Behind the Scenes at Secrets – Number 5 – Fall, 2009

The Voice Coil

I am sure that, as readers of Secrets, you have heard the term “voice coil” and most of you know its function. But, perhaps you are not aware of all the details of how it is designed and manufactured. So, here is a short course.

The voice coil is a wire that is wound around a “former” or “bobbin”, which may be made of something like Kapton, but in any case, something that can withstand high temperatures.

The finished voice coil is mounted to the rear end of the speaker cone, and is suspended between the poles of a large permanent magnet. When current from the power amplifier passes through the voice coil wire, that creates an electromagnet, and the voice coil then is pushed or pulled through the magnet’s pole pieces depending on the direction of the current. This moves the cone, and the cone moves enough air for you to hear the sound.

Winding the voice coil is a critical process because having the coil situated as close to the magnet pole pieces will move the coil farther than if the coil were not so close. Even the shape of the wire itself is an issue. By making the wire flat, this allows the individual windings to be more tightly packed than if the wire were round. This produces a stronger electromagnetic field, so again, the coil will move farther for a specific amount of current passing through it.

The insulation for the wire has to be special too, because the coil gets very hot when you are watching those action movies at high volume. Otherwise, the coil will burn and become defective. A lot of technological research has gone into developing methods of keeping the coil from getting too hot, including special venting of the pole pieces.

There are different ways the coil can be designed with respect to the magnet’s pole pieces. One way is called “Overhung”, where the coil is longer than the gap height (the ends of the pole pieces that are the closest to the voice coil). With this design, as the coil moves back and forth, it always has windings that are within the gap, so it produces less distortion at high volume. Its disadvantage is that it has higher coil mass, and with speakers, it is desirable to have the cone and its coil as low mass as possible.

The other design is called “Underhung”, where the coil is the same length as the length of the magnet gap. For this design, you have low mass, but the disadvantage is that at high volume, the coil can move completely out of the gap, so it produces more distortion.

The mass of the coil can be reduced by using aluminum instead of copper, and some manufacturers do this. Aluminum has 1/3 of the mass of copper and 2/3 of the conductivity of copper, so it is worthwhile to use it. A variation on this is using copper coated aluminum.

Currently, there seems to be a contest brewing as to who can make the most powerful subwoofer, and the voice coil is a very important part of the design considerations. Some of them are several inches long, and as such, aligning the coil within the gap is very difficult, because if it is slightly off center, it will rub against the magnet’s pole pieces.

In order for the speaker to move a lot of air, such as is required with a subwoofer, the voice coil has to travel a large distance back and forth, called its “excursion”. The mathematical term is Xmax, usually stated in millimeters. The new subwoofer designs will have Xmax numbers in inches!

It is going to be very interesting to see how this little “war” pans out, as some of the voice coils being put into this new generation of subwoofers are not only very long, but are also several inches in diameter. All I know at this point is that they will likely be able to blow your windows out. But actually, the real advantage will be that, at normal listening volume, they will have very low distortion – lower than we have ever seen before.

Below is a photo of a tray of finished voice coils at the Paradigm speaker factory. Click on the photo to download a high definition video (Windows Media Player file: *.wmv) of a Paradigm factory worker winding them. It will take a minute or so for it to buffer in your player because it is full 1920 x 1080 HD.