Figure 1. The decibel scale compresses a range of numbers from 10-to-100,000 to 10-to-50 using this technique. The Richter scale for earthquakes is similar.


Anyone who has been in the vicinity of an unenclosed diesel-driven emergency generator system can appreciate how much noise is produced. These gen sets often need to be contained for the comfort of personnel or noise-sensitive neighbors. Containing the noise is a challenge since the engine/generator also produces heat.

Somehow the noise must be contained while shedding thermal energy-a design challenge. For smaller gen sets (up to about 500 kW), manufacturers typically offer skid-mounted, sound-attenuating enclosures that can be procured as a tested system. Larger gen sets may be obtained with custom-fabricated acoustical enclosures. Sometimes, however, gen sets are located within buildings along with less noisy mechanical and electrical equipment. In these cases, the architect/engineer team is responsible for noise control.

Sound/Noise Fundamentals

The sound produced by real noise sources, the noise control afforded by various devices, and the sensitivity of human hearing all differ with frequency. Low frequencies (in musical terms: bass) are the most difficult to control and to hear. Therefore, engineering a noise control solution requires information about the strength of the noise source, the capability of noise control devices, and the allowable sound levels in terms of frequencies, which are often clustered together in groups called octave bands.

Figure 2. Limits in New Jersey that apply at residential receptors

Acousticians use a logarithmic scale, similar to the Richter scale, to compress the wide magnitude of audible sound powers and pressures to a set of every day numbers. Figure 1 shows how the decibel scale is used to compress a range of numbers from 10-to-100,000 to a range of 10-to-50 decibels (dB).

Manufacturers often provide acoustical data in the form of sound power levels or sound pressure levels, both in decibels. These are measures of sound magnitude. Sound power levels represent the overall sound produced by a noise source. Sound pressure levels are measured using a sound level meter and depend not only on the amount of sound produced by the source but also environmental factors such as the distance from the source to where the measurement is made. For example, light falling on a desk will depend on how far the surface is from the light source and the color and reflectivity of the office walls.

A-weighting is a way of combining sounds across the audible spectrum in a manner similar to the way the human ear detects sound, de-emphasizing low frequencies. A-weighted sound levels are denoted using the label dB(A) when using the decibel scale.

Outdoors, where sound does not build up with reflections from room surfaces, sound from a point source will fall off with distance. The fall-off follows the inverse square law, also applicable to light, proportional to distance squared. After taking into account the logarithmic scale, the fall-off with distance amounts to a 6 dB drop for each doubling of distance [10 log (2)2]. So an enclosed gen set that produces 72 dB(A) at 23 feet (ft) will contribute 66 dB(A) at 46 ft.

Sound levels cannot be added to sum up noise from several sources. Two equal noise sources each contributing, say 60 dB, total 63 dB together. And when the sources to be summed are 10 dB or more apart, the energy from the lower level source will barely affect the sum. For example, 40 dB + 50 dB ≈ 50 dB.

The Community

Deciding how much noise is too much at off-site receptors in the community requires a common-sense approach. Designers must first identify noise-sensitive receptors near a proposed gen-set installation and then review local or state noise regulations. Noise-sensitive receptors include residences, churches, hospitals, and nursing homes, as well as offices or schools that rely on open windows for ventilation during some seasons. Emergency generators are noise sources that operate intermittently, either during real emergencies or scheduled tests. Complaints about noise, activity interference, and code issues mostly come into play when gen sets are exercised during periodic run-up. New Jersey’s statewide noise regulation (N.J.A.C. 7:29) is a prime example.

In addition, codes tend to be much more stringent at night [50 dB(A) at receptor] than during the day [65 dB(A) at receptor]. If gen-set tests can be run during the day, fewer noise control measures will be needed. The actual limits in New Jersey at residential receptors are shown in figure 2.

Where there are no applicable regulations, prevailing ambient sound at the noise sensitive receptors and time of day and duration of gen-set exercises should be considered when developing a goal for sound emissions.

Typically, communities will tolerate an intermittent source that does not lift background ambient sound levels by more than about 5 dB(A). On the other hand, a rise of 20 dB(A) at night is very likely to elicit strong complaints.

Sources of Gen-Set Noise

When a gen set is placed indoors with its engine exhaust piped outdoors, the engine block or casing creates most of the noise. The engine exhaust is the second major noise source. Manufacturers usually provide ratings for the unsilenced exhaust.

The presence of an engine turbocharger lowers exhaust sound pressure levels by about 5 dB as it uses energy in the exhaust stream to pressurize combustion air. An 800-kilowatt (kW) gen-set driven by an 1800 rpm, turbocharged, reciprocating 12-cylinder diesel engine in an open field will produce levels as follows:


Component


A-weighted sound pressure level
at 23 ft
(dB re 20mPa)


Unsilenced, open exhaust


102


Engine casing


94


 




Ignoring reflected sound for a moment and relying on distance alone to control noise, the inverse square law says that it would take about 650 ft for the noise from this single 800-kW engine casing to meet a level of 65 dB(A). Sound-attenuating hardware is needed to meet the codes in most applications.

Auxiliary noise sources associated with gen-set installations are radiator fans and fans associated with load banks. Radiators are either skid-mounted to be integral with the gen set or can be remotely located outdoors, for example on a rooftop.

Noise Control Strategies

Engine exhaust silencers are available in a number of styles and with different grades of sound attenuation. Silencer manufacturers typically offer at least three grades of silencing which are often qualitatively described as follows:


Manufacturer’s Silencer Description


Typical Degree of Sound Attenuation
(dB(A))


Industrial


15-18


Residential


20- 25


Critical


25- 35


Super Critical or Hospital


35- 40


 



Some manufacturers can also provide silencers with performance above 40 dB for special circumstances. Engine silencers should preferably be located close to the engine to quiet this component in the initial section of the exhaust system.

The steps used to specify silencer performance for the 800-kW gen-set example to meet New Jersey’s daytime limit of 65 dB(A) if there were a residence within 100 ft can be seen in the table.


Strength of noise source; unsilenced exhaust


102 dB(A) at 23 ft


Contribution at residence (100 ft), about two doublings of distance so 12 dB distance loss


90 dB(A) at 100 ft


Goal for exhaust noise - use 65 dB(A) less some safety factor (3 dB) less a second safety factor (3 dB) since other gen-set noise will also contribute


59 dB(A)


Minimum exhaust silencer performance


31 dB(A)




 




Figures 3

A more thorough approach requires an analysis in octave bands.

A Critical-grade silencer may be adequate; a Hospital-grade unit is a safer choice. A Residential-grade silencer will not be sufficient.

The best practical solution is a traditional expansion-chamber-style engine exhaust silencer similar to those used for automotive applications. These units reflect sound back toward the engine to partially cancel out pulsations, especially the lower-frequency, engine-firing noise. Newer “space-saver” silencers offered as a less expensive alternative do not perform as well at low frequencies. Reputable engine silencer manufacturers include Universal/Nelson, GT-Exhaust Systems, and Maxim.

Figures 3 and 4 show the inside of a gen-set room that includes some important noise control measures

Sound radiated from the engine casing presents challenges because of the need to cool the gen set, which is usually done by driving outdoor air through the gen set room to reject the heat to the outdoors. To achieve noise-control objectives, the ventilation air intake and discharge openings to the exterior need to incorporate acoustical louvers or splitter-type dissipative silencers. Both devices use baffles comprising perforated metal over sound absorptive materials and are commonly available in high, medium, and low-pressure drop versions having between 25 and 50 percent open or free area for airflow. Selecting silencers or acoustical louvers for a specific application involves a trade-off among needed sound attenuation, space available, and allowable pressure drop for the airflow to be handled. Remotely located radiators can drastically reduce the air flowing through the gen-set room and is often more cost effective.

The interior of the gen-set room should be moderately sound absorptive. In critical situations, all available wall and ceiling surfaces should be treated with 2-to-4-inch thick unfaced, glass fiber board insulation, optionally protected with perforated or flattened, expanded metal mesh.

Some people intuitively believe that sound cannot propagate upstream and wonder why it is necessary to silence the air-intake path. But sound can easily travel through air intake openings, given that the speed of sound in air is about 1100 ft per second and that the free areas of air intake paths are characterized by air velocities no more than 1100 ft per minute to have reasonable pressure drops.

The best means of controlling fan noise reduction from remote radiators or load banks is to specify equipment having slower speed fans. Remote radiators are typically rated using A-weighted sound pressure levels at 25 ft. Radiators with 1800 revolution per minute (rpm) fans can be expected to produce about 80 dB(A) at 25 ft; however with 900 rpm fans, the level would be reduced to 70 dB(A). A bigger footprint may be needed to obtain the same heat rejection. The next choice is to screen the radiator from noise-sensitive receptors using a noise control barrier. Fan-speed reduction is more common since effective barriers require structure to handle wind load and can become bulky and unwieldy. For equipment-intensive applications or where noise-sensitive receptors are at close range, both techniques may be needed in tandem to meet goals.

Many people, including professionals in other disciplines, are surprised to learn that noise control measures can be anticipated and integrated into designs, just as other systems are engineered. Where there are regulations to be met or any concern about whether noise from a planned gen set installation will result in environmental impact, the advice of an acoustical engineer should be sought.

Table 1.

SIDEBAR: Case Study

A data center program called for four 1250-kW diesel-powered gen-sets indoors in a dedicated room, 80 ft from a commercial property line in New Jersey. The facility would circulate engine-cooling water to a cooling tower cell that would be powered during emergencies. This reduced the required airflow through the generator room. The generator room was also rendered highly sound absorptive via application of effective sound absorptive glass fiberboard.

Analysis showed that duct silencers were needed for room air intake and outlet ventilation paths, and fans were provided to drive air through the room. A screen wall blocked line-of-sight from the ventilation air intakes and outlets to vantage points at the property line. Tables 1-3 provide calculations that show:

Table 2.

  • How engine casing noise from four engines is controlled by room sound absorption, ventilation path silencers, and screen walls to contribute about 59 dB(A) at the property line 80 ft away (see table 1).

  • How the exhaust noise of four exhausts is controlled to meet 61 dB(A) at the property line via selection of appropriate exhaust silencers (see table 2).

  • How the sum of both components meets code limits (see table 3).


Table 3.

Room Size: 64 ft by 32 ft in plan, 12 ft high

Room Absorption: 1500 ft2 of 2-in thick sound absorptive material, 500 ft2 of 4-in thick treatment

Room Air Intake: 120,000 cfm through 3-ft long, 25-percent open knocked-down silencers, (23 dB loss at 250 hertz)

Room Air Discharge: 110,000 cfm through 4-ft long, 25-percent open knocked-down silencers, (28 dB loss at 250 hertz)