ENVIRONMENTAL CONSIDERATIONS — dB(A)
Noise Consideration
Because noise effects the surrounding environment, it is important to consider noise factors when installing a genset. The following is a brief approach to evaluating noise sources and noise level reduction.
Noise requires a source, a path, and a receiver. In a standby system, the genset is the source, the path is air or air and a structure which transmits the noise vibrations, and the receiver is a person in the vicinity (including the operator). Since little prevention can be done with the source or the receiver, the treatment method is to manipulate the pathway of noise.
The three main components of noise from an engine- generator set are:
1.Engine exhaust (low frequency sound)
2.Engine moving parts (low and high frequency sound)
3.Radiator discharge air (high frequency sound).
Noise Laws and Regulations
There are many state and local codes establishing maximum noise levels. Most noise regulations specify the maximum allowable noise level at the property line. Table 8 is an example of typical maximum allowable noise levels. OSHA has specific noise regulations where persons working in a generator room will be required to wear ear protection.
Noise Level Measurement and Decibel / dB(A) Units
To measure noise properly, the subjective response of human hearing is substituted by an objective measurement of sound measured by a meter. The unit of measurement for sound is the decibel (dB). The decibel is a convenient number on a logarithmic scale expressing the ratio of two sound pressures, comparing the actual pressure to a reference pressure.
Noise regulations are written in terms of "decibels 'A' scale" or dB(A). This term means the sound pressure level has been adjusted to duplicate how the imperfect human ear hears noise. The human ear can only hear within a range of frequencies. The dB(A) weighted scale tries to simulate human loudness perception. Loudness is dependent on sound pressure level (amplitude) and frequency. See Figure 2 on page 28 for a dB(A) comparison.
Decibel tests are conducted in a "free field". A free field is a sound field in which the effects of obstacles or boundaries on sound propagated in the field are negligible. A "reverberant field" is a sound field in which the effects of obstacles or boundaries on sound propagated in the field are not negligible.
Accurate noise measurements require the microphone to be placed outside the "near field". The near field is defined as the region within one wavelength or two times the largest dimension of the noise source, whichever is greater. Noise cannot be measure accurately for compliance with specifications calling for measurements within the near field.
Noise measurements should be made using a sound level meter and octave band analyzer. The microphones should be placed in a circle of 23 feet (7 meters) radius centered on the generator set.
Table 8. Typical Criteria for Outside Noise Levels
Noise Zones | Peak Day | Peak Night | Continuous | Continuous | ||
Day | Night | |||||
dB(A) | dB(A) | |||||
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Urban — | Residential | 62 | 52 | 57 | 47 | |
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Suburban — | Residential | 57 | 47 | 52 | 42 | |
Very Quite Suburban or | 52 | 42 | 47 | 37 | ||
Rural Residential | ||||||
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Urban — Nearby Industry | 67 | 57 | 62 | 52 | ||
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Heavy Industry | 72 | 62 | 67 | 57 | ||
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INDUSTRIAL GENERATOR SETS — APPLICATION & INSTALLATION MANUAL — REV. #4 (09/07/07) — PAGE 27