Here's some stuff on measuring a twisted pair cable for inductive coupling to itself via coiling, and to the external environment. Upper right is what theory predicts will happen for a single layer orderly wound cable on a non conductive bobbin, L vs bobbin circumference.
This page is a direct consequence of discussions with Ed Simon, where he mentioned how a mike cable (speaker cable as well) behaves differently when coiled. My drawing is designed for testing of that observation.