Ever heard a hum on your phone line? A phenomenon known as Capacitive Coupling is often the culprit, where wires running in parallel for more than 3 feet induce energy from one wire onto the parallel wire, creating that hum or noise on the line that you hear.

One recent example we ran across was a situation where the cabling for a client’s speaker system ran in parallel to an unused phone cable, causing the internal phone wiring in the building to come alive with a hearty, continuous hum. With the click of a telephone jack, this problem disappeared as we removed the unused telephone cable that was running in parallel to the speaker system’s cabling.

If your struggling with noise on the line in your internal wiring, start by unplugging all phones, cables, fax machines, alarm systems and other devices from your internal wiring. When the sound of silence appears on your test handset, start adding devices and cabling back one by one.

In especially troublesome scenarios, you might start disconnecting jacks at the patch panel or NID of the building, isolating the run that is experiencing capacitive coupling or is bridged by accident to an external power source.

Note that only analog phones (powered by an Analog Telephone Adapter) are affected by this class of issue, deskphones and wireless DECT & WiFi handsets do not fall victim to Capacitive Coupling as the signals are all digital until shortly before they arrive at your device.