Machinery Noise, Noise from Industrial Plants
Mechanized industry creates serious noise problems, subjecting a significant fraction of the working population to potentially harmful sound pressure levels of noise. It is responsible for high noise emissions indoors as well as outdoor of plants. In industrialized countries it has been estimated that 15-20 % or more of the working population is affected by sound pressure levels of 75-85 dBA. This noise is due to machinery of all kinds and often increases with the power of the machines. The characteristics of industrial noise vary considerably, depending on specific equipment. Rotating and reciprocating machines generate sound that is dominated by tonal and harmonic components; air moving equipment tends to generate sounds with a wide frequency range. The highest sound pressure levels are usually caused by components or gas flows that move at high speed (e.g., fans, steam pressure relief valves) or by operations involving mechanical impacts (e.g., stamping, riveting, road breaking).
In industrial areas, the noise usually stems from a wide variety of sources, many of which are of complex nature. Various types of machinery are involved and they represent artificial noises which are of concern because they may contain predominantly low or high frequencies as well as tonal components, they may be impulsive and also present unpleasant and disruptive temporal sound patterns.
Machinery that moves air are of special interest because it usually creates noise with a large component of low frequencies. Unlike noise containing mainly higher frequencies, low-frequency noise is less attenuated by walls or other structures and it can cross great distances with little energy loss due to atmospheric and ground attenuation.
In residential areas, noise may stem from mechanical devices (e.g., heat pumps and ventilation systems, traffic) as well as voices, music and other kinds of noises generated by neighbors (e.g., lawn movers, vivid parties, and other social activities). Due to low-frequency characteristics, noise from ventilation systems in residential buildings may cause considerable concern even at low and moderate sound pressure levels.
Sound generation mechanisms of machinery are reasonably well understood. The technical requirements for low noise output in new machinery can usually be specified but the noise declaration of machinery, which describes the noise output of the machine, is not yet used efficiently. The noise declaration should preferably be used for selecting and purchasing the machine which is least noisy. The difficulty of reducing the sound output and the noisiness of existing equipment is a serious obstacle to the improvement of working environments (e.g., jack hammer or shooting range). Machinery should preferably be silenced at the source. Noise from fixed installations such as factories or construction sites, heating pumps and ventilation system plants on roofs, can also affect the nearby communities. To reduce the sound output from such sources, either the use of quieter plant and equipment is encouraged, or through zoning to separate industrial land uses from the more noise-sensitive residential areas. As last resorts, insulation or restriction of operation time may be used.