VC microphonics significant?

[capslock]

Two parallel conductors that carry current in the same direction will repel each other. In a coil, this means that two turns which are side by side in the same winding layer as well as turns which are in different layers will repel each other. This is exactly why high quality inductors for crossovers are either vacuum soaked in some resin (e.g. Mundorf vacuum soaked) or baked together by using wire with a thermoplastic coating and sending a current pulse throught the coil after winding (Mundorf BL "baked laquer" or Intertechnik Airtherm). Copper foil inductors have similar advantages, here it is the tight winding and the soft foil that keep vibrations to a minimum

There was an article by Bernd Timmermanns of HobbyHifi magazine who measured microphonics of various inductors and found significant advantages for soaked, baked and copper foil inductors compared to conventional air core inductors that just had a tight winding with no glue and with our without adhesive tape around the outer layer.



Now, if microphonics of XO inductors can be easily measured and are sometimes audible (I have an air coil that literally sings), what about loudspeaker VCs? If the individual turns of the VC rattle against each other, this noise can potentially be transmitted by the former and radiated by the cone. There was a guy on a German forum who once claimed that some of the peaks seen in metal cone drivers are actually not cone modes but VC vibrations which are transmitted particularly well by an aluminum former in combination with a metal cone.

Now, all the VCs I have looked at seem to be pretty tightly wound, but there is no sign of them being soaked with some sort of resin or glue. Often, there is a layer of paper between former and first layer, sometimes also between first and second layers. And considering the need to withstand high temperatures, it would probably not be a wise thing for manufacturers to use wire with a thermoplastic coating.

Does anybody know if VC microphonics is a signifcant issue, and if so, how it is addressed?

[AndrewJ]

All voice coils are in fact bonded with high temperature adhesives.
The wire is either pre-coated or "wet-wound" and then the coil is baked. The quality of the coil and its power rating is determined by the temperature rating of the adhesive. Modern coils can easily withstand 300 deg C plus.
Without the adhesive they would just fall apart as the cone moves.
Some of the modes seen in the high frequency response can be attributed to coil vibration, but it is a bulk deformation mode of the coil assembly. This is why edge wound coils can be superior, because not only do they make better use of the magnet gap, but they also tend to be stiffer.

Andrew

[capslock]

Hi Andrew,

it is reassuring to learn that all VC are glued and baked. Just by looking at them, I would never have guessed.

What is an edge wound coil?


Regards,

Eric

[pinkmouse]

Originally posted by capslock
What is an edge wound coil?

AFAIK, one wound with rectangular wire rather than round.

[Joules]

Two parallel conductors that carry cursrent in the same direction will repel each other. In a coil, this means that two turns which are side by side in the same winding layer as well as turns which are in different layers will repel each other.

On the contrary two parallel conductors that carry cursrent in the same direction will ATTRACT each other. Two parallel conductors that carry cursrent in opposite direction will REPEL each other. A coil will try to expand radialy and compress axialy.