What is the difference between NC and NR curves?

NC (Noise Criteria) curves are used primarily in North America and define acceptable octave band levels from 63 Hz to 8 kHz. NR (Noise Rating) curves serve the same purpose in international standards but use different reference values. Both systems assign a single-number rating based on the worst octave band. NC-30 is approximately equivalent to NR-30.

What background noise level is acceptable for a classroom?

ANSI S12.60 requires background noise not exceeding 35 dBA (approximately NC-30) for classrooms with volumes up to 283 cubic meters. Larger rooms may have slightly relaxed criteria. These levels ensure adequate speech intelligibility for learning environments.

How does background noise affect acoustic measurements?

Background noise limits the dynamic range and low-level accuracy of acoustic measurements. For RT60 measurement, the noise floor must be at least 35 dB below the initial decay level for valid T20, or 45 dB for T30. High background noise also reduces STI measurement accuracy by degrading the modulation transfer function.

What is the L90 noise level?

L90 is the sound level exceeded 90 percent of the measurement period. It effectively strips away intermittent noise events and represents the underlying steady background noise. L90 is used in BS 4142 for industrial noise impact assessment and is generally the most appropriate single-number descriptor for background noise.

Can I reduce background noise with acoustic treatment?

Acoustic absorption reduces reverberation but does not significantly reduce background noise unless the noise source is within the room. HVAC noise requires mechanical solutions (duct silencers, isolation mounts, reduced fan speed). External noise requires improved building envelope (better windows, sealed gaps). Treatment helps with noise from within the room only.

What is the difference between NC and NR curves?

NC (Noise Criteria) curves are used primarily in North America and define acceptable octave band levels from 63 Hz to 8 kHz. NR (Noise Rating) curves serve the same purpose in international standards but use different reference values. Both systems assign a single-number rating based on the worst octave band. NC-30 is approximately equivalent to NR-30.

What background noise level is acceptable for a classroom?

ANSI S12.60 requires background noise not exceeding 35 dBA (approximately NC-30) for classrooms with volumes up to 283 cubic meters. Larger rooms may have slightly relaxed criteria. These levels ensure adequate speech intelligibility for learning environments.

How does background noise affect acoustic measurements?

Background noise limits the dynamic range and low-level accuracy of acoustic measurements. For RT60 measurement, the noise floor must be at least 35 dB below the initial decay level for valid T20, or 45 dB for T30. High background noise also reduces STI measurement accuracy by degrading the modulation transfer function.

What is the L90 noise level?

L90 is the sound level exceeded 90 percent of the measurement period. It effectively strips away intermittent noise events and represents the underlying steady background noise. L90 is used in BS 4142 for industrial noise impact assessment and is generally the most appropriate single-number descriptor for background noise.

Can I reduce background noise with acoustic treatment?

Acoustic absorption reduces reverberation but does not significantly reduce background noise unless the noise source is within the room. HVAC noise requires mechanical solutions (duct silencers, isolation mounts, reduced fan speed). External noise requires improved building envelope (better windows, sealed gaps). Treatment helps with noise from within the room only.

How to Measure Background Noise

6 steps15-20 min readUpdated 2026-03-20

Quick Answer

Measuring background noise means capturing the ambient sound level in a room with all intended noise sources (HVAC, equipment, external intrusion) operating normally and all occupants and intentional sound sources silent. The result, expressed as dBA Leq, NC rating, or NR rating, determines the room's suitability for its intended use and impacts speech intelligibility, measurement quality, and occupant comfort.

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Equipment Needed

✓SonaVyx SPL Meter with octave band analysis
✓Measurement microphone (calibrated)
✓Tripod or mic stand at ear height
✓Timer or stopwatch for measurement period control
✓Room floor plan for documenting measurement positions

Step-by-Step Guide

Prepare the Room

Set the room to its normal operating condition: HVAC running at typical settings, all mechanical equipment active, windows and doors in their normal state. Remove any temporary noise sources (construction, cleaning equipment). Ask all occupants to leave or remain silent. The goal is to capture the noise environment that exists during normal use, not a best-case or worst-case condition. If the room serves different purposes at different times, measure under each relevant condition.

Position the Microphone

Place the measurement microphone at the listener or occupant position, 1.2 to 1.5 meters above the floor, at least 1 meter from any surface. For rooms with multiple seating areas, measure at 3 to 5 representative positions. For rooms assessed per BS 8233 or ISO 1996, specific position requirements apply: typically at the center of the room and 1 meter from the facade. Avoid positions directly in front of HVAC diffusers where local air velocity creates false readings.

Select Measurement Parameters

In SonaVyx's SPL meter, select A-weighting and Slow time constant for the primary dBA measurement. Also enable octave band analysis from 63 Hz to 8 kHz for NC/NR curve assessment. Set the measurement duration to at least 5 minutes for steady-state noise, or longer if the noise fluctuates. Enable statistical analysis (L10, L50, L90) to characterize noise variability. For environmental assessments, 15-minute to 1-hour measurements provide more representative data.

Capture the Measurement

Start the measurement and remain silent and still during the capture period. Any movement, speaking, or equipment noise in the room will contaminate the reading. If unexpected noise events occur (phone ringing, door slamming, aircraft flyover), note the time and either extend the measurement or use the L90 value, which excludes the loudest 10 percent of the measurement period and represents the underlying steady background noise level.

Analyze NC/NR Rating

From the octave band data, determine the Noise Criteria (NC) or Noise Rating (NR) value. NC curves, widely used in North America, define maximum octave band levels for different room types. NR curves serve the same purpose in European and international standards. SonaVyx automatically calculates the NC and NR rating by comparing your measured octave band levels against the reference curves. The NC rating equals the highest NC curve that is not exceeded by any octave band measurement.

Compare Against Standards

Evaluate the measured background noise against applicable criteria. Concert halls and recording studios should meet NC-15 to NC-20. Classrooms per ANSI S12.60 must not exceed 35 dBA (approximately NC-30). Open offices typically target NC-35 to NC-40. Bedrooms per BS 8233 should achieve 30 dBA Leq at night. If the measured noise exceeds the criteria, identify the dominant source using the octave band spectrum: HVAC noise peaks at 63 to 250 Hz, traffic noise at 125 to 500 Hz, and electrical noise at exact harmonics of 50 or 60 Hz.

Understanding Background Noise Impact

Background noise affects room usability in multiple ways. It directly reduces speech intelligibility by masking consonant sounds, primarily in the 1 to 4 kHz range. It limits the usable dynamic range for music performance and recording. It causes occupant discomfort, fatigue, and reduced productivity. And it establishes the noise floor that limits the quality of acoustic measurements. For every 10 dB of background noise reduction, speech intelligibility improves by approximately 0.05 STI units and measurement dynamic range increases proportionally.

Steady vs Fluctuating Noise

Steady-state noise from HVAC and building services is typically constant in level and spectrum, making it straightforward to measure and characterize. Fluctuating noise from traffic, aircraft, and external sources varies continuously. For fluctuating noise, the Leq (equivalent continuous level) provides the energy-average over the measurement period, while L90 gives the steady background component. The difference between L10 and L90 quantifies the fluctuation range. Standards like BS 4142 use the L90 as the representative background noise level for noise impact assessment.

Noise Source Identification

The octave band spectrum of background noise helps identify the dominant source. HVAC rumble appears as elevated levels at 63 and 125 Hz. Duct-borne noise shows peaks at multiple low-frequency bands. Lighting dimmer noise appears as narrow peaks at harmonics of the mains frequency (100/120 Hz, 200/240 Hz). Traffic noise has a broad peak from 125 to 500 Hz. Identifying the source is the first step toward effective noise control, which is far more cost-effective than masking noise with treatment.

Common Mistakes to Avoid

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Measuring with HVAC off, which produces an unrealistically low background noise reading that does not represent actual conditions

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Standing too close to the microphone during measurement, adding body-reflected noise and breathing sounds

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Using too short a measurement period for fluctuating noise, producing an Leq that varies between measurements

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Reporting only dBA without octave band data, which prevents NC/NR evaluation and noise source identification

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Measuring at a single position and assuming it represents the entire room, missing HVAC hot spots and corner effects

Applicable Standards

Standard	Clause	Relevance
ISO 1996-2:2017	Clause 7	Residual sound measurement methodology
BS 8233:2014	Clause 7	Indoor ambient noise level criteria for different room types
ANSI/ASA S12.60-2010	Clause 5	Maximum background noise levels for classrooms

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