Before an acoustic consultant can design a room, recommend treatment, or predict speech intelligibility, they need to know one thing: how loud is the background? The air conditioning hum, the traffic rumble, the electrical transformer buzz — these ambient sounds set the noise floor against which everything else is heard. A background noise survey quantifies that floor, and it is the starting point for almost every acoustic assessment.
TLDR
A background noise survey is a systematic measurement of ambient sound levels in a space, conducted in octave bands (typically 31.5 Hz to 8 kHz) to characterise the frequency spectrum of the background noise. The survey is performed with a calibrated sound level meter or acoustic analyser, with all intended noise sources operating (HVAC systems, equipment) and no occupant activity. Results are plotted against standard noise criteria curves — NC (Noise Criteria), NR (Noise Rating), or RC (Room Criteria) — to determine whether the space meets its design target. Background noise directly affects STI (speech intelligibility), sleep quality, concentration, and occupant comfort. Standards including ASHRAE, ANSI S12.60, and WHO Environmental Noise Guidelines specify maximum background noise levels for different room types. The survey is typically the first measurement taken in any acoustic assessment project.
Real-World Analogy
Imagine trying to have a conversation at a quiet dinner table versus next to a busy highway. The words you speak are the same volume in both locations, but the highway noise masks your voice, forcing you to shout. A background noise survey measures the "highway" — the existing noise in a room before anyone speaks. The quieter the background, the easier communication becomes. The survey tells the designer how much of the acoustic "budget" is already consumed by noise before the room's primary function (speech, music, sleep) even begins.
Technical Definition
Measurement Equipment
- Sound level meter: Class 1 (precision) or Class 2 (general survey), per IEC 61672-1:2013. Must have octave-band analysis capability (IEC 61260-1:2014).
- Calibrator: Pistonphone or electronic calibrator per IEC 60942, used before and after every measurement session.
- Microphone: Free-field or diffuse-field type, positioned 1.2 to 1.5 m above floor level (seated ear height) or 1.5 to 1.7 m (standing).
- Windscreen: Always used, even indoors, to prevent air currents from HVAC registers affecting readings.
Measurement Procedure
Per ASHRAE guidelines and common practice:
- Schedule: Measure during normal operating conditions (HVAC on, typical equipment running) but without occupant activity. For offices, this means HVAC in normal cooling mode, computers and lighting on, no people.
- Duration: Minimum 1 minute of continuous measurement per position. For fluctuating noise, 5 to 15 minutes using Leq (equivalent continuous level).
- Positions: Multiple positions within the occupied zone, typically on a 2 to 3 metre grid. Minimum 1 metre from any wall or surface to avoid near-field effects.
- Parameters recorded: Octave-band Leq at 63, 125, 250, 500, 1000, 2000, 4000, 8000 Hz. Also record dBA (A-weighted overall level).
- Documentation: Record date, time, HVAC mode, external conditions (traffic, weather), meter calibration values, and measurement positions on a floor plan.
Interpreting Results with Criteria Curves
Measured octave-band levels are plotted against the relevant criteria curve:
- NC (Noise Criteria): The NC rating is the lowest NC curve that is not exceeded by any octave band measurement. Example: if the measured 250 Hz level is 42 dB and all other bands are below the NC-40 curve, the room is rated NC-42 (the 250 Hz band controls).
- NR (Noise Rating): Similar to NC but uses ISO 717-defined curves. More common in Europe.
- RC (Room Criteria): Includes a spectral balance assessment (rumble, neutral, hiss) beyond just the overall level. Per ASHRAE, RC is the preferred metric for mechanical system noise.
Typical Target Values
| Room Type | NC Target | NR Target | dBA Target |
|---|---|---|---|
| Recording studio | NC-15 to NC-20 | NR-15 to NR-20 | 20-25 |
| Concert hall | NC-15 to NC-20 | NR-15 to NR-20 | 20-25 |
| Private office | NC-30 to NC-35 | NR-30 to NR-35 | 35-40 |
| Open-plan office | NC-40 to NC-45 | NR-40 to NR-45 | 40-45 |
| Classroom | NC-25 to NC-30 | NR-25 to NR-30 | 30-35 |
| Hospital ward | NC-25 to NC-30 | NR-25 to NR-30 | 30-35 |
| Restaurant | NC-40 to NC-45 | NR-40 to NR-45 | 40-48 |
Why It Matters for Design
Background noise affects every aspect of acoustic design:
Speech intelligibility (STI): The modulation transfer function is degraded by background noise in proportion to the signal-to-noise ratio at each octave band. Per IEC 60268-16, every 3 dB increase in background noise at a given frequency reduces the modulation transfer by a measurable amount. A room with perfect RT60 but background noise of NC-45 will have significantly lower STI than the same room at NC-25.
Sound masking design: In open-plan offices, a controlled, uniform background noise (NC-40 to NC-45) actually improves privacy by masking distant conversations. The noise survey tells the designer what the existing noise spectrum looks like, so they can design the masking system to fill gaps rather than add to already-loud bands.
Compliance: Standards like ANSI S12.60 (classrooms, max 35 dBA) and WELL v2 Feature S02 (max NC levels by room type) require background noise surveys as evidence of compliance. Without a survey, there is no certification.
How AcousPlan Uses This
AcousPlan's room builder accepts background noise levels as inputs — either as a single dBA value or as octave-band levels. The platform uses these values in the STI calculation and compares them against NC, NR, and RC criteria for the selected room type. If the noise level exceeds the target, the AI co-pilot flags the issue and suggests mitigation strategies (HVAC silencers, duct lining, equipment relocation). The noise criteria chart in the results dashboard plots your measured or estimated levels against the reference curves.
Related Concepts
- What is an NC Rating? — Noise Criteria system
- What is an NR Rating? — Noise Rating system
- What is an RC Rating? — Room Criteria system
- How is STI Measured? — How noise affects intelligibility
- What is STI? — The intelligibility metric noise degrades
Calculate Now
Input your background noise levels into AcousPlan and see how they affect STI, NC compliance, and overall room acoustic performance.