Anyone looked into variable sensitivity speaker technology, like Eminence FDM?

[Jeff Gehring]

Specifically, I found myself wondering while looking at the Eminence site's page on the Maverick speaker: where does the power go? With a conventional attenuator, it's pretty straightforward that the shunted power is dissipated as waste heat. With an FDM speaker, there is no load resistance burning off the excess power, the magnetic flux is just progressively decoupled.

As the speaker is increasingly decoupled, I'd guess it appears significantly less inductive to the amp, with a change in the resultant impedance the amp sees. Maybe it just begins to approach the DC value of coil resistance, and the most of the power load is borne by the voice coil?

Anybody got the lowdown?

 

[Malcolm Irving]

... .
As the speaker is increasingly decoupled, I'd guess it appears significantly less inductive to the amp, with a change in the resultant impedance the amp sees. Maybe it just begins to approach the DC value of coil resistance, and the most of the power load is borne by the voice coil?
...

Yes, I think that must be right, but keep in mind that very little of the power going to a speaker comes out as sound energy. (The efficiency is only 7% or whatever.) So most of the power going into a speaker is dissipated by heating the voice coil (via its DC resistance) anyway.

Another way to look at this is when a speaker appears inductive (or capacitive) there is still no power dissipated in the inductance (or capacitance). Power can only be dissipated by a resistive element. The equivalent circuit of a speaker should include (somewhere) a small resistive component that represents power dissipated as sound, in addition to the voice coil resistance (in which power is always dissipated as heat). Equivalent circuits don't usually seem to show such a 'sound energy dissipation resistance', which is a pity since it is the desired end result of whole thing. I guess the reason they don't usually show it, is that it is a relatively small resistance.

 

[Steppin' Wolfe]

I am thinking that the eminence approach is purely mechanical. Are they varying the voice coil gap or the relationship between the magnet and the coil??? I don't know. The Flux tone method uses some form of a filed coil thing unless I am mistaken. They have an AC voltage supply to the magnet....and their product is much, much more expensive.

 

[Jeff Gehring]

I think Eminence's deal is purely electromechanical, you are moving the magnet, (or maybe a mu metal shield?) to reduce the flux through the gap where the voice coil is. Definitely no external power supply needed, as it is marketed as a replacement speaker.

 

[pdf64]

With the FluxTone, the electrical magnitude of bass resonance decreases as the mag strength is lowered.
I don't find that it has a noticable audible effect on the speaker's tonal balance.

kg posed a similar query in post #103 of http://music-electronics-forum.com/t21491/

 

[TEPR]

Would love to know more about the FDM...

 

[eclecticsynergy]

I think there must be more going on there. According to the specs, the speakers have a significantly higher impedance at max attenuation. Although nominal impedance is listed as 8Ω, they read just under 6Ω at full output, and a bit over 12Ω at max attenuation.

http://www.eminence.com/speakers/speaker-detail/?model=Reignmaker
http://www.eminence.com/speakers/speaker-detail/?model=Maverick

 

[Mayhem13]

If the magnet is physically offset, then yes, they could attenuate the sensitivity of the motor but Qms and Vas as well as Le would change with every adjustment. This would wreak havoc in a sealed cabinet for bass response and I imagine the harmonic distortion profile of the speaker would change considerably as well so not really tonally transparent....which doesn't make it a bad thing. Clearly in practicality it's for open systems only.

I'd have to take one apart to see what's going on or at least measure the T/S parameters at variable points on the dial to get a better picture. Getting a Bl curve wouldn't be too bad either.