The subject of Alnico vs ferrite magnet drivers comes up fairly regularly on our site. The following is my attempt to clarify some of the issues involved. Before I get into a discussion on the merits and disadvantages of each magnet type, it is important to separate fact from fiction. To understand how this debate began, it is necessary to know a bit about loudspeaker history. Up until the late 70’s, most high end speaker manufactures used Alnico magnets due to their greater energy/weight ratio. Starting in 1978, all major manufactures (JBL, EV, Altec, Tannoy etc.) switched to ferrite drivers. That was when the myths began.
Myth #1 – Speaker manufactures switched to ferrite as a way to lower their production costs and cheapen the quality of their drivers.
Fact – The switch to ferrite was in response to a crisis situation whereby Alnico became totally unavailable. A civil war in Zaire led to the complete embargo of the world’s only source of commercial cobalt used in Alnico. There was no choice but to switch. This is why, in less than one year, every major Alnico speaker manufacturer had switched to ferrite.
Myth #2 – Due to the lower energy/weight ratio of ferrite, drivers using this material have lower total flux and lower flux densities compared to the previous Alnico drivers.
Fact – The initial ferrite conversion had the exact same magnetic energy of Alnico drivers they replaced. For JBL, the initial conversion effort focused on bass drivers since that represented their largest consumption of magnets. They had sufficient magnet stock on hand to continue Alnico compression drivers for a number of months.
To be able to continue production of the speaker systems in their catalogs, the ferrite bass drivers had to be the exact sonic equivalents of the Alnico drivers they replaced. Otherwise, the entire systems would have to be re-engineered and there was no time to do this. To give you an example, the L300 Summit, both before and after the ferrite bass driver conversion, used the exact same Alnico tweeter, Alnico compression driver, enclosure and network. The only change was that the 136A driver had its Alnico motor replaced with a ferrite motor to become the 136H. The basket, cone and suspension remained identical. The only way this could work was if the ferrite motor had the exact same magnetic energy as the Alnico motor.
As the demand for high power drivers increased, the magnetic energy of the ferrite drivers began to exceed the former Alnico systems. As an example, the last ferrite version of the Altec 515 had a flux density of 15kgauss compared to the 14kgauss of the Alnico version. The Alnico embargo proved short lived. Alnico became available in limited quantities after a year or so, but at a much higher cost. This happened before the compression drivers were converted and it was decided to continue their production to save the costs of redesign. Therefore, Alnico HF drivers remained in production for another three or four years, until it became too cost prohibitive to continue. Around 1983, they were converted to ferrite motors as well.
Nonetheless, the costs of the ferrite replacements in constant dollars remained about the same as the Alnico drivers before the civil war broke out in Zaire. Any savings in cheaper magnetic materials were outweighed by the sheer size of the magnets and the need for a large pole piece to accommodate the external magnet topology.
Now to factual differences. There are three main advantages of Alnico over ferrite:
1) greater immunity to flux modulation
2) greater heat stability.
3) greater suitability to shielded applications
There is also one significant disadvantage – Alnico is susceptible to demagnetization due to large voice coil currents.
None of these differences are absolute. It is possible to design out all of the limitations of each material. However the issue becomes one of cost.
Ferrite designs can equal or exceed an Alnico magnet’s flux stability with the addition of a copper shorting ring around the pole piece. With the use of vented cooling and heat sinking, you can manage the heat build-up on a ferrite driver to where it stays below the threshold of non-linear response. Finally with the addition of secondary magnets, you can shield the motor of a ferrite driver to the same degree as an internal ferrite equivalent.
In the same manner, Alnico drivers can be engineered to be immune to demagnetization from overpowering. JBL has done this though the use of a series of flux stabilization rings in their new 1500AL driver. However it is extremely expensive.
All in all, the major speaker manufacturers have found it more cost effective to engineer out the limitations of ferrite drivers than to do so with Alnico drivers and hence the dominance of ferrite designs.
In conclusion, I believe the modern ferrite drivers are superior to the vintage Alnico designs. This is not because ferrite is inherently superior to Alnico. Instead, manufacturers have been able to engineer out any limitations of ferrite and apply the advantages of 25 years of technological progress in driver designs unrelated to magnets. For example, cone materials, suspension design and construction has progressed significantly since the last Alnico drivers were made.