Noise Power Spectrum for Firecrackers

Introduction

Sound level, its frequency spectrum and its variation over time characterize noise (Alam, 2006). Noise can also be characterized by its frequency content. This can be assessed by various types of frequency analysis to determine the relative contributions of the frequency components to the total noise (Berglund, 1999). Conventionally the range of frequency of audible sound is considered to be from 20 Hz to about 20,000 Hz (Carl, 2006). There are individual variations in the frequency of sound that can be heard by different individuals. The study of sound related noise includes the study of noise levels at different frequencies. A standard well known method to find out noise power associated with different frequencies is to use the Fourier transform technique (Berglund, 1995).

Fourier transform of sound reveals the amplitude of noise at the constituent frequencies from which one can estimate the corresponding power levels. The main principle of frequency analysis is that any selected frequency range is divided into a number of consecutive and discrete analysis bandwidths, such that the amount of energy present in each analysis bandwidth can be determined.

Firecrackers are used all over the world to celebrate different social as well as religious occasions. The firecrackers traditionally used for celebration are another major source of excessive noise. They are used indiscriminately in residential areas, next to hospitals, schools, with little consideration for the effect on the well being of persons unable for a variety of reasons to bear the high level of noise created. Bombs, chain bombs, etc. are permitted for manufacture provided they do not exceed 125dB – the level of a jet engine taking off at 25 meters (Noise Free Mumbai). Firecrackers generate instantaneous impulsive noise, which when measured in free field condition gives high sound pressure level. An impulse is much more harmful than a continuous noise (Khopkar, 1993). A sudden noise generated with high pitch or intensity but with a life-time of less than one second is called as impulse. Impulses caused by an exploding bomb (190 dB), naval gun-shooting, firing crackers and metal beating, are capable of producing noise to the extent of 140 dB. An unexpected thud of sound with a short life has high impulse and is dangerous. It is quite obvious than a number of crackers when bursting serially can easily form a band of continuous noise in the presence of reflecting surfaces (West Bengal Pollution Control Board, 2005). Noise pollution due to bursting of firecrackers during Diwali was surveyed at 11 cities in the state of Maharashtra by Maharashtra pollution control board in the year 2005 and found that the noise levels in Aurangabad city are higher than the stipulated limits (Deshpande, 2005). To make matters more complicated, the frequency of noise can affect human beings in different ways. Fatigue and nausea often result from low-frequency vibration, while high frequencies are likely to cause pain and hearing loss. A number of adverse effects of noise in general may be greater for low frequency noise than for the same noise energy in higher frequencies (Goldstein, 1994).