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Metrics of Noise
Researchers measure noise in terms of frequency, pressure, and intensity. Frequency, or pitch, is measured in Hertz (Hz) and reflects the number of sound waves propagated through the air per second. The range of frequencies heard by the human ear range 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 (infrasound), and others, such as bats,  above 20,000 Hz (ultrasound) to echolocate.

Sound pressure level (SPL) represents the amount of pressure relative to atmospheric pressure during sound wave propagation, measured in Pascals (Pa). 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. Researchers use different weights to account for noise frequency with intensity, as humans do not perceive sound level pressures at the same loudness level. Therefore, noise levels are weighted, using three categories: 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.

Understanding sound pressure levels is key to assessing measurements of noise pollution. Several metrics describe noise exposure:


 * 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.1 LAeq can be further broken up into different types of noise based on time of day:
 * Day-Night Level, DNL or LDN: This measurement assesses the 24 hour average for sound, with a 10 dB(A) penalty or weight added to nighttime noise measurements (10:00 pm-7:00 am) given the increased sensitivity to noise at night.
 * Nighttime Level, LAeqN: This measurement assesses nighttime noise between 10:00 pm and 7:00 am.
 * Maximum level, LAmax: This measurement represents the maximal noise level when examining point sources or single events of noise.1 Additionally, penalties are added to nighttime noise to account for increased sensitivity and perception of loudness.1
 * 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.