canny about where that occurs, you do get bass loudness low down, but maybe not quite right to relate that to efficiency in the strict sense of the term.
B.
Sensitivity figures are given at constant voltage and it has almost nothing to do with power conversion efficiency, especially at low frequencies.
"is 1000watts enough for this sub?" is actually pretty nonsense question because of the impedance peaks and fluctuating phase angle (inductive/resistive/capacitive load). Even that 1kW amp might be delivering only 100watts at clipping, so at that point amplifier is running out of voltage rather than avaliable power.
Speakers are very complicated electromechanical instruments to work with and leads to many compromises no matter how good a design.
In some cases, highly damped woofers might be actually less efficient at bass than low BL woofers. When speaker cone is electrically damped (as it is more or less), the braking force is wasted as a heat in typical amplifiers output stage?! But who wants to listen speaker resonance peaks over the original music signal?
"is 1000watts enough for this sub?" is actually pretty nonsense question because of the impedance peaks and fluctuating phase angle (inductive/resistive/capacitive load). Even that 1kW amp might be delivering only 100watts at clipping, so at that point amplifier is running out of voltage rather than avaliable power.
Speakers are very complicated electromechanical instruments to work with and leads to many compromises no matter how good a design.
In some cases, highly damped woofers might be actually less efficient at bass than low BL woofers. When speaker cone is electrically damped (as it is more or less), the braking force is wasted as a heat in typical amplifiers output stage?! But who wants to listen speaker resonance peaks over the original music signal?
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There's no need to bring Re into it, if you're only looking at BL. The whole thing just gets blown up otherwise when you consider alternate conductor materials. e.g. what happens if you use aluminum instead of copper?
Many knowledgeable posts. But now let's focus the abstract discussion into meaningful conclusions. For fidelity seems to me, the core issue is control of the motion of the cone and especially transient motions.
Let's say you were buying a driver. Wouldn't part of your decision be based on motor strength (just Bl) relative to cone assembly weight (plus maybe air mass varying with diameter)?
Let's say you were buying a driver. Wouldn't part of your decision be based on motor strength (just Bl) relative to cone assembly weight (plus maybe air mass varying with diameter)?
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