Does a voice coil have to be round?

[Tim Link]

A planer voice coil, e.g Magnepan, is a good example of a non-round voice coil. But as others have highlighted, a rectangular or other design leads to a lot of complications in design and construction and usually degraded performance. Many, many years ago I asked a B&W (I think) designer about a rectangular woofer they had tried and rejected. One thing I remember was that corner stress and flex led to much higher distortion and worse long-term reliability, with additional undesired modes forming as driver excursion and frequency increased.

A (one) drawback of planer magnetic designs is that, with a one-sided magnet structure, linear excursion is fairly limited so as output increases distortion rises quickly. Using magnets front and back helps, but rather than the voice coil being "immersed" in the magnetic field by wrapping around the magnet, you still have a rapidly-falling magnetic field with distance that makes large excursion impractical (IMO). Perhaps when we have superconductors and much stronger magnets...

I had an idea many years ago of creating a magnet in the form of a tube or rectangle, pierced by holes so more of a screen, then suspending a conductive membrane inside so you had a planer speaker enclosed by the magnet. The idea was two magnets in the form of channels with the membrane between: [ | ] so magnet - diaphragm - magnet. A little thought shows it is not significantly better than a conventional approach though should offer higher field strength. Should have patented it just for fun.

Thanks for sharing that. I like to know that what others are thinking about these problems. It's precisely that issue with the planar magnetic that interested me. How can we get those wires embedded in a tight magnetic gap and still keep a long planar membrane moving linearly? It seems you have to add a 3rd dimension to the membrane, which will make it heavier and stiffer, but should give you significantly increased motor strength to go along with that. Maybe it's a net loss. Too much extra weight for the added motor force.

It kind of reminds me of the idea of trying to make a vacuum balloon. Could you make a structure strong and light enough that you could suck enough air out of it to make it buoyant in air at standard temperature and pressure without it collapsing? In principle it should work. In reality, I don't see things like that, so I'm assuming we don't have materials strong and light enough. Similar to the vacuum balloon is the idea of a vacuum sound absorber, an object that tries to collapse under air pressure, but has a spring inside to balance the forces. If the air pressure goes up a bit, the spring collapses a bit. It's basically an air pressure gage, but it has to respond super quickly so that it can actually act as a sound absorber by rapidly expanding and shrinking its volume. I think the spring and structure would be way too heavy to respond quickly enough except for maybe very slow pressure changes.

When it comes to the design of drivers, my knowledge is weak. Maybe just for fun I should try to make a simple paper driver, playing with wire length, wire gauge, voice coil size, etc. to better understand what all the engineering trade-offs are. I have noticed that non-coil drivers can present an almost purely resistive load to the amplifier, so that seems like a reason to consider alternatives to a coil.

 

[DonH56]

Disclaimer: I am not a speaker designer.

Thanks for sharing that. I like to know that what others are thinking about these problems. It's precisely that issue with the planar magnetic that interested me. How can we get those wires embedded in a tight magnetic gap and still keep a long planar membrane moving linearly? It seems you have to add a 3rd dimension to the membrane, which will make it heavier and stiffer, but should give you significantly increased motor strength to go along with that. Maybe it's a net loss. Too much extra weight for the added motor force.

Planer magnetic speakers use multiple strips of magnets and conductors (wire or foil patterns on a lightweight membrane) to drive the entire diaphragm evenly. That way you do not necessarily need a heavy membrane, but lots of wiring and magnets fairly closely-spaced (depends upon membrane properties and frequency band it covers -- they usually break the membrane into multiple pieces to cover multiple octaves). Excursion is still an issue since field strength falls off rapidly (~1/r^2) as the electrical conductors move away from the magnets.

It kind of reminds me of the idea of trying to make a vacuum balloon. Could you make a structure strong and light enough that you could suck enough air out of it to make it buoyant in air at standard temperature and pressure without it collapsing? In principle it should work. In reality, I don't see things like that, so I'm assuming we don't have materials strong and light enough. Similar to the vacuum balloon is the idea of a vacuum sound absorber, an object that tries to collapse under air pressure, but has a spring inside to balance the forces. If the air pressure goes up a bit, the spring collapses a bit. It's basically an air pressure gage, but it has to respond super quickly so that it can actually act as a sound absorber by rapidly expanding and shrinking its volume. I think the spring and structure would be way too heavy to respond quickly enough except for maybe very slow pressure changes.

When it comes to the design of drivers, my knowledge is weak. Maybe just for fun I should try to make a simple paper driver, playing with wire length, wire gauge, voice coil size, etc. to better understand what all the engineering trade-offs are.

Vacuum balloons are outside my area of expertise. I made an example of my idea using some mylar material, copper tape, and magnetic sheets (rubbery magnetic material easy to form and cut). It worked as a proof of concept but certainly was not a commercial product.

I have noticed that non-coil drivers can present an almost purely resistive load to the amplifier, so that seems like a reason to consider alternatives to a coil.

I think that is mainly because the long wires required across the panels are basically resistors, so resistance dominates the load except for a minor blip through the crossover region(s). The efficiency is very low, however, leading to low sensitivity, and overall resistance is also low (for various reasons including wanting a consistent drive voltage across the panels), thus a good deal of amplifier power is required to play at moderate volume (and above). The vast majority of modern amplifiers (heck, very old amplifiers, too!) I have seen are very robust and load-tolerant so the trade may not be worth it. I have owned both and am happy with my current conventional design. It provides a pretty seamless sound from lows to highs and has much greater dynamic range than my Maggies.

FWIWFM - Don