Glossary

Added masking (deliberate masking sound)
Intentionally introduced background sound designed to raise the ambient level and render nearby speech less intelligible; used in open-plan environments to reduce distraction by blurring distant voices rather than by creating silence.
Agnotology
The study of culturally or institutionally produced ignorance — how not-knowing is manufactured or sustained. Used in 11  Governing sound as a lens on how an unmeasured harm becomes an unrecognised one. See 11  Governing sound.
A-weighting (dB(A))
A frequency weighting applied to a sound-level measurement that discounts each frequency in rough proportion to the ear’s reduced sensitivity to it, shaped as the inverse of the ~40-phon equal-loudness contour (Fletcher & Munson, 1933; International Organization for Standardization, 2023) and standardised in IEC 61672-1 (International Electrotechnical Commission, 2013). It leaves 1 kHz unchanged but subtracts about 3 dB at 500 Hz, 16 dB at 125 Hz, 26 dB at 63 Hz and over 30 dB at 50 Hz — approximating perceived loudness for ordinary mid-frequency sound while heavily discounting low-frequency and tonal content. A-weighted levels are written dB(A). See 2  Measuring the invisible.
Circumplex (pleasant–eventful)
The circular arrangement of the eight ISO soundscape descriptors — pleasant, vibrant, eventful, chaotic, annoying, monotonous, uneventful and calm — around two underlying, largely independent axes, pleasantness and eventfulness, identified by Axelsson et al. (2010) and formalised as Method A of ISO/TS 12913-2 (International Organization for Standardization, 2018), with the geometry that converts a set of ratings into a single coordinate specified in ISO/TS 12913-3 (International Organization for Standardization, 2019). Neighbouring attributes on the circle sit forty-five degrees apart, opposites a hundred and eighty. See 1  What counts as noise.
Cost-effectiveness (per decibel)
A metric expressing the reduction in noise level achieved per unit of expenditure, used to rank abatement measures independently of their absolute cost or visibility; in 10  The economics of quiet, the organising principle of the abatement hierarchy, by which source standards outperform barriers and insulation despite delivering a thinner benefit per household.
Disability-adjusted life-year (DALY)
A composite metric expressing the loss of one year of full health, combining years lost to premature mortality with years lived with a disabling condition; in 10  The economics of quiet, the unit underlying the European Environment Agency’s valuation of transport noise, permitting death and morbidity to be aggregated onto a single axis despite the conceptual violence this entails.
Disability weight
A coefficient, ranging from 0 (full health) to 1 (equivalent to death), expressing the relative severity of a non-fatal health state for use in DALY calculations; illustrated by the contested values of 0.02 for annoyance and 0.07 for sleep disturbance, where small downward revisions can shift aggregate cost estimates by billions of euros.
Distraction distance
The distance from a talker at which their speech falls below a specified Speech Transmission Index threshold (commonly 0.50) and therefore ceases to be sufficiently intelligible to cause serious disruption to a neighbour; used to set spatial and acoustic design targets in open-plan offices.
Environmental noise
Unwanted or harmful outdoor and indoor sound arising from all sources (traffic, industry, ventilation, building services, etc.) that affects people’s health, comfort or performance; in indoor contexts it is treated as an acoustic background that competes with target signals such as speech.
Environmental Noise Directive (END)
EU Directive 2002/49/EC, requiring strategic noise mapping and action planning for major sources and larger agglomerations; it mandates assessment but sets no binding limit values.
Exposure–effect relation
An empirically derived association linking the magnitude or pattern of noise exposure to a quantifiable change in an outcome (for example, reading comprehension or working-memory development); used to predict population-level burdens from measured acoustic conditions.
Exposure–response function (ERF)
The modelled relationship between exposure level and the probability or rate of an outcome; here, typically assumed linear in \(L_\text{den}\) above a reference category, with extrapolation below that category flagged as such.
Health-cost (burden-of-disease) method
A valuation approach that quantifies the cost of noise by enumerating health outcomes, weighting each by its disability severity, and pricing the resulting healthy years lost; distinguished by the transparency of its component steps, which simultaneously exposes each disability weight and exposure-response function to legitimate challenge.
Hedonic pricing method
A valuation technique that infers the implicit price of a non-market good — such as quiet — from observed differences in the market prices of otherwise comparable goods, typically housing; the basis of the Noise Depreciation Index, valued for resting on real transactions rather than stated intentions, but limited to capturing only preferences revealed through actual purchasing power.
Implied price of a healthy year
The monetary value per DALY derived by dividing an aggregate economic cost estimate by the corresponding number of healthy life-years lost; in 10  The economics of quiet, calculated at approximately €73,500 from the EEA’s 2025 figures, serving as the conversion rate underpinning all downstream cost comparisons.
Intelligibility
The degree to which spoken language can be understood by a listener in a given acoustic environment; determined by the preservation of speech modulations and the listener’s ability to discriminate phonetic elements, not by absolute sound level alone.
\(L_\text{den}\) (day–evening–night level)
The long-term A-weighted noise indicator used across the EU for strategic noise assessment, defined in Annex I of the Environmental Noise Directive (European Parliament and Council of the European Union, 2002). It averages the A-weighted level over a representative year across three periods, adding a +5 dB penalty to the evening and +10 dB to the night, on the reasoning that the same sound intrudes more when the surroundings are quiet. Under the Directive’s assessment methods it is computed over the 63 Hz–8 kHz octave bands, so sound below 63 Hz — including infrasound — lies outside the calculation entirely. See 2  Measuring the invisible.
\(L_\text{night}\)
The long-term A-weighted average sound level over the eight-hour night period (23:00–07:00), used to assess sleep disturbance; defined in Annex I of the Environmental Noise Directive (European Parliament and Council of the European Union, 2002). See 2  Measuring the invisible.
Listening effort
The cognitive and physiological resources a listener must deploy to decode and understand speech in adverse acoustic conditions; increased listening effort reduces capacity for concurrent tasks and, when chronic, imposes a measurable developmental cost on learning.
Low-frequency noise (LFN) / infrasound
Low-frequency noise is sound energy in roughly the 10–200 Hz range; infrasound is sound below about 20 Hz, at or beneath the nominal threshold of hearing (Leventhall, 2004). This is the band that A-weighting discounts most steeply and that EU assessment, below 63 Hz, omits altogether. Reviews find that A-weighted metrics underestimate low-frequency annoyance and can lead regulatory authorities to incorrect decisions (Leventhall, 2004) — the measurement mismatch at the centre of the book’s argument. See 2  Measuring the invisible.
Low-noise road surface
A path-based abatement measure consisting of porous or specially engineered pavement that attenuates tyre-road noise, typically reducing levels by one to five decibels; noted for its favourable cost per decibel relative to barriers, offset by a benefit that degrades within roughly six years as surface pores clog.
Masking
The perceptual reduction of one sound’s prominence by another; in 12  Designing the soundscape, the deliberate use of natural sound to augment a soundscape rather than to suppress the source. Masking operates spectrally and temporally and is a principal mechanism by which background noise degrades comprehension.
Noise barrier
A physical along-the-path structure, such as a wall or embankment, designed to block the direct transmission of sound between a source and nearby receivers, typically achieving reductions of ten to fifteen decibels; in 10  The economics of quiet, characterised as the most effective single path intervention but also the most costly per mile, and limited in reach to the first rows of housing behind it.
Noise Depreciation Index
The percentage reduction in a property’s market value associated with each additional decibel of ambient noise exposure, derived from hedonic pricing analysis of housing transactions; in 10  The economics of quiet, estimated at approximately 0.4 per cent per decibel for road traffic and 0.6 per cent for aircraft noise, though it captures only the preferences of those with the means to choose their residence.
Noise fluctuation (peaks)
Short-term increases in sound level above the background mean (transient peaks or swings); such fluctuations disrupt ongoing cognitive processing more than steady background levels and are strongly associated with impaired classroom progress.
Noise redistribution
The displacement of noise exposure from one location to another as a side-effect of an abatement measure, rather than its net reduction — for instance, rerouting heavy traffic from one street onto a neighbouring one; in 10  The economics of quiet, flagged as a limitation of several low-cost, non-engineering interventions such as lorry bans and rerouting schemes.
Open-plan office
A workspace layout characterised by large, shared areas with minimal partitioning; acoustically, such spaces are prone to continuous low-level noise (ventilation drones, equipment, speech) that reduces SNR and increases cognitive load for occupants.
Private soundscape
The individualised acoustic environment created by personal audio devices or headphones that both restores control over one’s immediate auditory input in noisy settings and, when used at high volumes, converts the user into an active source of potentially harmful exposure.
Rate of decay (sound attenuation with distance)
The rate at which a speech signal’s level decreases with each doubling of distance from the source; a high rate of decay (steep attenuation) reduces long-range intelligibility and is achieved through absorption, screening and layout.
Relative risk (RR)
The ratio of the incidence of an outcome in an exposed group to that in a reference group. In 3  The body under sound, RR ≈ 1.08 per 10 dB \(L_\text{den}\) for ischaemic heart disease means each additional 10 dB multiplies incidence by about 1.08 relative to the reference category.
Reverberation time (RT or RT60)
The time, measured in seconds, required for sound to decay by 60 dB after the source has stopped; long reverberation times smear successive speech sounds and reduce temporal clarity, thereby impairing intelligibility.
Safe-listening threshold and dose
The combination of a sound-level limit and an associated exposure duration that together define a tolerable weekly dose for hearing (for example, 80 dB for adults over a working week), where higher levels rapidly reduce safe exposure time and increase the risk of noise-induced hearing loss.
Severe sleep disturbance
A WHO-defined health outcome category denoting substantial, noise-induced impairment of sleep quality or continuity, distinguished by disability weight from milder annoyance-related outcomes; identified as the single largest contributor to Europe’s noise-related DALY burden.
Sonic injustice
The unequal distribution of noise-related harm, understood on the fullest account as two distinct axes rather than one: unequal exposure to unwanted sound, and unequal access to wanted sound — quiet, restorative and pleasant acoustic environments, the plain ability to govern what one hears (Trudeau et al., 2023). The two axes need not point the same way. In Barcelona (2007–2014) and under London’s aircraft corridors, exposure to unwanted sound ran higher among the more affluent, the inverse of the deprivation pattern typical of other environmental harms — an inversion treated as a genuine finding rather than an anomaly to be explained away. On the access axis, by contrast, advantage runs the ordinary way: the affluent can buy their way to quiet that the poor cannot. See 9  Sonic injustice.
Soundscape
The acoustic environment as perceived or experienced by people in context, as distinct from the physical sound field. Formalised in ISO 12913-1:2014. The concept that makes perception, and therefore design, part of the object.
Speech-to-noise ratio (SNR)
The difference in decibels between the level of a target speech signal and the level of the background noise at the listener’s ear; a positive SNR indicates speech is louder than the background and higher values generally improve comprehension.
Speech Transmission Index (STI)
A dimensionless index (range 0–1) that quantifies speech intelligibility by combining the effects of SNR and reverberation across modulation frequencies; values above roughly 0.75 are typically classed as excellent and values below about 0.45 as poor (International Electrotechnical Commission, 2020).
Stated-preference method (willingness-to-pay)
A valuation approach that elicits, through direct survey questions, the amount individuals report they would pay to avoid a given level of noise exposure; capable of reaching non-market values that hedonic pricing misses, but vulnerable to framing effects and the known divergence between hypothetical statements and actual behaviour.
Sustained attention
The ability to maintain focus on a task or signal over extended periods; in classrooms, prolonged acoustic interference taxes sustained attention and diminishes the resources available for learning activities.
Tonal penalty (correction term)
An addition to a measured A-weighted level, applied when a source has a discernible tonal or low-frequency character, intended to make an otherwise character-blind metric more sensitive to what a steady average discounts. Spain’s correction terms under RD 1367/2007 are the book’s working example (BOE, 2007): capped in magnitude and applied on top of an A-weighted base, they can push a borderline source into breach, but a source that remains just under the corrected limit is as untouchable as if the correction did not exist — the frame the correction sits inside, not the correction itself, decides what can be reached. See 11  Governing sound.
Triangulation (of valuation methods)
The practice of comparing results from multiple independent valuation approaches — hedonic, stated-preference, and health-based — and treating their divergence as a measure of genuine uncertainty rather than a methodological failure; proposed as the appropriate response to the fact that the three methods price fundamentally different things (market behaviour, stated belief, and epidemiological attribution) and do not converge on a single figure.
Unoccupied background level
The A-weighted sound pressure level measured in a room when it is empty and no intentional sound sources are active; used as a baseline for assessing suitability for learning and other tasks (for classrooms, recommended ≤ 35 dB(A)) (Acoustical Society of America, 2010).
Working memory
The limited-capacity system that temporarily holds and manipulates information (for example, fragments of a sentence) while a listener reconstructs missing elements; sustained demands on working memory from degraded speech slow learning and reduce comprehension.