Generator sets, especially those installed in or near areas where people live or work, are often subject to specific noise control requirements to minimize impact to human health or quality of life. In the US, for example, The Occupational Safety and Health Administration’s (OSHA’s) Noise standard (29 CFR 1910.95) requires employers to have a hearing conservation program in place if workers are exposed to a time-weighted average (TWA) noise level of 85 decibels (dBA) or higher over an 8-hour work shift. 85 dBA is approximately as loud as the average lawnmower. Equipment installed in residential areas is often subject to more rigorous requirements, some as low as 70 dBA which is about as loud as the average dishwasher, or even lower.
Noise Control Policy
Most cities have ordinances regarding the maximum permissible sound levels in given locations. This is straightforward for base load systems, but it is sometimes unclear how a standby generator set, which runs one hour a month for maintenance or during the occasional power outage is defined as a noise source. It is best to check with with the local government to learn how the laws are perceived in a given jurisdiction before finalizing noise control strategies.
Noise Control Enforcement
Various authorities at the national, regional and municipal levels enforce noise control guidelines and limits. Though noise control has been typically regulated at the state and local level, a growing trend is that noise guidelines are becoming more frequently enforced under the US EPA’s Noise Control Act of 1972 and the Quiet Communities Act of 1978. Though the EPA’s funding to enforce these statutes was eliminated in 1982, the laws were never rescinded.
The chart below shows the effects of prolonged exposure for different noise levels.
A number of standards exist 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 sound level meter (decibel meter) is the most common instrument used in measuring noise sources. A sound level meter works by using a microphone to sense sound pressure, and electronic circuitry to convert sound pressure to an SPL reading. Other measurement options include real-time sound analyzers and sound intensity probes.
Genset Sound Attenuation Strategies
One of the most effective strategies for meeting a genset noise requirement is optimizing the site layout for noise. For example, if the requirement is be lower than 70 dBA at the property line, then sometimes the requirement can be met relatively easily if the genset is located in an area with natural obstructions. Buildings, trees and earth slopes can affect the amount of noise present at a particular measurement point.
In addition, common noise attenuation options on genset packages include acoustic enclosures. These enclosures are typically steel and incorporate sound panels into the walls, roof, and doors. The sound panels typically incorporate fiberglass or mineral wool insulation to increase insertion losses. Acoustic genset enclosures come in a variety of configurations, including “drop-on” enclosures that mount onto the genset skid to minimize footprint, and “drop-over” enclosures that mount directly to the pad. Though “drop-over” enclosures typically perform better from an acoustic perspective, the reduced footprint of a “drop-on” enclosure is sometimes a necessity. Acoustic skid cladding, incorporating insulation, can be added to the skid of “drop-on” enclosures to further reduce emitted noise.
Moreover, most genset packages include silencers on air intake and exhaust systems. These silencers work via one or a combination of 3 device types:
- Reactive silencers, often referred to as “chamber” silencers;
- Absorptive silencers, often referred to as “packed” silencers, and
- Resonators, which are sections of exhaust pipe that expand to a larger diameter and allow the sound waves to reflect off the walls and cancel out.
Sound Insulation Blankets & Curtains
Gensets and other industrial machinery are often equipped with sound blankets or curtains. In some cases these blankets are used in lieu of an enclosure, but they can also be used together. For example, custom genset sound blankets can be fitted to the outside of high-heat areas of the genset package such as the exhaust bellows, silencer and stack to further enhance noise absorption. They can also be fitted to the genset machinery itself and have been shown to reduce noise by up to 40 dBA depending on the frequency. These blankets are completely removable and reusable, easy to install and are capable of being used in high temperature applications.