It's a kind of miracle that Magnepan have a sensitivity of about 86 db at 4 ohms because their magnetic system is somewhat weak for at least three reasons. First they use flexible magnets instead of ceramic strontium magnets to alnico or neodymium used in regular and high end speakers this means the flux density(gauss) is weaker and the total magnetic power (Maxwell) are limited . Secondly the back plate is thin compared to the back plate usually found in speakers in general, moreover on the Magnepan, the "back plate is perforated to let sound pass through. Thirdly there is no top plate as it is found on speakers in general. The role of these back and front plate is to help the magnetic lines to concentrate and to avoid to escape.
In a magnepan the magnetic lines create semi circle magnetic lines and the conductor is placed in the magnetic lines. The main problem with the magnepan magnetic structure is the assymetry of the magnetic field relative to the movement of the conductor in the magnetic field. When the membrane and the conductor goes outward, it leaves the magnetic field and when the membrane goes backward it enters in the magnetic field where the concentration of the magnetic field (gauss) is higher. BUT nothing being perfect, dynamic cone speakers have their own flaws, and suffer from linearity problems caused by their magnetic circuits, mechanical non linearity, In the case of magneplanars, their weaker magnetic structure is the trade-off that limit their efficiency but help for their transparency due to the light weight of the membrane conductors assembly also due to the perforated metallic plate.
