Dear loudspeaker motor experts out there,
What would be a minimum thickness of a copper sleeves on the inner (or outer) polepiece of a loudspeaker motor? We see various types, layout and thicknesses being applied by manufacturers.
Maybe a better question would be: how to determine the minimum thickness of such a device? After all, it looks as if the thicker the sleeve, the weaker the magnetic field in the gap.
What would be a minimum thickness of a copper sleeves on the inner (or outer) polepiece of a loudspeaker motor? We see various types, layout and thicknesses being applied by manufacturers.
Maybe a better question would be: how to determine the minimum thickness of such a device? After all, it looks as if the thicker the sleeve, the weaker the magnetic field in the gap.
Copper is a diamagnetic element. This means that there are more line forces of magnetic field ouside of it than inside, as opposition to iron, where inside are much more than outside. But what is your worry about it? Supposedly the speaker manufacturer had studied it properly.
Hello Osvaldo,
Thanx for chiming in. The idea is somewhere in the future to do do some experimenting with sleeves on loudspeaker motors that do not have sleeves. As I mentioned, I see a wild variety amongst manufacturers. Should the sleeve be simulated in e.g. FEMM?
Thanx for chiming in. The idea is somewhere in the future to do do some experimenting with sleeves on loudspeaker motors that do not have sleeves. As I mentioned, I see a wild variety amongst manufacturers. Should the sleeve be simulated in e.g. FEMM?
Their role is to shield the magnet against the field variations forced by the VC. It keeps the magnetic field more constant, and prevents the dissipation of eddy currents in the magnet and pole pieces.
It could be treated as the secondary of a transformer, and you would need a sufficiently high time-constant, L/R ratio to keep it effective down to low frequencies, thus in practice a thick enough section to lower the resistance, since L is fixed by the geometry
It could be treated as the secondary of a transformer, and you would need a sufficiently high time-constant, L/R ratio to keep it effective down to low frequencies, thus in practice a thick enough section to lower the resistance, since L is fixed by the geometry
Like on other areas, practical/manufacturing reasons rule.
Normal is 0.1mm copper sheet
And yes, even that lowers useful flux
Sorry but no, they do not influence each other since they are at 90 degrees (VC field is axial, speaker own is radial)
The main function is to short VC inductance, control lower mid-hf impedance rise and allow way stronger highs.
First PA system which blew my head off was an Italian Semprini system, cabinets were 6x12" "bedframes" on poles, loaded with copper capped Goodmans twin cone full range speakers.
We are talking 1968/69
They destroyed then standard Shure Vocal Master systems.
Thanks for the replies so far, but I am basically aware of the function of the sleeve(s) a.k.a. shortening rings. So 0.1 mm sheet would already do the trick?
The VC is bathed in a radial field, but the symetry of the pole pieces is axial, and the reaction caused by the VC affects them
It is way to paraphrase what I said: the ring is shorted turn which tries to keep the static field as constant as possible, ideally to leave the motional impedance as the sole dominating parameter
More details here for example:
https://www.lexiaudio.com/news/the-role-of-faraday-ring-in-speaker.html
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