Ambient Air Quality Monitoring

Care has to be taken to choose the location so that it is representative of the target sample area. Undue traffic, proximity to roadside, or material handling yard will give the true results for that location, but not “ representative” for the area in general. As such the objective of Ambient Monitoring has to be clear before selecting the location.
If you are trying to measure the impact of an industry on the surrounding environment, the location has to be in the downwind direction, where the emission from that industry is probably hugging the ground. This distance varies depending on weather conditions. It is generally within 50 to 100 times the stack height.

The duration of sampling has to match with the one specified in the standard being used for comparison. Depending on the concentration of various pollutants, the sampling duration may have to be reduced to shorter interval. As such the monitoring may be continued for 24 hours, by changing the absorbing solutions after every 8 hours. The average daily results can be calculated for comparison. Gaseous pollutants are analyzed by standard methods in the laboratory.

Sound Level Monitoring

Noise v/s Sound: These two words are quite often used interchangeably, which is not very correct. Noise has been defined as unwanted sound. A good lyrical song may be liked by us and hence is not a noise. But the same song can be unwanted during examination study time. What we measure with these instruments is the general total sound level in the area, and not Noise. Noise is subjective concept.

Noise Level Measurements: A young, audiometrically healthy normal male adult responds to sound waves of frequency range of 20 to 16,000 Hz, where as children and women have capacity to respond to 20,000 Hz. The speech zon lies in the range of 500 to 2,000 Hz. The human ear is most sensitive in the range of 2,000 to 5,000 Hz.

Weightings: The writers of the acoustical standards have established three weighting characteristics and these are A, B & C. The “A” weighting net work filters out very low frequencies very severely, “B” moderately and “C” hardly filters out any.
Time Weightings: The two time weightings are called Fast and Slow responses. Slow responses refer to averaging time of up to 1 second to 0.5 seconds (500 ms) and the Fast response is 125 ms to 200 ms. However, it may vary as per requirement of the purchaser specifications. dB(A) Fast meets the requirements of Human ear sensitivity.

Noise Rating Systems: The human response to Noise depends upon the frequency of the sound, the type of noise (continuous, intermittent or impulsive) and the time (day or night) it occurs. Thus the Noise rating system should take frequency into account, differentiate between day time and night time noise and be capable of describing the cumulative noise exposure. The following two systems are

Presently in vogue: The LN Concept: If measurements are made over a period of time, the parameter LN represents or indicates as to how frequently a particular sound level is exceeded. Say L30 = 70 dB A, then it represents that 70 dB A was exceeded 30% of the measuring time.

The Leq Concept: The equivalent continuous equal energy level, Leq can be applied to any fluctuating Noise Level. It is that constant Noise Level that over a given time, expends the same amount of energy as the fluctuating level over the same time period.

General Sound Level Monitoring: Sound Level Measurements at workplace are done using simple instruments capable of having ‘A’ and ‘C’ weightings and ‘Slow’ and ‘Fast’ response selection switch. The sound level measurement has to be done where human employee is facing the sound or at employee’s place of working. It may be near or far from the actual Noise Source.

Standards: These are depending on the location and period of day. Industrial areas obviously have somewhat higher acceptable sound levels than those prescribed for residential areas. The collected night standards are stringent than the day time standards.