How to Diagnose and Fix RF Interference in Audio

Symptoms

RF interference in audio produces a variety of distinctive sounds depending on the source. Cell phone interference creates a rhythmic buzzing pattern (the "GSM buzz") when a phone is transmitting near audio cables. Wi-Fi interference causes a rapid clicking or digital hash sound. LED dimmer drivers create a high-pitched whine or buzzing that correlates with light level. Switching power supplies introduce broadband hash with energy concentrated above 10kHz. FM radio stations can be faintly audible through audio equipment in areas with strong transmitters. The interference may be intermittent, corresponding to when wireless devices transmit.

Common Causes

Audio equipment and cables act as unintentional antennas. Long unbalanced cable runs are particularly susceptible because the shield-to-center conductor geometry creates an effective receiving antenna. Poor shield connections allow RF to enter the signal path. Cable connectors with broken or intermittent shield contact create RF entry points. Equipment with inadequate RF filtering on input stages demodulates the RF signal into audible frequencies. Proximity to high-power wireless transmitters, cell towers, or broadcast antennas increases RF field strength. Switch-mode power supplies in LED fixtures, laptop chargers, and digital equipment generate conducted and radiated emissions.

Measurement Procedure

1. Open SonaVyx RTA mode at maximum resolution (1/24 octave or unsmoothed).
2. With no program material, observe the noise floor spectrum.
3. RF interference typically appears as spectral content above the normal thermal noise floor, often with a distinctive pattern.
4. Cell phone interference shows as periodic bursts at 217Hz repetition rate.
5. Switching supply interference shows as energy peaks at the switching frequency and harmonics (typically 50kHz-200kHz, aliases may appear in the audible range).
6. Systematically power off potential RF sources to identify the culprit.

Interpretation

RF interference is identified by its distinctive spectral signature and its correlation with external events. If the noise appears when someone makes a phone call, it is cell phone RF. If it correlates with LED fixtures, it is conducted or radiated EMI from the drivers. If it appears randomly, scan for nearby wireless equipment or transmitters. The key diagnostic is to systematically eliminate potential sources by powering them off one at a time and monitoring the RTA for changes.

Solutions

Replace unbalanced connections with balanced. Use high-quality cables with continuous braided shields rather than spiral wrapped shields — braided shields provide 30-40dB better RF rejection. Add ferrite chokes to cable ends, particularly on long analog runs and near RF sources. Keep audio cables away from power cables, LED drivers, and wireless equipment. Use RF filters on audio input stages where equipment does not have adequate built-in filtering. For persistent cell phone interference, establish a "no phones near FOH" policy and route audio cables away from audience areas. Shield sensitive equipment in RF-blocking enclosures if the environment has high ambient RF levels.

Verification

After treatment, monitor the RTA noise floor. The RF-related spectral content should be gone or reduced below the thermal noise floor. Test with known RF sources (hold a transmitting phone near treated cables) to verify the shielding is effective.

Open RTA to identify RF interference