I'd like to post a warning that I meant to put in my recent letter to SLOG. I have had several cases recently where the bias control was turned to maximum, where heavy crackling occurs, and left for an extended period in order to "clean" the panel. This procedure is not necessary and it can damage the membrane. If the procedure is followed for setting the bias as mentioned in the owner's manual it is safe. In brief, it states that the bias control is advanced to where a crackling sound is just heard and then it states to reduce the bias to where it is just barely below the point where crackling begins.
I'd like to touch upon the mechanism that causes crackling. The stator insulation is capable of preventing electrons from passing through it until the electrostatic field is increased to the point where the attraction force is great enough to permit the electrons to push their way through the molecular lattice structure of the insulation. This is an elastic deformation of the lattice and is totally reversible - it is not a breakdown where the lattice structure is permanently damaged. However, electrons passing through the insulation under high field conditions collect on the surface of the insulation and when the collective charge reaches a critical level it "jumps" to the membrane, causing a little "tick" sound. Consider that this is randomly occuring over the hundreds of square inches of insulation area. Thus, a continual series of random "ticking" sounds are occuring, which gives the sound of "crackling" The energy of each little tick isn't much, but considering thousands of these occuring a minute over a long period of time, it is capable of removing the conductive coating on the membrane. The final result is a loss of sensitivity. The solution is the installation of a new membrane.
There are situations where environmental factors and power line voltages are such that advancing the bias control to full does not make the speaker crackle. This doesn't mean that the speaker is defective. Under these conditions it is OK to leave the bias turned to maximum. Just don't leave the bias set where crackling is constant. An occasional crackle is fine and nothing to worry about. This can occur when the line voltage temporarily varies.
The intent of the bias control is to permit users to compensate for environmental factors and line potentials in order that the speakers can have optimum sensitivity. Other manufacturers avoid this problem by setting the bias low enough so that crackling never occurs. This is one approach, but we are interested in achieving the best possible sound, which requires that the bias be set to just barely below the threshold where crackling begins.
Please let me know if you have any questions on this topic.