Noise pollution

Noise pollution, or sound pollution, is the propagation of noise or sound with potential harmful effects on humans and animals. The main sources of outdoor noise worldwide are machines, transportation, and propagation systems. Poor urban planning may give rise to noise pollution. Side-by-side industrial and residential zones can result in noise pollution in residential areas. Some of the main sources of noise in residential areas include loud music, transportation, lawn care maintenance, construction, electrical generators, wind turbines, explosions, and other human activity.
Documented problems associated with noise in urban environments go back as far as ancient Rome. Research suggests that noise pollution in the United States is the highest in low-income and racial minority neighborhoods, and noise pollution associated with household electricity generators is an emerging environmental degradation in many developing nations. A national study of modeled transportation noise in the contiguous United States found that census tracts with higher proportions of Black, Hispanic, and Asian population and higher poverty had higher average noise levels than wealthier and mostly White areas. The study found that residential segregation and urban land use practices correlate with the high levels of traffic and aviation noise in many communities of color and low-income neighborhoods.
High noise levels can contribute to cardiovascular effects in humans and an increased incidence of coronary artery disease. In animals, noise can increase the risk of death by altering predator or prey detection and avoidance, interfere with reproduction and navigation, and contribute to permanent hearing loss.

Noise assessment

Metrics of noise

is quantified in terms of sound pressure levels measured in decibels, a logarithmic scale used to relate physical sound intensity to human perception. Everyday environmental sounds such as road traffic and construction typically range from about 70 to over 100 dB, and repeated exposure above approximately 85 dB is associated with an increased risk of hearing damage. Metrics such as the equivalent continuous sound level and the day–night average sound level are commonly used in regulatory and public health contexts to describe long-term community noise exposure.
Researchers measure noise in terms of pressure, intensity, and frequency. Sound pressure level represents the amount of pressure relative to atmospheric pressure during sound wave propagation that can vary with time; this is also known as the sum of the amplitudes of a wave. Sound intensity, measured in Watts per meters-squared, represents the flow of sound over a particular area. Although sound pressure and intensity differ, both can describe the level of loudness by comparing the current state to the threshold of hearing; this results in decibel units on the logarithmic scale. The logarithmic scale accommodates the vast range of sound heard by the human ear.
Frequency, or pitch, is measured in Hertz and reflects the number of sound waves propagated through the air per second. Humans generally hear frequencies from 20 Hz to 20,000 Hz; however, sensitivity to hearing higher frequencies decreases with age. Some organisms, such as elephants, can register frequencies between 0 and 20 Hz, and others, such as bats, can recognize frequencies above 20,000 Hz to echolocate.
Researchers use different weights to account for noise frequency with intensity, as humans do not perceive sound at the same loudness level. The most commonly used weighted levels are A-weighting, C-weighting, and Z-weighting. A-weighting mirrors the range of hearing, with frequencies of 20 Hz to 20,000 Hz. This gives more weight to higher frequencies and less weight to lower frequencies. C-weighting has been used to measure peak sound pressure or impulse noise, similar to loud short-lived noises from machinery in occupational settings. Z-weighting, also known as zero-weighting, represents noise levels without any frequency weights.
Understanding sound pressure levels is key to assessing measurements of noise pollution. Several metrics describing noise exposure include:

Energy average equivalent level of the A-weighted sound, LAeq: This measures the average sound energy over a given period for constant or continuous noise, such as road traffic. LAeq can be further broken up into different types of noise based on time of day; however, cutoffs for evening and nighttime hours may differ between countries, with the United States, Belgium, and New Zealand noting evening hours from 19:00–22:00 or 7:00 p.m.–10:00 p.m. and nighttime hours from 22:00–7:00 or 10:00 p.m.–7:00 a.m. and most European countries noting evening hours from 19:00–23:00 or 7:00 p.m.–11:00 p.m. and nighttime hours from 23:00–7:00 or 11:00 p.m.–7:00 a.m.). LAeq terms include:
* Day-night average level, DNL or LDN: This measurement assesses the cumulative exposure to sound for a 24-hour period of the year, with a 10 dB penalty or weight added to nighttime noise measurements given the increased sensitivity to noise at night. This is calculated from the following equation :
*Day-evening-night average level, DENL or Lden: This measurement, commonly used in European countries, assesses the 24-hour average in a year ; however, this measurement separates evening from night hours and adds a 5 dB penalty to evening and 10 dB penalty to nighttime hours. This is calculated from the following equation :
*Daytime level, LAeqD, or Lday: This measurement assesses daytime noise, usually from 7:00–19:00, yet may vary by country.
*Nighttime level, LAeqN, or Lnight: This measurement assesses nighttime noise, depending on country cutoff hours discussed above.
Maximum level, LAmax: This measurement represents the maximal noise level when examining point sources or single events of noise; however, this value does not factor in duration of the event.
Sound exposure level of A-weighted sound, SEL: This measurement represents the total energy for a particular event. SEL is used to describe discrete events in terms of A-weighted sound. The difference between SEL and LAmax is that SEL is derived using multiple time points of a particular event in calculating sound levels rather than the peak value.
Percentile-derived measurements : Noise may be described in terms of its statistical distribution over a set time, in which investigators may obtain values, or cut-points, at any percentile level. The L90 is the sound level that exceeds 90% of the time period; this is commonly referred to as background noise.

Researchers with the US National Park Service found that human activity doubles the background-noise levels in 63 percent of protected spaces like national parks, and increases them tenfold in 21 percent. In the latter places, "if you could have heard something 100 feet away, now you can only hear it 10 feet away".

Instrumentation

Sound level meters

Sound can be measured in the air using a sound level meter, a device consisting of a microphone, an amplifier, and a time meter. Sound level meters can measure noise at different frequencies. There are two settings for response time constants, fast or slow. Sound level meters meet the required standards set by the International Electrotechnical Commission and in the United States, the American National Standards Institute as type 0, 1, or 2 instruments.
Type 0 devices are not required to meet the same criteria expected of types 1 and 2 since scientists use these as laboratory reference standards. Type 1 instruments are to study the precision of capturing sound measurements, while type 2 instruments are for general field use. Type 1 devices acceptable by the standards have a margin of error of ±1.5 dB, while type 2 instruments meet a margin of error of ±2.3 dB.

Dosimeters

Sound can also be measured using a noise dosimeter, a device similar to a sound level meter. Individuals have used dosimeters to measure personal exposure levels in occupational settings given their smaller, more portable size. Unlike many sound level meters, a dosimeter microphone attaches to the worker and monitors levels throughout a work shift. Additionally, dosimeters can calculate the percent dose or time-weighted average.

Smartphone applications

In recent years, scientists and audio engineers have been developing smartphone apps to conduct sound measurements, similar to the standalone sound level meters and dosimeters. In 2014, the National Institute for Occupational Safety and Health within the Centers for Disease Control and Prevention published a study examining the efficacy of 192 sound measurement apps on iOS and Android smartphones.
The authors found that only 10 apps, all of which were on the App Store, met all acceptability criteria. Of these 10 apps, only 4 apps met accuracy criteria within 2 dB from the reference standard. As a result of this study, they created the NIOSH Sound Level Meter App to increase accessibility and decrease costs of monitoring noise using crowdsourcing data with a tested and highly accurate application. The app is compliant with ANSI S1.4 and IEC 61672 requirements.
The app calculates the following measures: total run time, instantaneous sound level, A-weighted equivalent sound level, maximum level, C-weighted peak sound level, time-weighted average, dose, and projected dose. Dose and projected dose are based on sound level and duration of noise exposure in relation to the NIOSH recommended exposure limit of 85 dB for an eight-hour work shift.
Using the phone's internal microphone, the NIOSH Sound Level Meter measures instantaneous sound levels in real time and converts sound into electrical energy to calculate measurements in A-, C-, or Z-weighted decibels. App users are able to generate, save, and e-mail measurement reports. The NIOSH Sound Level Meter is currently only available on Apple iOS devices.