US Patent 7880087 Toneable conduit with loose toning signal wire

A toneable conduit includes an elongate polymeric tube having a sidewall with an interior surface and an exterior surface. The interior surface defines a main channel. A sub-channel, smaller in cross sectional area than the main channel, extends longitudinally between the exterior and interior surfaces of the tube. A continuous toning signal wire is located within the sub-channel. The sub-channel has a larger cross sectional area than a cross sectional area of the toning signal wire, so that an air gap exists within said sub-channel around portions of the toning signal wire.