Voice Coil dimensions

[thadman]

Why are traditional thermally optimized voice coils (ie Prosound) in possession of a large diameter coil relative to a large height coil?

A 4" diameter 5mm coil has the same surface area (heat dissipation?) as a 1" diameter 20mm coil.

The magnetic gap for such a long coil would obviously have to be larger to accomodate the additional height, but are there other aspects related to this solution that offer benefits I may be overlooking?

Thanks,
Thadman

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Other than thermal issues, I'd guess that's the compromise among many other factors such as magnectic circuit optimizations: gap length/width, fux density, and the physical things such as excursion required. And in the end, the overall cost to get to the design goal.

I think of AE, which mostly uses 2.5in VC and very thick top plate and extended pole piece. They are said to be excellent in thermal management. The full copper sleeve helps a lot in lowering inductance and non-linear distortion, while I'd guess the sleeve is much more essential in such long gap motor design which is probably lower in flux density.

A thinner top plate (and smaller diameter) should be higher in flux density if all others being equal. Such motor can also be adequate for high performance (low distortion) if only the power is kept low. Wide rangers with high sensitivities (including those vintage ones) are in this camp, I guess.

[thadman]

If we observe a magnetic cylinder, at the midpoint, the vector corresponding to flux will be orthogonal to the surface. However, once we depart from the midpoint, a vertical component will be observed because the components are no longer symmetrical. Is this a reason we observe non-linearities?

If so, how can we relate this additional vector component to motor non-linearities?

[Ron E]

Originally Posted by thadman

Is this a reason we observe non-linearities?
If so, how can we relate this additional vector component to motor non-linearities?

Prosound coils are short because making them this way increases efficiency. A long coil is throwing away a lot of power as heat. And yes, managing the fringe field is important to linearity.

[thadman]

Originally Posted by Ron E

And yes, managing the fringe field is important to linearity.

How do we manage the fringe field?

Assuming the vertical vector component has some relationship with motor non-linearity, could a motor ever achieve full linearity? It would appear that curvature of the flux vector is intrinsic to 3 dimensional motor geometry and is unavoidable in all circumstances.

[gedlee]

Originally Posted by thadman

How do we manage the fringe field?

Assuming the vertical vector component has some relationship with motor non-linearity, could a motor ever achieve full linearity? It would appear that curvature of the flux vector is intrinsic to 3 dimensional motor geometry and is unavoidable in all circumstances.

An infinitly linear motor is physically impossible. Hence the question to ask is, "Given that nonlinearity is inevitable, what is the least audible shape?" Probably not what you would think.

A larger shorter coil offers more efficiency potential than a tall slender one. The greater the total copper mass the better the thermal transient response. The long term steady state response depends on the motor structure design. "Dynamics" depend mostly on short term thermal response.

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Ah! I think of some time ago you've mentioned the rounded (or tapered?) shapes of top plate and pole piece...

I've also noticed this in some vintage drivers!

[thadman]

Originally Posted by gedlee

An infinitly linear motor is physically impossible. Hence the question to ask is, "Given that nonlinearity is inevitable, what is the least audible shape?" Probably not what you would think.

A larger shorter coil offers more efficiency potential than a tall slender one. The greater the total copper mass the better the thermal transient response. The long term steady state response depends on the motor structure design. "Dynamics" depend mostly on short term thermal response.

After reading Steve Mowry's article on Air Core motors, I do not see why it is not used in many designs (lack of efficiency?). The lack of non-linear effects related to traditional motor topologies is very compelling.

Dr. Geddes,

What are your impressions of this particular topology relative to traditional topologies utilizing steel and/or aluminum/copper in their construction?

If we cannot approach infinite linearity, I believe suppressing the most offensive orders would be optimal. Are 2nd or 3rd orders to be optimized? How should we approach the optimization of a motor for a particular harmonic order?

[gedlee]

Originally Posted by thadman

If we cannot approach infinite linearity, I believe suppressing the most offensive orders would be optimal. Are 2nd or 3rd orders to be optimized? How should we approach the optimization of a motor for a particular harmonic order?

The physics of the problem dictates that the field and force Vs displacement be symmetric, which means that only odd orders of nonlinearity will be present. Some asymmetry is possible and not problematic as long as it is gradual, but by necessity these will generally be low. Now we want to maximize the lowest odd orders and minimize the higher ones. Its not difficult to see that a sort of bell shape or Guassian shape is the most ideal. The idea of a perfectly flat BL with sharp falloff on either side is clearly NOT ideal.

[thadman]

Originally Posted by gedlee

The idea of a perfectly flat BL with sharp falloff on either side is clearly NOT ideal.

Assuming the driver was engineered properly and operated within its limits (ie within the linear BL field), wouldn't the flat curve be more desirable relative to the gaussian curve?