Noise Pollution

Noise pollution refers to the propagation of unwanted or harmful sound that can adversely affect human health, animal behaviour, and environmental quality. It is predominantly caused by machinery, transportation systems, and poorly planned urban environments, and it constitutes a growing concern in both developed and developing regions. With modern cities experiencing increasing industrialisation, traffic density, and residential expansion, noise has become a widespread environmental stressor requiring systematic assessment and regulation.

Sources of Noise Pollution

Noise pollution arises from a wide array of outdoor and indoor sources. In urban contexts, transport systems represent the most persistent contributors, including motor vehicles, railways, and aircraft. Aircraft noise is a significant issue near major airports where landing and take-off routes pass close to residential zones, as exemplified by flights approaching Heathrow Airport. Road traffic in large cities such as São Paulo produces continuous background noise, intensified by high vehicle density and congested infrastructure.
Industrial operations contribute heavily to noise levels when factories, workshops, or construction sites are situated adjacent to residential areas. Poor urban planning exacerbates this by allowing incompatible land uses to coexist without adequate sound buffers. Common neighbourhood sources include loud music, emergency sirens, lawn maintenance equipment, electrical generators, wind turbines, and explosive sounds from various activities.
Historical records indicate that noise was recognised as a problem as early as ancient Rome, reflecting the longstanding human concern with disruptive sound. Contemporary research also demonstrates socioeconomic disparities: noise exposure in the United States tends to be higher in low-income districts and communities populated by racial minorities. Furthermore, reliance on household electricity generators in many developing nations has produced new forms of localised environmental noise pollution.
Noise also affects wildlife, interfering with communication, mating behaviours, predator–prey dynamics, and navigation. For some species, persistent noise can lead to increased mortality risk and long-term auditory damage.

Physical Principles and Metrics of Noise

The scientific assessment of noise pollution relies upon physical measurements of sound pressure, intensity, and frequency. Sound pressure level (SPL) represents the variation in air pressure caused by sound waves relative to atmospheric pressure. As sound pressure fluctuates over time, SPL provides a quantitative measure of these variations. Sound intensity, measured in watts per square metre, indicates the energy passing through a given area. Although pressure and intensity are distinct concepts, both are commonly expressed using the decibel (dB) scale, a logarithmic unit that accommodates the broad range of pressures detectable by the human ear.
Frequency, expressed in hertz (Hz), describes the number of sound wave cycles occurring per second. The typical audible range for humans spans from 20 Hz to 20,000 Hz, although sensitivity to higher frequencies decreases with age. Some species detect frequencies outside this range: elephants can perceive infrasound below 20 Hz, while bats rely on ultrasound above 20,000 Hz for echolocation.
Because the human ear perceives frequencies differently, frequency weighting is applied when measuring noise. The most common weighting systems include:

• A-weighting, which reflects human hearing sensitivity across 20–20,000 Hz, emphasising mid to high frequencies.
• C-weighting, used for peak and impulse noise, capturing low-frequency content more accurately than A-weighting.
• Z-weighting, or zero-weighting, which applies no frequency modification and represents the raw sound level.

Measures of Environmental Noise Exposure

Various metrics describe noise levels over time, especially where noise fluctuates or persists across daily cycles. These metrics are widely employed in environmental impact assessments, regulatory frameworks, and urban planning.

• LAeq (Equivalent Continuous Sound Level): Represents the average A-weighted sound energy over a specified period, typically used for constant noise sources such as road traffic.
• Day–Night Average Sound Level (Ldn): Calculates a 24-hour noise average with an additional 10 dB penalty applied to nighttime hours due to increased human sensitivity during sleep.
• Day–Evening–Night Level (Lden): Common in European standards, separating daytime, evening, and nighttime periods and applying a 5 dB penalty for evening noise and a 10 dB penalty at night.
• Daytime Level (LAeqD/Lday): Measures noise during standard daytime hours.
• Nighttime Level (LAeqN/Lnight): Measures noise during designated nighttime periods.
• LAmax: Indicates the maximum sound level reached during a noise event, though it does not account for event duration.
• Sound Exposure Level (SEL): Describes the total energy of a discrete event, incorporating multiple time points rather than relying solely on the peak value.
• Percentile-based measures (e.g., L10, L50, L90): Provide statistical descriptions of noise distribution, with L90 often used to represent background noise.

Studies by the US National Park Service suggest that anthropogenic activity has significantly altered natural soundscapes, doubling background noise levels in the majority of protected areas and increasing them tenfold in a substantial minority. Such intensification masks natural sounds and disrupts ecological dynamics.

Instruments and Methods of Noise Measurement

Noise assessment relies on specialised instruments designed to capture, quantify, and analyse sound in various environments. These technologies are essential for regulatory compliance, occupational health monitoring, and environmental research.
Sound Level Meters (SLMs) are the most common devices used for environmental measurements. They consist of a microphone, amplifier, and signal processor capable of operating with A- or C-weighted frequency filters. SLMs allow for two response time settings: a fast response approximating human hearing (0.125 seconds) and a slow response (1 second) used to stabilise readings in fluctuating noise conditions. Standards for SLMs are set by the International Electrotechnical Commission and by the American National Standards Institute, classifying instruments as Type 0, Type 1, or Type 2. Type 0 devices serve as laboratory reference instruments, Type 1 devices provide high precision for detailed studies, and Type 2 devices are generally used for field measurements. Type 1 instruments typically allow a margin of error of 1.5 dB, whereas Type 2 devices permit up to 2.3 dB.
Noise Dosimeters are compact devices worn by individuals, predominantly in occupational settings such as factories, construction sites, or industrial plants. They record sound levels continuously throughout a work shift and calculate measures such as percent dose and time-weighted average exposure. Dosimeters are particularly valuable where workers are mobile or exposed to varying noise environments.
Smartphone Applications have recently emerged as accessible tools for basic sound measurements. Advances in sensor technology and calibration techniques have improved the reliability of smartphone-based measurements, though specialised instruments remain necessary for regulatory compliance and high-precision assessments. For informal monitoring or educational purposes, well-designed applications offer a useful supplement to professional equipment.

Health and Environmental Impacts

Exposure to high levels of environmental noise has been linked to numerous adverse health outcomes. In humans, prolonged or intense noise exposure can contribute to cardiovascular strain, elevated stress levels, sleep disturbances, and increased risk of coronary artery disease. Chronic noise can impair concentration, reduce productivity, and diminish overall quality of life. Sudden or extreme noise events may cause temporary or permanent hearing loss.
For animals, noise pollution poses significant ecological challenges. It may interfere with communication signals, crucial for mating and territorial behaviours, or impair the ability to detect predators and prey. Marine and terrestrial species alike can suffer long-term consequences, including changes in migration patterns, reproductive success, and survival rates.