Audio connections longer than 8 inches require shielded cable to prevent electromagnetic hum pickup

An unshielded wire acts as an antenna, picking up 50/60 Hz hum from power lines, computer switching noise, and radio interference. Shielded audio cable wraps the signal conductor in a braided or foil ground conductor, which intercepts the incoming electromagnetic fields before they reach the signal wire. For connections between amplifiers and speakers (low-impedance, high-current), shielding is unnecessary; for all other audio connections—microphone outputs, contact mike cables, mixer connections, anything high-impedance—shielding is required. Rule #9: ‘Use shielded cable to make all audio connections longer than eight inches, unless they go between an amplifier and a speaker.’ Shielded cable has one or more inner conductors (signal) plus a shield (ground).

Examples

A 2-meter patch cable from a contact mic to a mixer uses shielded cable; a 30cm cable from an amp output to a speaker does not need shielding. Hum when touching the cable suggests a broken shield connection.

Assessment

When is shielded cable required and when is it not? What is the role of the shield in the cable, and what happens if the shield is not connected at one end?