... .25", 1.5", 2", 2.5", 3", 4" 2. Material: Aluminum, Pure Oxygen-free Copper 3. Lay: 2 lays and 4 ... internal side of the bobbin essentially an internal / external voice coil. For multi layers of four or more. For flat wire ... special winding designs in order to meet the customers needs. High quality Voice Coil