Q: “Why are two conductors of my balanced mic cable twisted together?”
A: First, check out the WFTD for Balanced to understand that these are the positive and negative conductors of your cable. The interference-canceling characteristic of a balanced line is based on the premise that unwanted external noise is induced into both signal conductors equally. Minimizing the distance between the two conductors by twisting them together helps to equalize their reception of external interference and improve the common-mode rejection ratio of the line.
The two conductors also form a sort of “loop antenna” that picks up stray magnetic fields. The farther apart the two conductors are the larger the “antenna” becomes, and the more interference it picks up from sources such as transformers, fluorescent lighting ballasts, stage lighting, etc. Minimizing the loop area of the cable helps to reduce the unwanted hum and buzz from this type of interference, which the cable’s shield is almost totally ineffective against.
The distance between the twists is called the lay of the pair. Reducing the lay (increasing the number of twists) improves its common-mode rejection, and also improves its flexibility. The typical pair lay in microphone cables is about 0.75″ to 1.5″. Shortening the pair lay uses more wire and more machine time to produce the same overall finished length, so of course it increases the cost of the cable.