Flux Density values

[stevem]

Does anyone know if Maxwells can be converted to Teslas or Gauss units of Flux Density measure? If so, What are the conversion values? Thanks.

[Don McRitchie]

The short answer is no. Maxwell (or its SI equivalent Weber) is a measure of magnetic energy or flux. Tesla is a measure of flux density. In other words, the total flux divided by the area over which it is acting. In a loudspeaker, the area of concern is that of the voice coil gap. The gap area is defined by the circumference of the gap multiplied by the height of the gap. The total flux at the pole piece in Maxwells or Webers (depending on units) is divided by the gap area to determine the flux density in the gap.

This is a very simplified explanation. The total flux at the gap is not genereally equal to the total flux of the magnet due to losses in the circuit and the issue of saturation at the coil gap. Magnetic conductors (such as iron) will only conduct a finite flux density, after which, adding more magnetic energy has no effect. For the most efficient design, the magnet is generally specified to be just large enough to saturate the pole piece at the voice coil gap. The gap width also has a significant impact since magnetic engergy is dissipated exponentially with distance. Wider gaps mean less flux density.

It should be noted that neither measure is a good reflection of a loudspeakers performance. Depending on the geometry of the magnetic motor, you can design a loudspeaker with a low flux density that is very efficient and vice versa. For example, JBL's 1500AL has a flux density of only 5k Gauss compared to a 2235 with 12k Gauss, yet their sensitivity is about the same. The difference is that the 1500AL has a magnetic gap that is 6 times deeper than the 2235 and a magnet that is much larger. However, the gap is much wider as well so that it tends to balance out in overall motor strength.

[stevem]

Thanks for the very thorough reply! I was confused by the different nomenclatures, but now I understand that they are different measures.