How to Measure and Reduce Background Noise

Symptoms

Background noise is the constant ambient sound present in a room when no program material is playing. It includes HVAC systems, traffic, electrical hum, refrigeration equipment, building systems, and outdoor environmental sources. When background noise is too high, listeners must strain to hear speech, recording quality suffers, and the sound system must be driven louder to achieve adequate signal-to-noise ratio, which increases the risk of feedback and listener fatigue.

Common Causes

HVAC systems are the dominant background noise source in most buildings, contributing broadband noise through ductwork and vibration. External traffic noise penetrates through windows, doors, and lightweight wall constructions. Electrical systems contribute transformer hum and fluorescent light ballast buzz. Refrigeration and kitchen equipment in adjacent spaces transmit through shared walls. Computer equipment, projectors, and AV racks create fan noise. In open-plan spaces, speech from adjacent areas contributes to the overall noise floor. Building mechanical systems like elevators, pumps, and generators transmit vibration through the structure.

Measurement Procedure

1. Open SonaVyx SPL Meter with A-weighting and octave band analysis.
2. Turn off all sound system equipment so only background noise remains.
3. Measure LAeq over 60 seconds at the primary listening position.
4. Record the 1/1 octave band levels from 63Hz through 8kHz.
5. Compare the octave band profile to NC curves in the SonaVyx display.
6. The NC curve that your octave band levels just touch (tangent method) determines your room NC rating.
7. Compare this to the target NC rating for your room type.

Target NC Ratings

Each room type has an appropriate background noise level. Recording studios: NC-15 to NC-20. Concert halls: NC-15 to NC-25. Theaters and churches: NC-25 to NC-30. Classrooms: NC-25 to NC-35. Conference rooms: NC-25 to NC-30. Open offices: NC-40 to NC-45. Restaurants: NC-40 to NC-50. If your measured NC rating exceeds the target for your room type, noise reduction is needed.

Solutions

Address the loudest source first for maximum impact. For HVAC, install duct silencers, reduce fan speed, or add vibration isolation. For traffic noise, upgrade windows to double or triple glazing, seal air gaps around doors and windows, and add mass to lightweight walls. For electrical hum, replace magnetic ballasts with electronic ballasts, distance transformers from the listening area, and check for ground loops. For equipment noise, relocate noisy equipment to a separate room or closet with adequate ventilation. For inter-room noise, add mass to shared walls (additional drywall layers) and seal all penetrations. Prioritize the octave band that exceeds the NC target by the most — that band determines the overall NC rating.

Verification

After each intervention, remeasure the octave band background noise. The NC rating should improve (decrease) toward the target for your space. Measure STI as well, because reducing background noise directly improves speech intelligibility. A 10dB reduction in background noise can improve STI by 0.15 to 0.25 points.

Open SPL Meter for background noise measurement