Sources of generator set noise
An engine-generator is the combination of an electrical generator and an engine mounted together to form a single piece of equipment. This combination is also called an engine-generator set or a “Gen-Set.” In many contexts, the engine is taken for granted and the combined unit is simply called a generator.
Like any motor, a generator motor creates a lot of heat and needs a cooling system to prevent overheating. Standby generators can be either air-cooled or liquid-cooled. The major difference is that air-cooled systems are louder and not quite as effective. Liquid cooled systems are quieter and more dependable – and also more expensive to purchase and to maintain.
The decibel (often listed as dB) rating on a generator, is a number that is given that explains the noise level generated by the engine running. Some noisy generators can reach sound levels ranging from 80-100-plus decibels, which at 100 plus decibels translates to the noise of a jack hammer at 10 meters (32.8 feet); 110 decibels is the noise equivalent of a plane taking off at a 10 meters; 115 decibels translates to a jet’s screeching whistle at 10 meters, and is also the threshold of noise-induced pain. Sustained noise levels above this can cause hearing damage in short a short amount of time. Sustained noise at 100 dB can cause long term health problems. A higher quality and less noisy generator is going to be in the 70’s decibel rating while a really good generator will be in the 60’s decibels range.
According to a Cummings White Paper by Senior Acoustics Specialist Dennis Aaberg, generator set (residential and commercial generators) noise is produced by six major sources:
• 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.
Noise from “portable” generators comes primarily from two sources, the engine block and the exhaust system. With an air cooled engine there is little you can do about blocking noise. Some engines can have a larger muffler attached or make other changes to the exhaust system. Merely mounting exhaust pipe vertically will noticeably reduce noise.
Reducing Noise “Inside” the Generator Enclosure
With the growth of standby, prime and peaking power installations in densely populated areas, it’s become important to focus attention on understanding how generator noise is propagated and controlled.
Typically, there are two main methods for controlling the airborne noise in a power generator:
• Blocking airborne noise via a weighted barrier
• Absorbing airborne noise via acoustical absorbing insulation.
Aaberg’s white paper expands on this claim. It recommends that standby generator enclosures incorporate the following types of materials for best results and often with cost and performance improvements as compared to more traditional solutions:
• Flexible non-lead barriers (like Acoustiblok) in weights ranging from 1/4″ lbs./ft2 to 2 lbs./ft2
• Faced acoustical foams in thicknesses ranging from 1/4″ to 2″ and with numerous facings, including reflective and reinforced facings
• Decoupled barrier composites and barrier/absorber composites in a range of barrier weights, decoupler thicknesses and absorption layer thicknesses faced with numerous films
• Damping composites comprising a damping layer to manage structure borne vibration
• Combined with acoustical foam layers to absorb airborne noise.
Significant noise control can be achieved by lining the generator’s sheet metal enclosure with a weighted barrier, or a decoupled weighted barrier (composite of barrier over decoupling foam) to help block noise. Ideally, at least 90 percent of the enclosure should be lined. For optimal effect the enclosure openings must be minimized.
Absorption reduces airborne noise due to mechanical sound energy by converting it into low grade heat energy. As air is pushed into the absorbing material by the sound pressure wave, viscous forces dissipate the mechanical sound energy as heat.
Most power generation equipment requires several openings in the metal enclosure – for air intake, exhaust and heat release. These openings are generally detrimental to the performance of barriers and decoupled barriers as they can allow noise to escape unhindered. By incorporating acoustical absorbers as a lining for louvers or by creating a path for airflow, noise can be absorbed before it escapes the enclosure.
In recent years, a wide range of elastomer innovations, like Acoustiblok, have been created that can be utilized in designing next generation Gen-Set enclosures. These materials must meet multiple design objectives including noise frequencies, operating temperature range and operating environment including thermal management, contaminant resistance and maintenance considerations.
Reducing Noise “Outside” the Generator
In a residential setting, the simplest type of noise control for generators or other gas or propane-powered engines is a noise barrier placed around it. Outdoor sound curtains or sound curtain noise barrier walls are an effective method of reducing noise generated by equipment, pumps, generators or other processes that are outside and are exposed to the elements.
It is important to take into account the source height of the generator, which can be fairly high, and to consider the height of the receiver if the impacted site has multiple floors. To be effective, a barrier wall must at least block the line-of-sight from the source to the receiver.
Sound barriers will make a substantial difference in the noise exposure levels from generators if the proper materials are used. They can reduce the sound by about 12-15 decibels, which is significant because a 10 decibel decrease in sound results in half the sound heard to the human ear. The more distance between the generator and your house or building, the better. Not all materials will work. It is important to do your research.
There are other soundproofing product solutions such as sound panels. Some soundproofing companies, like Acoustiblok, Inc. manufacture all-weather sound panels that are different from other conventional acoustical sound panels. In addition to being able to stand up to the most extreme environments, they not only absorb virtually all sound but they also contain a layer of noise absorbing viscoelastic material which provides exceptional noise blockage.
The sound absorbency of the panels brings down substantially the acoustical energy around the generator and by eliminating all sound reflections in the area, while the viscoelastic material in the panel allows it to also be an excellent sound barrier.
With increasing focus on noise in our communities, it’s important to take a proactive approach to solving your generator noise issues by using lab tested and proven acoustical soundproofing materials.