Noise pollution

Noise pollution is the propagation of sound that has potential harmful effects on humans and animals.

It occurs when sounds are unwanted or too loud for the environment. The main sources of outdoor noise worldwide include machines, transportation, and propagation systems. In cities, traffic is often the primary source of noise pollution.

Poor urban planning can also cause problems. For example, placing industrial zones right next to residential areas can lead to high noise levels in neighborhoods. Other common sources include airplanes, rail transport, construction, and lawn care maintenance.

High noise levels can lead to serious health problems for humans. Chronic exposure to environmental noise is linked to cardiovascular disorders and hypertension, which is high blood pressure. It can also cause high stress levels, anxiety, and sleep disturbances. Some people may experience tinnitus, which is a ringing in the ears, or permanent hearing loss.

Animals are also affected by noise pollution. It can increase the risk of death by making it harder for them to detect predators or prey. Noise can also interfere with animal reproduction and navigation. In some species, it can even lead to permanent hearing loss.

Scientists use the decibel (dB) to quantify noise exposure. The decibel is a logarithmic scale used to relate physical sound intensity to human perception. This scale is necessary because the human ear can hear a vast range of sound intensities. Sound pressure level (SPL) represents the amount of pressure relative to atmospheric pressure during sound wave propagation. Sound intensity is measured in Watts per meters-squared and represents the flow of sound over a particular area. While these two concepts differ, both can describe loudness by comparing the sound to the threshold of hearing.

Researchers also measure frequency, which is the pitch of a sound. Frequency is measured in hertz (Hz) and reflects the number of sound waves sent through the air per second. Humans generally hear frequencies from 20 Hz to 20,000 Hz. However, sensitivity to higher frequencies often decreases as people age. Other organisms hear different ranges. For example, elephants can register infrasound, which are frequencies between 0 and 20 Hz. Conversely, bats can recognize ultrasound, which are frequencies above 20,000 Hz, to help them echolocate.

Because humans do not perceive all frequencies at the same loudness, researchers use frequency weighting.

A-weighting is the most common method because it mirrors the human range of hearing. It gives more weight to higher frequencies and less weight to lower frequencies. C-weighting is used to measure peak sound pressure or impulse noise, such as loud, short-lived sounds from machinery. Z-weighting, or zero-weighting, represents noise levels without any frequency weights applied. These different weights allow scientists to capture how noise actually affects human perception.

To manage public health, several specific metrics are used to describe long-term noise exposure. The equivalent continuous sound level (LAeq) measures the average sound energy over a specific period, such as for constant road traffic. Another metric is the day-night average sound level (LDN). This assesses cumulative exposure over 24 hours and adds a 10 dB penalty to nighttime noise because people are more sensitive to sound at night. In Europe, the day-evening-night level (LDN) is used. It separates evening hours from nighttime hours and adds a 5 dB penalty to the evening and a 10 dB penalty to the night.

Specialized tools allow for precise measurement of these sound levels.

A sound level meter uses a microphone, an amplifier, and a time meter to measure sound in the air. These instruments are categorized by the International Electrotechnical Commission into Type 1 for precision studies and Type 2 for general field use. For individual workers, a noise dosimeter can be used. This is a smaller, portable device with a microphone that attaches to the worker to monitor noise throughout a full shift.

Even smartphones can now assist with monitoring. The NIOSH developed a specific app for Apple iOS devices that allows users to calculate various measurements, including the maximum level and the total dose of noise exposure.

Noise pollution also has significant impacts on protected natural spaces. Researchers from the US National Park Service found that human activity can double background-noise levels in 63 percent of protected areas. In 21 percent of these spaces, human activity increases background noise tenfold. This increase has a major impact on the environment. In these loud areas, if a person could have heard something 100 feet away, they can now only hear it 10 feet away.