How to Measure Noise Accurately

With increasing population, industrialization and urbanization, concerns over noise pollution have increased. The U.S. Environmental Protection Agency has set 55 decibels as the safe noise level, which takes several measures to maintain. Noise exposure time and the power level of noise are the important parameters that determine the damage to human ears.

The International Electrotechnical Commission (IEC) is the international body that decides the technical specification of noise measuring instruments such as a sound level meter and dosimeter because standardizing the technical values are important, so that incorrect measurements of different noise measuring instruments can be avoided.

It is also called 'an exponentially averaging sound level meter' because the AC (alternating current) signal from the microphone is converted into DC (direct current) by an RMS (root mean square) circuit. A basic sound level meter device consists of microphone, filters, amplifier, squaring device, etc.

Filters inside the device, alter the frequency spectrum of the noise signals that passes through it. At places like industries, airports, roads, etc., where sound levels are high, sound level meter is installed to ensure that the sound does not exceed a certain limit.

The microphone in the sound level meter converts the noise signals to electrical signals, which can be read by the sound level meter. The meter displays the sound level in decibels (dB), which is a logarithmic unit of sound intensity.

'HI' and 'LO' in the meter's display indicates whether the noise level is high or low, respectively with regards to the threshold level. Sound Pressure Level (SPL) is the measure used in sound level meters for calculating the noise. The formula for calculating the SPL is - 

SPL = 20 log₁₀ (root mean square sound pressure/reference sound pressure)

Root mean square sound pressure is calculated using standard frequency weighting and a standard time weighting standards. Usually, 20 micro pascals is the value of reference sound pressure, because this is the minimum value, which a human ear can respond to. It is the standard value used in all the noise measuring devices.

The frequency weightings that are used in sound level meter is associated with human ear responses. There are different types of frequency weightings. They are A weighting, C weighting and Z weighting.

• A weighting cuts off the low and high frequencies, which humans cannot hear. The dB (A) level determines the ambient noise level and noise intrusions in the environment.
• C frequency weighting has a flat frequency response ranging between 31 Hz and 8 kHz.
• Z frequency weighting has a flat frequency response of 20 Hz to 10 kHz.

Time weighting depends upon the rate of fluctuations whether it is fast, slow or impulsive and impulse mode is used only for auxiliary processing functions.

Dosimeter is used in places where the noise level is varying and not continuous. When the meter is powered, the display annunciators such as dose, criterion level (Lc), threshold level (Lt) and exchange rate (ER) are switched on. When the run button in dosimeter is switched on, the meter begins to measure the accumulated sound exposure.

Sound exposure level is a measure of acoustic energy in transient noise. The device is predominantly used in industries, where workers are exposed to high intensity sounds that can impair the human ear.

The dosimeter records the decibel values above the threshold level. The threshold level is the sound level at which the dosimeter begins to integrate the noise in terms of time variable.

Sound levels, below the threshold value will not be included in measuring the noise level. It is almost same as that of sound level meter except that, it measures only broadband noise level without frequency analysis.

The noise level exposure of an employee working for more than 8 hours per day can be calculated by using the following formula:
LAeq8h = LAeq + 10 log₁₀ [T/8]
where,

• LAeq is the equivalent continuous A weighted sound pressure level measured over a period of time.
• T, represents the number of shift hours of a worker in an industry.

Condenser microphones, dynamic microphones and ceramic microphones are some of the devices that are used to measure noise levels. Condenser microphone measurement ranges from 0.01 Hz to 140 KHz. Dynamic range of condenser microphone can reach up to 140 dB and it can be used in extreme conditions where sound intensity is very high.

In dynamic microphones, a diaphragm that deflects under pressure fluctuations of sound waves, is connected to a moving coil from where electrical signal is generated. In this way, the sound signals in the form of electrical signals are measured. Ceramic microphone is a device, which can measure high acoustic noise and pressure fluctuations.

It is a device that is used for monitoring and measuring environmental noise due to traffic on roads, aircraft noise in airports and other noises where statistical information is required.

These noise measuring devices are used to monitor and maintain the noise levels to a certain limit. So, determining the noise level can prevent noise induced hearing loss for people who work in high noise level environment. Hence, let's do our best to reduce the noise pollution and remember that it causes behavioral and psychological effects in human beings.