Variable-efficiency speaker?

[DonMoose]

How would a variable-efficiency speaker work? Could such a thing replace power scaling and hot plates as volume-tamers?

As I understand speakers, for a given cone/spider/coil set efficiency is about the size of the gap in the magnet in which the coil moves. I'm not clear if the critical dimensions are radial width or axial depth of the narrowest part.

Or do I fail to consider some critical thing?

[UR12]

How would a variable-efficiency speaker work? Could such a thing replace power scaling and hot plates as volume-tamers?

As I understand speakers, for a given cone/spider/coil set efficiency is about the size of the gap in the magnet in which the coil moves. I'm not clear if the critical dimensions are radial width or axial depth of the narrowest part.

Or do I fail to consider some critical thing?

In the old days there were field coils speakers. They didn't have permanent magnets but a big coil of wire that created the magnet field for the voice coil to operate within. They usually doubled as the power supply choke with high voltage on it. I don't remember who, but there was someone out there marketing a device that was like a field coil speaker with a variable power supply to drive the coil. This allows you to vary the amount of magnet field and also the efficiencey of the speaker.

[Ears]

There is a thread somewhere in here on this.

[Structo]

Yeah, some guy is marketing that.

If I recall he wanted something in the neighborhood of $1200 for it.

[LOUDthud]

The efficiency of a speaker is proportional to the strength of the magnetic field in the gap (flux density) times the length of wire in the gap times the current through the wire. Changing the dimensions of the gap or the strength of the magnet changes flux density and therefore the efficiency. In the 60's, JBL speakers had one of the highest flux densities, upto about 1.2 Tesla. I'm not sure if current production Neodymium magnet speakers exceed that number. I've read about a guy in the 30's who made speakers with electro magnets that had a flux density of 5 Tesla. He was trying to make a horn speaker system that would fill an auditorium with a 5 watt amplifier.

[LeeMo]

http://www.fluxtone-speakers.com/Home_Page.html

[DonMoose]

Innntresting (fluxtone).

I wonder if I'll find one to play with?