The most common measurement of environmental noise is the dB(A) level. It is measured using a simple sound level meter having an A-weighting filter. Frequency weighted filters are used in sound level meters to mimic certain conditions. A-weighting is a frequency-dependent curve (or filter) that is applied to sound pressure microphone measurements to mimic the effects of human hearing, i.e. in a frequency range between 20 Hz to 20 kHz. As far as loudness is concerned, humans can typically hear starting at 0 dB, and our hearing is most sensitive in 1000 - 5000 Hz frequency range. While studying long term trends in environmental noise, a single- value descriptor like Leq - Equivalent Level is used to define an entire day's noise history. Another useful set of parameters is the Ln values or statistical noise levels, such as L10, L50, and L90 Leq is the preferred method to describe sound levels that vary over time, resulting in a single decibel value which takes into account the total sound energy over the period of time of interest. Leq is the imaginary constant noise level that would result in the same total sound energy being produced over a given period. The meter initially converts the dB values to sound pressure levels and adds them all up, then divides by the number of samples and finally converts this equivalent level back to decibels. An equivalent continuous A-weighted sound pressure level is a common measurement used in the industry to characterize noise levels in loud environments. L10 is the noise level which exceeds only 10% of the time of the measurement duration. It is also called the ‘Average peak level’. This parameter is generally used to give an indication of the upper limit of fluctuating noise due to intermittent events such as traffic congestion. L50 is the noise level which exceeds 50% of the measuring time. It is statistically the middle point of noise readings that represents the median of the fluctuating noise levels. L90, also known as ‘average background level’, is the noise level which exceeds 90% of the measuring time. It generally indicates the ambient noise level of an environment.