High efficiency 15"

[gedlee]

Originally posted by BHTX
it's definitely audible.. to me, anyway.

I think that it needs to be audible to people in general under blind conditions using real world signals. All tests that I know of like this have found this to not be the case.

[Bill F.]

Instead of promoting eddy currents in shorting rings to reduce Le, ATC goes the opposite direction--minimizing eddy currents and increasing Le by lining the magnetic gap with a magnetic material that features resistivity (they call it SLMM).

Here's an excerpt from ATC's company profile and technical philosophy paper.

The third source of distortion is due principally to the inherently non-linear magnetic performance of steel. The alternating magnetic field created by the voice coil induces eddy currents into both the pole and front plate, adjacent to the coil, of the permanent magnet assembly. These eddy currents flow in such a way as to oppose the magnetic field producing them, (i.e. from the voice coil), and cancel out much of the self inductance. This mechanism is minimized in ATC bass and bass/mid drive units by the use of a new material, which has the unique properties of high magnetic permeability and saturation as well as low electrical conductivity.We call it a super linear magnet material (SLMM). With this material fitted to the pole and front plate adjacent to the voice coil the eddy currents are suppressed and the impedance (self inductance) increases. The result is that third harmonic distortion is reduced by between 12–15dB.

I haven't taken the time to run simulations to confirm this, but I find the idea facinating. Any comments?

[Bill F.]

In another document that doesn't seem to be on ATC's website anymore, they go into more particulars:

Experiments were performed on a blocked voice coil with the magnet left un-energised. It could be thought of as a cored inductor. A current was passed through the coil and second and third harmonic distortion components were measured. Mathematical analysis, in conjunction with the experiments, has revealed some surprising answers to the question of why replacing the steel regions with S.L.M.M. has such a dramatic effect on the distortion.
(Snip)
...the presence of the S.L.M.M. increases the self-inductance of the voice coil. When eddy currents are allowed to circulate in the system, they oppose the magnetic field producing them (i.e. that from the coil) and ‘cancel out’ much of the self-inductance. With the S.L.M.M. in place, eddy currents are suppressed and the self-inductance (i.e. the impedance) goes up. thirdly, whilst the impedance, and therefore the fundamental voltage across a blocked coil goes up when the rings are fitted, the harmonic components, that are induced back into the voice coil, stay the same. This is because they are dependent only on magnetic field, which as we have seen, doesn’t change very much. The net effect is a rise in the signal/distortion ratio.