Power Generation - Control unwanted noise from on-site power systems

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With the growth of standby, prime and peaking power installations in highly populated areas, design engineers have focused their attention on understanding how generator set noise is propagated and controlled. The high cost of Retrofitting a site for noise reduction makes it imperative to assess noise performance requirements early in the on-site power system design stage. By applying the principles outlined in this paper, power system designers and end users alike will be able to more easily control unwanted noise from their on-site power system.
*Part two of this feature will appear in Electrical Review May 09 and can be found at
www.electricalreview.co.uk

Like many types of rotating machinery, reciprocating engine-powered generator sets produce noise and vibration. Whether these generator sets run continuously in prime power applications or only occasionally in standby applications, their operating sound levels often must be reduced to comply with local, state or federal ordinances. In North America, maximum permitted overall noise levels range from 45 dB(A) to 72 dB(A), depending on location and zoning. In fact, recently, some states and communities have begun to specify property line noise restrictions using octave band frequencies to reduce the amount of low-frequency noise that reaches community neighborhoods. Since untreated generator set noise levels can approach 100 dB(A) or more, both the location of the generator set and noise mitigation take on great importance.

In general, two forms of regulations affect the volume of noise to which individuals or the public may be exposed state or municipal noise ordinances and Occupational Safety and Health Administration (OSHA) federal safety regulations. The former regulations address noise that may migrate beyond property lines and disturb the public but that is seldom sufficiently loud to  constitute a safety hazard. The latter addresses standards for noise exposure in the workplace to protect the health of workers. OSHA regulations normally only apply to workers who may be exposed to generator set noise that is above 80 dB(A) for any appreciable time. Workers can limit exposure by wearing proper hearing protection when working around operating generator sets. Europe and Japan, as well as numerous other countries, have also set standards to control noise in the workplace and in the environment at large.

Defining noise

Sound is what the human ear hears; noise is simply unwanted sound. Sound is produced by vibrating objects and reaches the listener's ear as pressure waves in the air or other media. Sound is technically a variation in pressure in the region adjacent to the ear. When the amount of sound becomes uncomfortable or annoying, it means that the variations in air pressure near the ear have reached too high an amplitude. The human ear has such a wide dynamic range that the decibel (dB) scale was devised to express sound levels. The dB scale is logarithmic because the ratio between the softest sound the ear can detect and the loudest sound it can experience without damage is roughly a million to one or 1:106. By using a base-10 logarithmic scale, the whole range of human hearing can be described by a more convenient number that ranges from 0 dB (threshold of normal hearing) to 140 dB (the threshold of pain). There are two dB scales:
A and L.

- The dB(L) unit is a linear scale that treats all audible frequencies as having equal value. However, the human ear does not experience all sound frequencies as equally loud. The ear is particularly sensitive to frequencies in the range of 1,000 to 4,000 Hertz (cycles per second), and not as sensitive to sounds in the lower or higher frequencies.
- Therefore, the "A-weighting filter," which is an approximation of loudness, is used to correct the sound pressure levels to more accurately reflect what the human ear perceives. This frequency-weighting results in the dB(A) scale, which was adopted by OSHA in 1972 as the official regulated sound level descriptor. Figure 1 shows typical noise levels associated with various surroundings and noise sources.

Sources of generator set noise
Generator set noise is produced by six major sources (see Figure 2):
- Engine noise - This is mainly caused by mechanical and combustion forces and typically ranges from 100 dB(A) to 121 dB(A), measured at one meter, depending on the size of the engine.
- Cooling fan noise - This results from the sound of air being moved at high speed across the engine and through the radiator. Its level ranges from 100 dB(A) to 105 (A) dB at one meter.
- Alternator noise - This is caused by cooling air and brush friction and ranges from approximately 80 dB(A) to 90 dB(A) at one meter.
- Induction noise - This is caused by fluctuations in current in the alternator windings that give rise to mechanical noise that ranges from 80 dB(A) to 90 dB(A) at one meter.
- Engine exhaust - Without an exhaust silencer, this ranges from 120 dB(A) to 130 dB(A) or more and is usually reduced by a minimum of 15 dB(A) with a standard silencer.
- Structural/mechanical noise - This is caused by mechanical vibration of various structural parts and components that is radiated as sound.

Measuring noise
Before you can begin to determine what mitigation might be required, you have to collect accurate sound measurements of both the existing ambient noise and the noise contributed by the generator set. Accurate and meaningful generator set sound-level data should
be measured in a "free field environment." A free field, as distinguished from a "reverberant field," is a sound field in which there are negligible effects from sound being reflected from obstacles or boundaries. Noise measurements should be made using a sound level meter and an octave band filter set, at a minimum to allow for more detailed analysis by acoustical consultants.

When measuring sound levels from a distance of 7 meters, microphones are placed in a circular array with measurement locations at 45-degree increments around the generator set. The measurement array is 7 meters from an imaginary parallelepiped that just encloses the generator set, which is usually defined by the footprint dimensions of the skid base or chassis.

When measuring sound power levels for European applications, a parallelepiped microphone array is typically used, as defined in International Standards Organization standard ISO 3744.

Sound performance data for generator sets from Cummins Power Generation Inc. are available on the company's design software CD (called "Power Suite").
Sound performance data is also available in the Power Suite Library on the company's Web site: www.cumminspower.com.

Initial noise measurements are usually made in eight octave bands from 63 Hertz to 8000 Hertz, although the highest sound power generated is typically in the range of 1,000 Hertz to 4,000 Hertz - the range of sound to which the human ear is most sensitive.
While measurements are taken across a spectrum of frequencies, the logarithmic sum of all the frequencies is the most important reading. However, when the overall sound level exceeds the allowable level for a project, frequency band data is used to determine what design changes are necessary to lower the overall sound level to comply with requirements.

Totaling all the sources of noise
The total noise level from a generator set is the sum of all the individual sources, regardless of frequency. However, because the dB(A) scale is logarithmic, the individual dB(A) readings cannot be added or subtracted in the usual arithmetic way. For example, if one noise source produces 90 dB(A) and a second noise source also produces 90 dB(A), the total amount of noise produced is 93 dB(A) - not 180 dB(A). An increase of 3 dB(A) represents a doubling of the sound power; yet, this increase is barely perceptible to the human ear.

Figure 3 illustrates how to add decibels based on the numerical difference between two noise levels. As in the example above, if there is no difference between noise source 1 and noise source 2, their combined dB(A) measurement would only increase by 3 dB(A) - from 90 dB(A) to 93 dB(A). If source 1 was 100 dB(A) and source 2 was 95 dB(A), their combined dB(A) measurement would be 101 dB(A).

Noise laws and regulations
In North America, state and local codes establish maximum noise levels that are permitted at the property line. Figure 4 shows some representative outdoor noise level regulations. Compliance with these noise regulations requires an understanding of the existing ambient noise level at the property line without the generator set running and what the resultant noise level will ultimately be with the generator set running at full load.

In Europe, regulation of generator noise is governed by the 2000/14/EC (Stage II) legislation that has been in place since 2006. For generators with a prime power rating less than 400 kW, the maximum sound power level permitted is calculated using the formula:
95 + Log Pel = dB(A)
(where Pel is the generator's prime power rating)

Generators of 400 kW prime rating and above are only required to be labeled with the LWA figure (European measurement of "acoustic power level") calculated from the manufacturer's developmental test results. For the European market, most generators from 11 kVA to 550 kVA are packaged in standard enclosures that make the units compliant with most legislation. Standard enclosures typically reduce radiated noise by a minimum of 10 dB(A).