SOUND ATTENUATED GENERATOR BASICS
During the installation of an electrical generating system, many field factors can add to deviations of actual Sound Power Levels (SPL: Total sound radiated from a source with respect to a reference power of Watts) versus predicted levels of sound. The noise that is present in the natural environment of the genset prior to installation is referred to as ambient noise. The ambient, or background noise, should be measured and calculated before the installation of equipment. Therefore, a margin of safety should be applied to calculated values if all Field Sound Attenuated CAT 2500 kWconditions are not fully studied. For example, buildings, walls, signs and auxiliary equipment commonly change the sound field. Obstacles within the sound path will partially reflect, absorb and transmit sound. It is important to study field conditions and know the local decibel laws before embarking on an electrical generation project.
Most generator applications are standby rated and surprisingly many municipalities have laxed restrictions for strictly standby units. Most laws are tougher when dealing with prime or continuous power or cogeneration applications because of their extended use of operation. If a particular noise level at the property line must be achieved, then the sound attenuated enclosure manufacturer should be told beforehand what the requirement is and how far the enclosure will be from the property line. Additionally, the generator enclosure manufacturer should be made aware of the layout of the surrounding buildings, equipment, ancillary structures and the topography. For example, a large structure nearby the genset, a grass-covered berm or heavy foliage around the site, or a hard surfaced parking lot can greatly influence the propagation of sound and therefore, the enclosure design needed for the project.
Noise guidelines are often enforced through the Environmental Protection Act (EPA). As stated above various authorities at the national, regional and municipal levels publish noise control guidelines and limits. A wide variation of standards exists for the measurement and calculation of SPL, SWL and other more complex acoustical parameters. ISO standard 8528-10, Measurement of Airbone Noise, by the Enveloping Surface Method, can be referenced as a standard procedure for determining overall SPL readings.
A real-time sound analyzer is an all-purpose sound measuring device that uses multiple processors to measure various sound levels at the same time. By using a real-time sound analyzer the user can observe sound properties over the entire spectrum of interest in real time, without loss of any data. A real-time analyzer can perform the work of many sound level meters by simultaneously measuring all octave or 1/3 octave bands instead of just one octave band at one time. Other options features of a real-time sound analyzer can include Fast Fourier Transform (FFT) measurements for discrete frequency analysis, and sound intensity measurements using an sound intensity probe. A sound intensity probe essentially acts as a device with two microphones seprated by a spacer and estimates the instantaneous sound intensity by simultaneously measuring pressures at both microphones. It is generally a good idea to conduct a sound intensity analysis because it determines the contributions of individual source elements to an overall sound level in a multi-source environment.
A word of caution for potential customers: Because sound is a wave phenomenon, there is a mathematical rule, the inverse square law, which is often applied as a rule-of-thumb to determine the effects of distance on sound level. Without exploring all of the underlying theory, the inverse square law simply states that for a point source of sound under free field conditions that the sound level will decrease by 6 dB each time the distance from the source is doubled. For example if one measured 100 dB (A) at 50 feet, then we would measure 94 dB (A) if we moved to 100 feet. It is also important to note that the term “free field” does not begin until 30-50 feet away from the generator.
The most overlooked aspect of choosing a sound attenuation enclosure system is that the as the amount of sound attenuation increases, ie the quitter the unit becomes, the larger the generator enclosure will be. The effect is most directly correlated with kW rating and air flow needed for the generator.
Once the sound attenuation approaches 40 dB (A) of reduction (40 dB (A) is considered the maximum economically viable reduction by a standard prefabricated enclosure) it is not unusual to see more enclosure space devoted to air handing as to the muffling of the engine itself.
A-weighted decibels, abbreviated dBA, or dBa, or dB (a), are an expression of the relative loudness of sounds in air as perceived by the human ear. In the A-weighted system, the decibel values of sounds at low frequencies are reduced, compared with unweighted decibels, in which no correction is made for audio frequency. This correction is made because the human ear is less sensitive at low audio frequencies, especially below 1000 Hz, than at high audio frequencies.
In computer systems, dBA is often used to specify the loudness of the fan used to cool the microprocessor and associated components. Typical dBA ratings are in the neighborhood of 25 dBA, representing 25 A-weighted decibels above the threshold of hearing. This is approximately the loudness of a person whispering in a quiet room.
This was last updated in March 2011
Contributor(s): Christoffer Holmstedt
Posted by: Margaret Rouse