Knowledge base

Know something about Noise

     Noise is most often and most simply defined as unwanted sound. In considering Environmental Noise the analyst studies, how affects the health or interferes with the work zone activities of the people. The principle concern is quantitative determination of the amount and type of noise to which people will be (or already are) exposed in a given situation.

     Almost all noise problems have three basic elements; (I) Noise Source, (II) the transmission path (III) the receiver. In order to analyze the noise problem and for the development of appropriate environment assessment, the dominant noise sources must be identified and characterized according to their loudness, frequency, duration and other characteristics. 

Why to consider noise control as a Big issue?

     Noise is a major occupational hazard. Short-term effects of noise exposure include temporary hearing loss, stress, annoyance, difficulty in verbal communication, and safety hazards. The primary long-term health effect of noise exposure is permanent hearing loss. Both short term and long-term effects can be prevented by timely recognition, evaluation and control of noise exposure. Apart from other pollutions noise are also spreading and strict regulations from government forcing the organization to think about control.

General Noise Standards for Industries

    Whether a point or line source, occupational health standards in most countries limit employees’ exposure to the noise. For example, the Occupational Safety and Health Administration (OSHA) sets 85 dBA over an eight-hour period as the maximum admissible noise exposure limit in the workplace. The OSHA standard is representative of a source noise limit. With this standard in mind, plant equipment is typically ordered to emit sounds of no more than 85 dBA at one meter (3 feet).

     Normally 10 to 12 measurements of the sound pressure around the periphery of a machine at one meter (3 feet) are taken to obtain the source noise level. However, the number of measurements varies by machine shape and size. National and international standard institutes, such as ASTM (American Society for Testing and Materials), ANSI (American National Standards Institute), CSA (Canadian Standards Association) and ISO (International Standards Organization) publish guidelines on how to construct a grid over equipment and gather point measurements at different frequencies.

Technical Discussions and Definitions

  • Some basic ACOUSTIC terms

1) Sound pressure :- Sound pressure (p) is expressed in dyne/cm2 and is the rms value of the pressure due to a pure sinusoidal wave.
2) Intensity :- The intensity (I) of a sound wave in a given direction is the sound energy transmitted per second in this direction through a unit area perpendicular to the specified direction. In the case of a sinusoidal plane or spherical wave, the Intensity is proportional to the mean square pressure exerted on an area at right angles to the given direction.
Function of the intensity but also varies with frequency and composition of the note being heard.
3) Octave Band (O.B.) :-A range of frequencies where the highest frequency of the band is double the lowest frequency of the band. The band is usually specified by the center frequency, i.e., 31.5, 63, 125, 250, 500 Hz, etc.
4) Acoustical Material :- A material used to alter a sound field. The material may be used to absorb, damp or block acoustical energy.
5) Airborne Noise :- A condition when the atmosphere is carrying sound waves.
6) Frequency Spectrum :- A graph or plot of the sound pressure level in each band from a set of octave or 1/3-octave bands.
7) Sound Level Meter :- An instrument used to measure sound pressure level. Sound level meters are commonly Type1, precision instruments, or Type 2, general-purpose instruments. Both types can have weighting and filter networks to provide dB readings by octave band in the A, B, or C scales.
8) Insertion Loss :- The reduction of sound power level attained by inserting a silencer or muffler in an acoustic transmission system.
9) Sound Power Level (Lw) :- A measure of the total airborne acoustic power generated by a noise source, expressed on a decibel scale referenced to some standard (usually 10-12 watts).
10) Loudness :- Loudness is the subjective human definition of the intensity of a sound. Human reaction to sound is highly dependent on the sound pressure and frequency.
11) Sound Pressure Level (Lp) :- A measure of the air pressure change caused by a sound wave, expressed on a decibel scale referenced to 20µPa.
12) Transmission Loss (TL) :- The reduction in sound power that is caused by placing a wall or barrier between the source and receiver. Transmission loss is expressed in decibels.