Every audio connection requires both a signal conductor and a ground return, and shielded cable protects longer runs from hum

An audio signal only has meaning relative to a reference voltage (ground). Every jack has a tip (hot/signal) and a sleeve (ground/shield). Shielded cable — a signal conductor wrapped in a braided or twisted shield — prevents the signal from picking up electromagnetic interference. The shield should be connected to sleeve/ground at both ends; reversing tip and sleeve causes persistent hum. Collins states this as Rule 9 (use shielded cable for audio connections longer than 8 inches) and Rule 10 (every audio connection consists of two parts: the signal and a ground reference). The exception is speaker wire, which carries a power-level signal that is less vulnerable to interference.

Examples

When building a contact mic, connect the Piezo disk’s hot wire to the cable’s inner conductor and its ground to the cable’s shield. Forgetting to slide the plug barrel onto the wire before soldering is the most common beginner mistake.

Assessment

Trace the signal path of a contact mic connected to an amplifier. Identify where the signal and ground travel and explain what would happen if the shield were left unconnected.