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I came across this interesting article on the net about Alnico magnets being used in speakers.


AlNiCo is a type of alloy magnet which was used extensively in loudspeakers between 1930 and 1960. As its name suggests this family of alloy magnets employ Aluminum, Nickel, and Cobalt. The most common ones are with marks 2, 5, and 8. Each has its own unique characteristics which generally has to do with how strong they are when magnetized, and how easily the magnet can be demagnetized. For speaker applications, AlNiCo 5 is the best choice in the AlNiCo family of alloy magnets. Its peak energy product is just right for loudspeakers where we need to concentrate high densities of magnetic flux in the gap around the voice coil. AlNiCo 5 is an alloy made up of 8% Aluminum, 14% Nickel, 24% Cobalt, and 3% Copper. AlNiCo 5 is almost universally chosen for loudspeaker use because it has a high flux, and under normal circumstances of use, the speaker wasn't intended to be driven hard enough to affect the magnetism of the magnet by the voice coil magnetism. The price of cobalt began to skyrocket, so the industry was forced to develop other types of magnets. It currently sells for about US $64 a kilo. Most of the worlds supply comes from Zaire. Besides that country controlling the market, cobalt is also a strategic metal used in missles and other weapons systems and cobalt magnets are still the most widely used magnets in the world, being employed in everything from weapons systems to analog meter movements, to debris separator grates in manufacturing processes.

Many people, who don't take AlNiCo cult for granted, ask themself why it should be better than ferrite with the same magnetic strenght. The answer lays in Alnico's smooth compression at high average levels. This effects sound simmilar to tube's soft clipping at high outpout powers. Alnico is an alloy magnet and all alloy magnets are easier to demagnetize than comparable ceramic (Strontium Ferrite) magnets. As the voice coil starts moving in response to the input signal, it generates a magnetic field of its own that tries to demagnetize the magnet. As its effect lowers the available magnetic field of the alnico magnet, the speaker becomes less efficient, the voice coil moves less, etc. The physics of it is that the magnet domains near the surface of the magnet poles begin to change state, or flip directions. The result is smooth compression, the same kind of operating curve compression that occurs in a tube amplifier. The ceramic magnet, on the other hand, doesn't compress or demagnetize as easily, so the voice coil moves to its mechanical limit and won't go any farther. This is why many say ceramic magnets sound a little edgey at high average levels as opposed to alnico. If we continue with tube/solid state comparison, then alnico behaves tube-amp like and louder average volumes can be achieved but with smooth compression. The compressing or demagnetization that occurs with the AlNiCo is not permanent. It's domains spring back to their initial state.

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I appreciate your posting that. Looking at an alnico magnet I wondered why anyone would build a speaker magnet like that.

I knew there had to be a reason.

Do you suppose they stopped doing that just because of cost?


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