Dayton Audio ND90-8 3-1 - Manual (Page 2)

The electrodynamic coupling factor, also called Bl-product or force factor Bl(x), is defined by the
integral of the magnetic flux density B over voice coil length l, and translates current into force. In
traditional modeling this parameter is assumed to be constant. The force factor Bl(0) at the rest
position corresponds with the Bl-product used in linear modeling. The red curve displays Bl over the
entire displacement range covered during the measurement. You see the typical decay of Bl when the
voice coil moves out of the gap.At the end of the measurement, the black curve shows the confidential
range (interval where the voice coil displacement in this range occurred 99% of the measurement
time). During the measurement, the black curve shows the current working range. The dashed curve
displays Bl(x) mirrored at the rest position of the voice coil – this way, asymmetries can be quickly
identified. Since a laser was connected during the measurement, a "coil in / coil out" marker is
displayed on the bottom left / bottom right.
More information regarding Bl(x) and its optimization can be found in the article “Optimal Voice Coil
Rest Position”