Background noise is a critical factor in the domain of acoustics, essential for effective noise control and environmental assessments. It is generally defined as noise that contributes to the total sound level but is not the specific source under investigation.
Definition: While the term "residual sound" is preferred in ISO 1996 terminology (ambient noise without the specific source noise), "background noise" is often commonly used. It is used as a value of a noise parameter, such as the LA90 (the level exceeded for 90% of the measurement time).
Measurement: Because background noise often contains complex characteristics like impulse noise or pure tones, a simple measurement of Sound Pressure Level (SPL) is usually insufficient, necessitating detailed frequency analysis (octave or 1/3-octave band) and energy-based metrics like the Equivalent Continuous Level (LAeq)
Noise Control Planning: Background noise often contributes significantly to the total noise in a workspace. Before making decisions about individual noise control measures, measuring background noise is necessary.
Environmental Influence: Background noise is one of the two main environmental effects that should be considered when noise measurements are carried out, the other being sound reflections.
Validity Threshold: Background noise at least 10 dB below the level of the noise emitted by the source being assessed is considered accurate to within 0.5 dB.
ISO 1996-2 Correction Requirement: When the residual sound pressure level is more than 3 dB below the measured sound pressure level, the level shall be corrected.
Foundational concepts such as ‘ambient’, ‘residual’, and ‘background’ noise are central to describing the acoustic environment, yet they are frequently a source of confusion and inconsistent application. This section provides a systematic analysis of these critical terms, clarifying their distinct definitions, contextual roles, and interrelationships as established by authoritative international standards.
Background noise is defined as noise from unwanted sources that may influence the accuracy of acoustic measurements of a specific sound source of interest. Background noise often arises from secondary sources, such as ventilation or compressors, and can contribute significantly to the overall total noise, particularly in industrial settings. In acoustics, the background sound refers to the noise level measured when the source being assessed is silent.
Ambient sound is the totally encompassing sound present in a given situation at a given time. It is composed of sound from all sources, both near and far. As noted in BS 4142, the ambient sound level is a measure of the combined effect of the residual sound and the specific sound source being assessed, when that source is active.
In practice, ambient noise is the total noise arising from all combined sources present within a specific environment. This total includes all noise components such as factory noise, road traffic noise, birdsong, and running water. The noise from a specific source under investigation is merely one component of the broader ambient noise. Because nearly every environment contains numerous sound sources, many different contributors generate ambient noise at any single measurement location.
Residual sound is the ambient sound that remains at the assessment location when the specific sound source under investigation is absent or suppressed to a degree that it is no longer a contributing factor. This is the existing acoustic environment or soundscape without the source of interest. The corresponding metric is the residual sound level, denoted as Lr = LAeq,T.
When a specific noise source cannot be switched off to directly measure the residual sound (the sound remaining without the specific source), alternative methods are required, and statistical analysis plays a vital role in quantifying the underlying noise environment.
For example, when assessing noise from a major highway, the source cannot be suppressed. In such cases, ISO 1996-2 (Annex I) notes that residual sound can be difficult to measure directly and may require alternative methods, such as using the L95 percent exceedance level as an approximation of the underlying noise floor.
BS 4142 provides an alternative method: conducting measurements at a different location where the residual sound is demonstrably comparable to that at the assessment location. This approach requires rigorous justification, demonstrating that the alternative location is acoustically comparable. Key factors include ensuring the location is the same distance from other dominant residual sound sources, is acoustically screened from the specific source, shares similar ground cover, and that measurements are conducted under identical meteorological conditions.
The background sound level (LA90,T), a critical measure for assessing impact, is inherently defined statistically as the A-weighted sound pressure level exceeded by the residual sound for 90% of a given time interval.
| Term | Definition by BS 4142:2014+A1:2019 |
|---|---|
| Ambient sound | Totally encompassing sound in a given situation at a given time, usually composed of sound from many sources near and far. NOTE: The ambient sound comprises the residual sound and the specific sound when present. |
| Ambient sound level, La = LAeq,T | Equivalent continuous A-weighted sound pressure level of the totally encompassing sound in a given situation at a given time, usually from many sources near and far, at the assessment location over a given time interval, T. NOTE: The ambient sound level is a measure of the residual sound and the specific sound when present. |
| Background sound level, LA90,T | A-weighted sound pressure level that is exceeded by the residual sound at the assessment location for 90% of a given time interval, T, measured using time weighting, F, and quoted to the nearest whole number of decibels. |
| Residual sound | Ambient sound remaining at the assessment location when the specific sound source is suppressed to such a degree that it does not contribute to the ambient sound. |
| Residual sound level, Lr = LAeq,T | Equivalent continuous A-weighted sound pressure level of the residual sound at the assessment location over a given time interval, T. |
| Specific sound level, Ls = LAeq,Tr | Equivalent continuous A-weighted sound pressure level produced by the specific sound source at the assessment location over a given reference time interval, Tr. |
| Specific sound source | Sound source being assessed. |
The most common point of confusion arises between ‘residual sound’ and ‘background sound level’. The distinction is critical: residual sound is the total acoustic environment when the specific source is off. The background sound level (LA90,T) is a specific statistical metric derived from the residual sound. It is not the average of the residual sound but rather a measure of its typical, underlying quiet periods, which BS 4142 uses as the primary reference for assessing the impact of a new industrial or commercial sound. Therefore, the logical hierarchy is: Residual Sound (the physical phenomenon) is measured to produce a Background Sound Level (the statistical descriptor).
| Equation / Note |
|---|
| Ambient Sound = Specific Sound Source + Residual Sound |
| The background sound level is a specific statistical metric derived from the Residual Sound, not from the Ambient Sound. |
The term noise floor refers to the minimum measurable signal or the level of internal noise inherently generated by the components of an acoustic measurement system, such as a sound level meter or analyzer. This intrinsic noise sets the limit below which valid acoustic measurements cannot reliably be made.
For example, the noise floor of a sound level meter system is composed of several factors associated with the measurement electronics (e.g. microphone self-noise, preamplifier noise). The relationship between the actual signal being measured and the instrument’s noise floor determines the signal-to-noise ratio (SNR).
Accurate acoustic measurements require the signal of interest to be well above this minimum level. By ISO 1996-2:20217, for all measurements, the background noise of a measurement system (sound level meter) should be at least 5 dB below the sound to be measured.
A fundamental metric derived directly from the concept of background noise is the signal-to-noise ratio (SNR). As defined in IEC 60268-16 (3.35), the Signal-to-Noise Ratio (SNR) is the difference between the sound pressure level of the signal (speech or test signal) and the sound pressure level of the background noise. This ratio is a primary determinant of measurement validity, whether quantifying the margin above the noise floor for a sound power test or modeling the perceptual clarity of speech.
Background noise is a critical factor in acoustic engineering, defined both conceptually as unwanted sound and technically as the residual sound field existing when the source of interest is inactive. Its proper identification, measurement, and correction are essential to ensure the validity and accuracy of sound level measurements.
For noise measurements (especially sound pressure levels) to be considered reliable and accurate, the specific noise produced by the source must significantly exceed the level of the unwanted background noise. Measurement procedures are entirely based on the difference between the total measured noise and the background noise alone.
The typical components (sources) of a measured background noise are diverse and can be categorized based on their origin:
Measurement Validity and Correction: Background noise (LN) is recognized as one of the main environmental effects that must be considered during sound pressure measurements.
Noise Control Planning and Implementation: Background noise must always be checked and mapped before making decisions about individual noise control measures, as it often contributes significantly to the total measured noise.
Sound Intensity Measurement Technique: Sound intensity measurements offer a distinct advantage regarding background noise compared to standard sound pressure methods.
Psychoacoustics and Safety: In high-noise environments, background noise presents an accident risk because the psychoacoustic phenomenon of masking can prevent warning signals or shouts from being heard.
Background noise, fundamentally defined as unwanted sound, poses significant and wide-ranging threats to human health across auditory, physiological, and psychological domains. The most severe physical effect is irreparable hearing damage caused by prolonged exposure to loud sounds, which damages the sensory hair cells of the inner ear, leading to progressively impaired hearing ability.
The risk increases with sound level and exposure duration; noise levels above 85 dB may cause hearing damage, ranging from a temporary threshold shift to permanent hearing loss. Furthermore, intense audible noise affects the cardiovascular and endocrine systems, influencing blood circulation and causing stress. Non-auditory bodily reactions include dilation of pupils, increased heart rate, and the production of hormones such as adrenaline and corticotrophine, alongside the constriction of blood vessels.
Noise is a worldwide environmental problem that causes widespread sleep disturbance, annoyance, and adverse health effects, reducing efficiency and causing tiredness. The degree of annoyance is highly subjective, depending on the listener’s attitude towards the noise source. Specific characteristics worsen these effects: abrupt impulsive noise causes greater annoyance and is very harmful to hearing, while low frequency noise (typical of large diesel engines) is hard to muffle and often causes more annoyance than anticipated from simple sound pressure measurements. Critically, noise can also present an accident risk by masking warning signals or shouts.
Background noise also severely compromises communication and psychological well-being, ranging from mild annoyance to significant interference with speech.
Speech intelligibility (STI) is significantly reduced by high levels of background noise, affecting communication and potentially masking warning signals. For normal conversation, the sound level should be at most 65 to 70 dB in a work area. Noise levels above N85 (a noise rating number) can interfere seriously with speech intelligibility. Noise levels can also create an accident risk by masking warning shouts or signals. Noise levels above N75 are typically considered unsatisfactory for telephone communication
Background noise primarily reduces the signal-to-noise ratio (SNR), making the desired signal less prominent and harder to perceive clearly. When background noise has similar frequency content to the signal (like speech), it can mask the signal, rendering parts of it unintelligible.
Background noise is highly significant in work and environmental acoustics due to the psychoacoustic effect of masking, which occurs when one sound is sufficiently loud that a second, quieter sound is effectively “drowned out”. This physical phenomenon is explained as a shift in the listener’s hearing threshold caused by the louder background noise. In the occupational and environmental safety, background noise presents a critical accident risk in working environments because it can be loud enough to mask essential warning signals or shouted instructions. Moreover, background noise levels, particularly those below an N85 Noise Rating, are known to interfere seriously with the intelligibility of speech. Because masking influences subjective perception, these effects are incorporated into advanced noise criteria and methods, such as those suggested by E. Zwicker, which aim to determine the overall loudness and annoyance caused by noise environments from objective sound pressure level measurements.
Noise Rating (NR) as defined by the old version of ISO 1996-1971 is a set of suggested noise control criteria used primarily to evaluate annoyance and health risks, in the form of sound pressure level spectrum curves (in octave frequency bands). For instance, the criterion N85 is particularly significant because noise levels below N85 may interfere seriously with the intelligibility of speech, while noise levels above N85 may cause hearing damage.
Acoustically, background noise can cause masking, reducing speech intelligibility, which is directly relevant to the communicative impact assessed by N85 criteria; and practically, before noise levels can be accurately determined and compared against any Noise Rating criteria (N), the background noise (noise from unwanted sources) must be measured and mathematically corrected for, as measurements are considered unreliable if the source noise is not at least 3 dB higher than the background noise alone. Furthermore, when calculating related parameters like the Rating Level (Lr), which quantifies annoyance, some assessment standards compare this calculated level directly to the background noise (often measured as L90) to establish relative limits.
Accurate quantification of ambient, residual, and background sound requires a rigorous methodological framework that involves standardized procedures for data acquisition, strategies for overcoming common field challenges, and a critical appreciation for measurement uncertainty, as detailed in standards such as BS 4142 and ISO 1996-2.
To ensure reliable and repeatable results, acoustic measurements must adhere to strict procedural guidelines. A typical setup includes:
Consistent measurement is key to understanding and managing background noise. The goal is to quantify the ambient sound level before attempting to reduce it. Because background noise often contains complex characteristics like impulse noise or pure tones, a simple measurement of Sound Pressure Level (SPL) is usually insufficient, necessitating use of time-averaged Equivalent Continuous Level (LAeq,T) or a detailed frequency analysis (octave or 1/3-octave band). Measurements should ideally be taken under relevant conditions (e.g., room unoccupied vs. occupied) using calibrated sound level meters (SLMs).
For sounds that fluctuate or are intermittent, selecting a measurement time interval that is truly representative is crucial. The duration must be sufficient to capture the typical acoustic conditions to derive a reliable value for the background sound level (LA90,T), as noted in BS 4142.
In traditional sound pressure level measurements for environmental impact (BS 4142) or building acoustics (ISO 16283-1), it is a source of contamination that must be carefully measured and corrected for. In contrast, for sound intensity measurements, the influence of steady background noise can be largely negated by the physical principles of the technique, allowing for accurate source characterization even in noisy environments.
| Metric | Description | Common Use |
|---|---|---|
| Leq | Equivalent Continuous Sound Level (average over time) | Environmental noise, workplace safety, room noise |
| L90 | Statistical level exceeded 90% of the time | Representing baseline background noise |
| SNR | Signal-to-Noise Ratio (difference between signal and noise dB) | Audio quality, speech intelligibility |
| NC / RC | Noise Criteria / Room Criteria (ratings based on octave bands) | HVAC noise specification, room design goals |
| Noise Floor | Lowest inherent noise level of a system or environment | Audio system performance, recording limits |
| Difference | Required Action | Result Reliability |
|---|---|---|
| Greater than 10 dB | No correction is necessary. | Measurement is accurate to within 0.5 dB. |
| Between 3 dB and 10 dB | A correction must be made. | An approximate correction can be calculated or derived from a correction chart. |
| Less than 3 dB | The background noise level is too high. | A reliable value for the source noise alone cannot be obtained. |
ISO 1996-2:2017 (Annex I) gives two ways to estimate residual (background) sound. First, when the test source contributes for 5% or less of the total measurement time, the level exceeded during 95% of the time (L95) may be taken as representative of the residual sound pressure level. Second, if the residual sound can be treated as Gaussian, the equivalent residual level can be calculated from percentiles using L50 and L90 or L95 accoring to the Leq,Gauss formula.
In the field of building acoustics, specifically the measurement of sound insulation as described in PN-EN ISO 16283-1, background noise is treated as a source of measurement contamination. Here, background noise is defined as the sound from all sources in the receiving room except for the test loudspeaker operating in the source room. Its presence can mask the sound being transmitted through the test partition, leading to an overestimation of the partition’s insulation performance.
The measurement uses 1/3 octave bands. In each one-third-octave or octave band, the background should be at least 6 dB—and preferably more than 10 dB—below the measured signal-lus-background level; if the noise is not steady, the averaging time is increased to obtain a stable estimate. When the background is not far enough below the signal, the measured level must be corrected by energy subtraction.
The standard specifies a clear correction procedure to account for this contamination:
For precision-grade measurements, such as those conducted in reverberation test rooms for sound power determination, specific and non-negotiable criteria for background noise are essential for ensuring accuracy. The ISO 3741:2010 standard defines the Background Noise Correction (K1) as a formal correction factor applied to measured sound pressure levels (ISO 3741:2010). The applicability of this correction is governed by strict criteria:
If the relative criteria cannot be met, the measurements can still conform to the standard if the background noise levels are below the absolute maximums specified in Table 2 of ISO 3741. This allows for valid measurements of quiet sources where achieving a 6 dB or 10 dB difference is not feasible.
The Speech Transmission Index (STI) is a metric designed to quantify the potential for human understanding. The STI model relies on several key concepts related to background noise:
In STI measurements (IEC 60268-16), the background noise must be accurately measured as the equivalent continuous sound pressure level (Leq) in each of the seven octave bands from 125 Hz to 8 kHz.
Compare measured levels (e.g., LAeq, LA90, NC rating) against established standards or target criteria for the specific space and its use (e.g., classroom, office, concert hall). If levels exceed targets, identify dominant frequency bands and sources to guide mitigation strategies. For audio, calculate SNR and determine if it’s sufficient for intelligibility (e.g., >15-20 dB often cited for speech).
Example: An office measures 45 dB(A) Leq during work hours. Speech at a nearby desk is 60 dB(A). The SNR is roughly 15 dB (60-45). If the target for good intelligibility is SNR > 20 dB, noise reduction or speech reinforcement might be needed.
Background noise originates from diverse sources both inside and outside a space, including building systems (HVAC), external environmental noise (traffic), equipment (electronics), and human activity. Reducing it effectively often involves a combination of controlling the source, blocking the transmission path, and optimizing the listening environment or recording setup. A common hierarchy for noise control is:
Room acoustics significantly influence perceived background noise. Factors like room size, shape, and surface materials affect how sound reflects and decays (reverberation), which can amplify or mask background noise.
HVAC: Check for rattling, clean filters, adjust fan speeds if possible, or consider duct lining/silencers
Equipment: Move noisy computers or appliances into cabinets or farther away. Use vibration isolation pads under machinery.
Activity: Establish quiet zones or times in shared spaces.
Absorption: Add soft furnishings (rugs, curtains), acoustic wall panels, or ceiling tiles to absorb sound reflections, reducing overall reverberation and noise buildup. Target first reflection points.
Isolation: Seal air gaps around doors and windows to block external noise. Improve wall/window construction for better sound transmission class (STC) ratings.
Layout: Position workstations or listening areas away from known noise sources. Use barriers or partitions in open-plan offices.
| Strategy | Action Examples | Target Stage |
|---|---|---|
| Source Control | Quieter HVAC fans, equipment isolation, scheduling noisy activities | Source |
| Path Control | Sealing gaps, insulation, sound barriers, improving wall/window STC | Path |
| Room Treatment | Acoustic panels (absorption), bass traps, diffusers, strategic layout | Room |
| Capture Optim. | Directional mics, closer mic placement, low-noise preamps, proper gain staging | Receiver/Capture |
Soundscape design extends beyond mere noise reduction; it involves balancing sounds to enhance the subjective experience of the environment by preserving desirable sounds (soundmarks) and combining and balancing sounds to create attractive and stimulating acoustic environments. Therefore, soundscape is a holistic framework for environmental planning that utilizes noise abatement measures to tackle background noise while simultaneously engaging the talents of scientists, social scientists, architects, and town planners to define principles for overall acoustic improvement.
An authorized SVANTEK consultant will help You with the details such as the required accessories for your noise monitoring task.
