Click here to purchase the entire book in PDF format. Magnetic field coupling Magnetic field coupling is determined by the mutual inductance between the source and receiver. Remember back to the chapter where we talked about the right hand rule and how, when we send AC through a wire, we generate a pulsing magnetic field around it whose direction is dependent on the direction of the current and whose amplitude (or distance from the wire) is proportional to the level of the current. If we place another wire in this moving magnetic field, we will induce a current in the second wire - which is how a transformer works. Although this concept is great for transformers, it's a problem when we have microphone cables sitting next to high-current AC cables. If there's lots of current through the AC cable at 60 Hz, and we place the mic cable in the resulting generated magnetic field, then we will induce a current in the mic cable which is then amplified by the mic preamplifier to an audible level. This is bad, but there are a number of ways to avoid it as we'll see. The level of this noise is proportional to: The loop area of the receiver (therefore it's best not to create a loop with your mic cables) The frequency of source The current of source The permeability of the medium between them The level of this noise is inversely proportional to: The square of the distance between them (so you should keep mic cables away from AC cables! - and if they have to cross, cross them at a right angle to each other to minimize the effects)