Noise is unwanted sound — any sound that interferes with the intended acoustic purpose of a space. The same sound can be noise in one context and perfectly acceptable in another. The hum of an air conditioning system is noise in a recording studio but a welcome source of background masking in an open-plan office. A conversation at a neighbouring restaurant table is noise if you are trying to have a private discussion but ambience if you are dining alone.
This context-dependent definition is what makes noise control both an engineering discipline and a design judgment. Reducing noise is not always about making things quieter — it is about ensuring that the sounds people want to hear are not obscured by sounds they do not.
Real-World Analogy
Imagine trying to read a book while someone shines a flashlight in your face. The light from the flashlight is not inherently bad — in a dark room, you would welcome it. But in this context, it overwhelms the light reflected from the page and makes reading impossible. The flashlight is "light noise."
Acoustic noise works the same way. Speech is the signal you want to read; background noise is the flashlight that washes it out. The louder the background noise, the harder it is to distinguish speech — not because the speech has changed, but because the noise has raised the threshold of what is audible.
Technical Definition
Physically, noise is simply sound — pressure waves propagating through air. What distinguishes noise from useful sound is entirely a matter of context and intent. However, acoustic engineering provides precise ways to quantify, classify, and control it.
Background Noise Level
The background noise level is the sound pressure level (SPL) present in a room when no intentional sound sources are active. It typically comes from:
- HVAC systems: fans, ducts, diffusers, compressors
- External sources: traffic, aircraft, rain, wind
- Building services: plumbing, lifts, electrical transformers
- Occupant activity: conversations in adjacent rooms, footsteps from upper floors
Noise Criteria Curves
Several standardised curve systems rate background noise:
NR (Noise Rating) curves, defined in ISO 1996 and widely used in Europe, specify maximum SPL values at octave band centre frequencies from 31.5 Hz to 8000 Hz. A room rated NR 30 has background noise below the NR 30 curve at every octave band.
NC (Noise Criteria) curves, developed by Beranek in 1957 and updated in ANSI/ASA S12.2, are the North American equivalent. NC curves run from 63 Hz to 8000 Hz and are widely specified in US building standards.
RC (Room Criteria) curves, also from ANSI/ASA S12.2, add spectral quality assessment — they flag spectra that are "rumbly" (excessive low-frequency energy), "hissy" (excessive high-frequency energy), or "neutral" (balanced spectrum).
Signal-to-Noise Ratio
The signal-to-noise ratio (SNR), expressed in dB, is the difference between the desired signal level and the noise level:
SNR = L_signal - L_noise
For speech intelligibility, an SNR of at least +15 dB is needed at the listener position. Below +10 dB, intelligibility degrades rapidly. This relationship is formalised in the Speech Transmission Index (STI) calculation per IEC 60268-16:2020, which computes the effective modulation transfer function at each octave band based on the SNR.
Why It Matters for Design
Noise control is not optional — it is the foundation on which all other acoustic design rests:
Speech intelligibility. In classrooms, meeting rooms, courtrooms, and healthcare facilities, background noise directly determines whether speech can be understood. ANSI S12.60-2010 limits classroom background noise to 35 dB(A) because decades of research show that children's speech comprehension drops sharply above this level.
Concentration and productivity. Open-plan office studies consistently show that irrelevant speech (noise from neighbouring conversations) is the primary source of distraction. Background noise levels and masking systems must be designed together to provide privacy without creating a distractingly loud environment.
Health and wellbeing. The WHO Environmental Noise Guidelines for the European Region (2018) identify noise exposure as a public health concern, linking chronic noise to cardiovascular disease, sleep disturbance, cognitive impairment in children, and annoyance. Building codes increasingly incorporate noise criteria for residential spaces to protect occupant health.
Acoustic privacy. In medical consultation rooms, legal offices, and HR meeting rooms, the goal is not just intelligibility for the occupants but unintelligibility for anyone outside. The background noise level in the corridor or adjacent room must be high enough to mask transmitted speech. Too little background noise can actually be a problem — silence makes even faint speech through a wall audible.
Design hierarchy. Professional acoustic design follows a clear hierarchy: first control the noise, then design the room acoustics. There is no point optimising RT60 for speech clarity if the HVAC system is producing 50 dB(A) of background noise that drowns out the speaker.
How AcousPlan Uses This
AcousPlan's noise criteria module evaluates background noise against NR, NC, and RC curves. You enter octave-band SPL values (measured or estimated from HVAC specifications), and the calculator determines the applicable NR/NC rating, identifies which octave bands exceed the target, and highlights whether the spectrum is balanced or problematically rumbly/hissy.
The STI calculator uses your background noise data as a critical input. It combines the noise levels with the room's reverberation characteristics to predict speech intelligibility — showing exactly how noise and reverberation interact to help or harm communication.
The speech privacy calculator works the problem in reverse: given the speech level, transmission loss, and background noise, it calculates whether speech transmitted through a partition will be intelligible, audible but unintelligible, or inaudible.
Related Concepts
- What is a Decibel (dB)? — The unit used to measure noise levels
- What is Sound Pressure? — The physical quantity noise measurements capture
- What Are Octave Bands? — The frequency framework for noise analysis
- What is Sound Masking? — Using controlled noise to improve privacy
- What is Transmission Loss? — How barriers reduce noise between spaces
Calculate Now
Assess your room's noise environment with the AcousPlan Room Calculator. Enter background noise levels, check NR/NC compliance, and see how noise affects speech intelligibility in your space.