Sound advice on attenuating genset noise, vibrations

Whether they’re used for prime power, standby, or emergency service, diesel generators make noise—lots of noise. Here’s how to lower genset noise to an acceptable level.

03/01/2011


Generator noise sources include exhaust noise, mechanical noise from the engine, noise from the generator rotor passing the stationary poles, cooling system noise, and vibration. All of these noise sources must be addressed to achieve a quiet generator installation (see Figure 2).

We will address each of these noise sources and methods to mitigate them in the following discussion. By meticulously adhering to these methodologies, the genset may be installed directly below or above the CEO’s office and you will not get a single complaint about the generator being too loud.

First things first

Before beginning design, the engineer must carefully analyze the generator installation requirements. Simply calling for the best attenuation can result in a very large enclosure and can be very expensive. Typical installations are normally satisfactory with a 35 dB to 40 dB attenuation level. However, quiet environments or sensitive locations can require greater attenuation. For a two megawatt genset, 35 dB of attenuation can cost around $110,000. However, reducing the attenuation to 25 dB reduces the relative cost to about $75,000, while increasing attenuation to 55 dB raises the price to more than $750,000.

Enclosure size also varies with attenuation level. A 12x30-ft enclosure can provide 25 dB of attenuation. For an attenuation level of 55 dB, the enclosure dimensions increase by 33% in width and double in length. This increase equates to 2.5 times the total area.

Sound pressure level (SPL) for a particular installation is easy to measure but much more difficult to calculate (see sidebar titled “Understanding dB”). Therefore, we will address the amount of attenuation for each element, not the resultant noise level. Factors that influence the noise level include generator manufacturer, loading, location, temperature, and humidity.

Noise ordinances

SPL may be regulated by local noise ordinances, which vary widely from one jurisdiction to another. In some areas, the SPL may be no more than 3 dB above the existing ambient at the property line, although in other areas the maximum SPL at the property line or 50 ft away from the source can vary from 50-80 dB.

This could be a serious problem if the ambient SPL is 55 dB and the maximum permitted is only 50 dB. Determining the requirements of your local ordinance should be one of the first tasks in the design process, since that will determine everything about the installation. One source of information is the website www.nonoise.org.

Exhaust noise

The noise most typically associated with genset operation is diesel engine exhaust. Engine exhausts are notoriously loud, but reduction of this noise source is really quite simple. The engineer can select an exhaust silencer that provides the level of sound attenuation that matches the attenuation level required by the installation.

Most silencers are designed for four levels of attenuation, with a few manufacturers providing a fifth level. An industrial-grade silencer that reduces exhaust noise by 20-25 dB is used in areas where there are high, ambient sound levels, since the generator noise will not be noticeable. Where ambient sound levels are lower, such as residential areas, the most common silencer used reduces the exhaust sound level by 25-32 dB. For very quiet environments where greater attenuation levels are needed, a critical-grade silencer reduces the exhaust noise by 30-38 dB. For health-care and other very sensitive locations, a super-critical silencer reduces the exhaust sound level by 35-42 dB. A super-critical-plus silencer can provide as much as 50 dB of attenuation.

When it comes to silencers, size really does matter and the engineer must take the silencer dimensions into consideration during his design. For example, a super-critical-plus silencer for a large genset can be more than 6.5 ft in diameter, more than 23 ft long, have an inlet diameter of 30 in., and weigh nearly 8,000 lb. Some diesel generators may require two silencers of this size, so space and weight are very important design considerations.

Mechanical noise

Two of the largest noise sources are the engine and generator, both of which produce mechanical noise. Movement of the engine components generates noise. The combined engine, combustion, and generator windage noises are the most difficult and expensive to mitigate. Windage is the sound generated by the passage of the generator poles through the surrounding air.

Stopping the propagation of mechanical noise is, simply, a question of mass. The greater the mass between the noise source and the listener, the more attenuation achieved. Thus, placing a concrete block wall will provide much more attenuation than placing a single, 0.5-in.-thick sheet of gypsum wall board. By filling the voids in the concrete block wall with tightly-packed sand, the attenuation is greater than leaving the voids empty or filling them with fiberglass insulation. When trying to provide maximum attenuation for critical installations, using two concrete block walls with sand-filled voids and separated by a small air space provides excellent noise control. As an alternative, similar attenuation levels may be achieved by using engineered enclosures specifically designed to mitigate generator noise.

Even with a remote radiator, since a genset’s engine must have a source of combustion air, and air for cooling the generator and the exterior of the engine, any sound-attenuating enclosure must have a means of ventilation. For the minimum attenuation installation, locating the intake and exhaust louvers on the top of the enclosure, which will direct the noise upward, will normally suffice. When the installation requires a higher level of attenuation, acoustically lined ducts for both the cooling air intake and the hot-air exhaust can provide up to 50 dB of mechanical and cooling noise attenuation. These air passages have multiple 90-deg turns with sound-absorptive linings to prevent sound from leaving the enclosure. However, these winding passages increase the pressure drop of the cooling air, and require that the cooling-air circulating-fan be sized larger to maintain the total cooling airflow required for the genset (see Figure 1).

Cooling noise

The noises associated with genset cooling emanate from the radiator fan—whether integral or remote. For the integral radiator, the noise source is closely coupled with the genset, and the radiator fan noise will be mitigated along with the mechanical noise from the generator and engine. When the radiator is remote, the fan noise levels may be reduced by oversizing the remote radiator and operating the cooling fan at lower speeds, which will reduce the fan-generated noise.

Vibration

After taking care of the exhaust noise, the engine noise, and the noise from the cooling system, the engineer is still left with vibration—the most pervasive source of noise produced by a genset. Vibration is transmitted through integral isolators to the skid-base and from there to the housekeeping pad, genset foundation, or building structure.

When the foundation is fully isolated from the building, vibration noise isn’t a big issue. But when a genset is mounted on a concrete housekeeping pad or structural steel base within the building, vibrations are transmitted to the building structure and can be heard anywhere the structure can vibrate.

The solution is to isolate the generator base from the structure or housekeeping pad, but this is much easier said than done. Note that for most modern gensets, vibration dampers are provided between the engine-generator and the base. Typically, these dampers are inadequate for quiet installations. They don’t dampen the vibrations enough to prevent them from being transmitted through the base to the structure. Therefore, the engineer can ignore the minimal effects that these integral dampers provide for installations requiring more than 25 dB of attenuation.

Vibration pads and springs are a good, first step to prevent transmitting vibration to the structure. The layered, multiple-material pads (which commonly use neoprene rubber and cork) seem to work better than the homogeneous pads. The density changes in layered pads are more effective in attenuating the higher frequency vibrations.

For low-frequency vibrations, the isolating springs work very well, particularly when the springs have a compression of at least 2 in. when loaded to their rated capacity. The high-compliance springs with more than 2 in. of compression under load give more opportunity for spring movement, thus transmitting less vibration to the structure.

A downside of the high-compliance springs is a decrease in stability. Instead, consider caged springs to reduce side movements during installation. Once they are installed, ensure the springs do not contact their cage in order to maintain optimum isolation. A combination of pads and springs will mitigate most vibrations for less critical installations. However, they must be supplemented for the quietest, most critical installations.

As with silencing airborne noise, mass is the key to reducing vibration-generated noise for critical installations. Since the engine, generator, and base are moving at the vibration frequency, there is a lot of mass in motion. To prevent this vibration from transferring to the structure, the engineer can place an intervening element with a large inertial moment—called an inertia base—between the genset base and the building structure.

This normally consists of a steel frame filled with reinforced concrete and weighs at least 150% of the total weight of the genset. By mounting the genset on the inertia base using vibration-dampening springs, and mounting the inertia base on the genset foundation using another set of springs, the genset vibration will not be transmitted through the inertial base. When the neoprene vibration pads are used in conjunction with the springs, isolation of both the low- and high-frequency vibration can be dampened to a negligible magnitude.

Other vibration paths

Isolation of the genset with springs, an inertia base, and vibration pads does not completely eliminate vibration transmission. This is because of the piping and conduit connections between the fixed building structure and the engine and generator. The fuel supply and return piping cannot be hard-piped, since vibrations can be transmitted through the pipes to the building structure. Likewise, the power, control, and monitoring conduits must have similar vibration and movement isolation so that even the smallest conduit does not impede genset movement or transmit vibrations.

For both the fuel and conduit connections, a flexible, metallic braided pipe/conduit can be used, which provides continued protection of the conductors, piping flexibility, and vibration isolation. While the appearances of the flexible pipe and flexible conduit are very similar, each is designed and manufactured for exceptional flexibility for its applications.

Combining systems

Each of the aforementioned mitigation methods reduces the noise from one or more sources of the total noise created by a genset. For a quiet, effective installation, a combination of these methodologies will address the genset noise. Unless an engineer is a sound-attenuation specialist, he should coordinate the various attenuating components with the enclosure or genset manufacturer so that all of the elements work together to achieve the total attenuation level desired. A rule of thumb is to use the appropriate silencer and enclosure along with one set of vibration isolation pads and springs for installations that require up to 32 dB of attenuation. If the genset is mounted on a separate, on-grade foundation, the achieved attenuation level may even be higher, since vibration is not as much of an issue as it would be within a structure. Within a building with a relatively flexible structure, a 32 dB attenuation installation may be difficult to achieve without using an inertia pad.

For the higher attenuation levels within, or on the roof of, a building, the inertia base becomes mandatory. Constructing the inertia pad large enough to accommodate the sound-attenuating enclosure so it will seal to the inertia pad—not the supporting floor—provides optimum attenuation and the best sound isolation (see Figure 3). Sealing every tiny hole, crack, crevice, and any other opening within the enclosure or inertia base with nonhardening elastomeric caulking ensures that the various attenuating components will function as they were designed.

Controlling genset noise is easily accomplished with today’s technology. However, meticulous attention to detail in both the design and construction of the installation is required to ensure that the desired attenuation level is reached.

Ignoring one small design element or using one incorrect item during construction can result in attenuation of 10-15 dB below the intended design level. This can make the difference between a successful project and a complete disaster (see the sidebars titled “Example: Appropriate attenuation” and “Example: Inadequate attenuation”).

Ken Lovorn, PE, is president of Lovorn Engineering Associates, Pittsburgh, Pa., and a member of the Consulting-Specifying Engineer Editorial Advisory Board.


Understanding dB

Decibel, or dB, is the unit used for sound pressure measurements. To quantify sound pressure, or loudness, one should understand the math behind dB measurement and the relationship between sound pressures. Sound pressure level (SPL) or Lp is a function of the ratio of the measured sound pressure to a reference sound pressure using the formula:

                                                                Lp = 10 log10 (pm2/pref2)

What this means in real terms is that to increase the sound pressure level by a factor of two, one must increase the Lp by the base-ten logarithm of two, or 0.30103. Thus, an SPL of 40 dB is half as loud as an SPL of 43 dB—not 80 dB as one might think.


Example: Appropriate attenuation

Unlike the other components of genset noise, vibration typically is not quantified since its mitigation has many more variables than the airborne noise. All of the aforementioned vibration mitigation techniques have been field tested and confirmed over many projects.

In particular, the use of an inertia pad for very sensitive installation has been employed in a number of projects with stunning success. In a project that was very similar to the apartment building example, a 250 kW genset was installed in a large garage bay on the lower level of a high-rise apartment building. All of the sound and vibration attenuating techniques noted in this article were used to minimize the impact of the generator in this location.

During a full-load test of the unit, the impacts of the genset noise and vibration on the apartment located directly over the garage were reviewed with the building owner. With the windows open, only a very slight hiss was noted from the exhaust pipe, located about 10 ft from the window. No other noise or vibration emanating from the genset was discernable. It was as though there was no generator in the vicinity.


Example: Inadequate attenuation

The consequences of having inadequate sound attenuation can be quite dramatic. A 250-kW generator was installed in a first-floor mechanical room in one wing of a local high-rise apartment building that needed emergency power. The genset was solidly bolted to a concrete housekeeping pad, which was installed on a steel-framed, concrete floor with the only vibration isolation being the isolators that were a part of the generator package.

There was no sound-attenuating enclosure included in the installation, and only a residential-grade silencer was used. The first time the genset was started, the vibration was so great in the apartments on the upper levels that the test had to be terminated. Subsequently, all tenants of the upper floors in that wing had to be relocated and the apartments remain unoccupied today.