Ways to Reduce Generator Noise Level
One of the most overlooked issues in large motor driven equipment is noise level. Sound wave; It can be defined as any disturbance that propagates in an elastic medium such as a solid, liquid or gas. Noise is unwanted sound waves perceived by human hearing senses. A person who is exposed to excessive noise for a long time may suffer from hearing loss. Accordingly, excessive noise in an environment is a potential hazard.
Noise control involves three key factors: sound, path and receiver. Before creating a solution to a complex noise problem, the dominant source of noise pollution must be determined, the characteristics of the transmission path must be discovered, and the noise level must be reduced to the allowable level.
During generator assembly, many factors cause the actual Sound Power Levels (Sound Power Levels ‘SPL’: Total sound emitted from a source relative to a reference power) to deviate from the estimated sound levels. The noise present in the environment before the generator installation is called ambient noise. Ambient or background noise should be measured and calculated before installing the device. If all field conditions have not been fully studied, a safety tolerance should be applied to the calculated values. E.g; buildings, walls and signage are common factors that change the sound field. Obstacles within the sound transmission path will partially reflect, absorb or transmit the sound. It is important to study site conditions and know local noise laws before starting a generator project.
Sound waves should not only be considered in air, but also in solid and liquid environment. Sound in air is usually created by vibration in solid or turbulence in liquid. Sound waves in solid or liquid travel long distances before producing the sound that can be heard in air. An example of vibration noise is when train sound waves are heard over rails from long distance before air transmission. It is this type of sound transmission type that is difficult to isolate the generator sets acoustically. Without adequate vibration isolation at the generator base, the effect of vibration cannot be eliminated.
Ideally, the generators used are; should be mounted on isolators or on concrete with sound insulation enclosures surrounding the unit. Even minor leaks in the system can affect the overall sound level. In order to prevent the noise produced by the fluids in the generator piping system, sound containment structures should be used surrounding these pipes.
Commonly Used Sound Absorption Materials
When sound waves hit a hard surface, they are naturally reflected. Installing an absorption surface on hard surfaces reduces the amount of reflected sound.
The majority of sound absorption compounds contain materials of varying density, which convert sound energy into heat through their pores. When researching sound enclosure isolators, it is best to look for materials with air ducts into which the sound wave can propagate. If the pores are closed, the material is usually a poor absorbent. any pore; must not be covered by paint, coating or protective coating.
A few key points should be noted when beginning the material evaluation process. The main criterion in sound absorption measurement is the ability to absorb energy, which is defined as the absorption coefficient. Absorption coefficient; mathematically, it is defined as the ratio of sound energy waves absorbed by a surface to sound energy on the surface. The absorption coefficient can vary between 0 and 1. For example, if the coefficient = 0.8, 80% of the sound energy will be absorbed. Another way to examine sound coefficient levels is to look through an open door or window. This analysis can be as the ratio of the sound wave absorbed from the window opening (100% coefficient=1) to the reflected one into the room. The absorption coefficient is entirely frequency dependent, and is studied for either the octave or 1/3 octave bands. Porous sound insulators are most efficient at high frequency, and can also increase low frequency absorption when the thickness or mass of the material is improved.
When low frequency sound absorption is required, the solution is usually to use panel sound absorption materials. Thin, flexible panels should be mounted away from the wall, creating a shallow air gap between the two materials. The air gap between the panel and the wall is mostly conducive to low frequency sound absorption. Sound waves of the frequency of interest create a resonating effect that causes the panel to vibrate through the air gap. The sharpness of the setting can be reduced by simply filling the gap with a secondary porous material.
The traditional approach to sound reduction uses a sound-absorbing material sandwiched between the porous lining and the outer structure. Porous coating; It consists of different materials with evenly distributed small pores that effectively absorb sound at basic “tuning” frequencies. Medium and large pores are used for low, high frequencies; but its use is not common. The porous surface is mounted on the sound absorbing porous material. Depending on the thickness, the gap and the gap size increase the absorption of the whole structure at the main frequencies. Due to the reflection of solid surfaces, most high-frequency sounds are significantly attenuated using this system. A porous surface with a cavity surrounding at least 20% of the entire material will not be able to significantly reduce the absorption of high frequency sounds. Any value above 20% is effective in absorbing the entire sound.
Structures of Canopies of Generator Sets
Mechanical data and exhaust data must be determined before purchasing the canopy. This information is typically expressed in decibels at a predetermined distance. These data may include noise spectrum analysis. It is also important to include the radiator noise level.
It is important to know that for a given kW rating, dimensions, noise and airflow conditions vary for each manufadcturer. E.g; A 2013 production 800 kW generator will not produce the same noise as a 2004 800 kW generator. Each unit may contain slightly different. If a cabin is to be used to accommodate more than one generator set, whether it is a sound insulation cabin or a weather protection cabin, it is recommended to choose the worst case to meet each generator.
Law of Mass Fundamentals
Law of mass; refers to the conduction loss of particles in solid panels, and for a limited frequency range the magnitude of the loss is fully controlled by the mass per unit area of the wall. The basic principle of the law of mass states that when the frequency is doubled, the transmission loss will increase by 6 decibels. For example, a sheet metal surface has a transmission loss of 13 dB at 63 Hz, 19 dB at 125 Hz, and 25 dB at 250 Hz. When the thickness of the sheet is increased from 1/16 in. to 1/8 in., the transmission loss at 63 Hz becomes 13+6=19 dB. A composite layer is created by using the combination of light material and mass layers and the desired sound level is achieved.
All materials, whether man-made or natural, have a natural mode of vibration known as the resonant frequency. The resonant frequency formulation relies on many properties, including mass. Lightweight skid structures at the base of the generator can sometimes cause high noise levels due to the vibration of the base with engine frequency waves. Choosing the appropriate vibration isolator is very important when choosing a generator. Good insulators have a serious effect on damping the motor force frequency and isolating other parts of the structure.
Most machine parts and multi-pipe systems come standard with thermally insulating coatings to protect both operating personnel from burns and the machine from excessive heat loss. Large equipment such as generators or turbines require service connecting pipe, which can be a source of intense noise. It is possible to provide both acoustic and thermal insulation with a single composite structure towards the outside of the piping system or metal material. The most commonly used insulation material in generators is foam composites or chemical sprays with which the piping system is painted.
Mufflers / Noise Silencers
Silencers are the most effective way to control noise from the engine, fan and blower. Silencers are divided into 3 groups: reactive, absorptive and reactive/absorptive combination. Absorptive silencers are good at sound reduction at high frequencies, while reactive silencers are good at low frequency absorption. Eliminating a wide range of acoustic effects requires a system that includes both reactive and absorptive elements. Choosing a suitable muffler; It depends on different factors such as flow rate, noise spectrum, temperature, humidity.
The Most Common Canopy Groups
The most commonly used cabinets in generators are acoustic cabinets. Typical sound insulation generator cabinets are a multi-layer structure consisting of a porous sound absorption material facing the interior of the equipment alongside the waterproof layer. The main absorption layer is the waterproof layer that blocks the passage through which the sound energy from the generator is transmitted. The porous sound absorption layer disperses the trapped sound energy and provides heat-insulation properties. A typical soundproofing cabinet is maintained through hinged doors and air intake louvers.
Protective Cabins Against Normal Weather Conditions or Severe Weather Conditions
It should be determined whether a cabin is required for weather conditions in addition to the sound insulation cabins. The main point here is whether the customer needs a cabin for normal weather conditions or severe weather conditions. If the person only needs to protect the generator from weather conditions such as rain or snow, the solution is a protective cabin against normal weather conditions.
A full protective cabin is needed for extreme weather conditions. In extreme weather conditions such as strong wind, heavy rainfall, seismic activity or extreme temperature, a full protective cabin should be used against harsh weather conditions. Full protection; means that the alternator or engine is not damaged in weather conditions such as rain, snow, sleet or hail.
Generator Cabin Construction
Generator cabins; It is manufactured according to various preferences such as durability, sound insulation and cost. The most popular designs include:
Bolted – A simple generator cabinet; It consists of metal panels that are riveted or screwed together.
Welded – The surface of the skeleton welded with fabricated or structural metal elements is covered with a metal sheet by means of bolts or rivets.
Prefabricated Panels – The panels come pre-fabricated and are assembled to form the roof and sides of the cabinet. Suspended door assembly is utilized, wall and roof panels usually include thermal or acoustic insulation with metal surface in the generator inner lining.
Generator Cabin Materials
When acquiring a generator cabinet, it is important to consider the best materials for the short and long term. The balance factor between start-up costs, generator geolocation, and long-term considerations for maintenance is important. See the table below for the most commonly used cabinet materials:w
|-Low Initial Cost
-Conducive to Noise Reduction
|-medium Initial Cost
-more Durable Than painted Steel
|-occasional Bad Paint Adhesion
|Factory-Prepainted Coated Steel
|-Reasonable Initial Cost
|-Painted Surface Semi-Gloss
-Aesthetically Pleasing Surfaces
|-Higher Initial Cost Than Steel
-Additional Mass Requirement For Noise Control With Isolation
|“300” Stainless steel
|-Resistance to Harsh Chemical Environmental Conditions
-No Paint Required
|-Very High Initial Cost
|“400” Stainless steel
|-Same As Plated Steel
|-Muffler Level” Stainless Requires High Carbon Content Paint For Good Looks And Longevity.
|Powder Coated Steel
|-Basically The Same As Plated Steel
-More Color Options Than Prepaint.
-More Material Thickness Than Prepaint
|Powder Coated Aluminum
|-Basically The Same As Prepainted Aluminum, Being More Resistant To Corrosion.
Noise Reduction Recommendations
One of the key points to consider when purchasing a generator is how much noise reduction is needed. This decision is important when researching the generator as it will affect cabinet size, air options, and casing materials. It is useful to know that the sound pressure unit measurement (decibel dB) is logarithmic when performing a noise test. The basic principle can be related to the amount of sound reduction in a cabin. As the size, weight and air handling capacity of the generator increase, more sound insulation is required and the cost increases.
At the beginning of the generator project, it is important to create an accurate table according to the noise requirements instead of spending excessively on sound insulation. Most cities have guidelines for maximum noise levels in living areas, but a noise source that operates once a month during maintenance or power outages, such as a standby generator, obscures this. It is important to learn the maximum sound level laws before purchasing a generator cabinet.
Most diesel generators are used as standby, and many municipalities have easing restrictions for standby units. For generators used as prime or continuous power, these restrictions are more stringent due to long-term use. If it is desired to provide a certain sound level in the living area, the cabin manufacturer must be informed of the requirement and the distance of the generator set from the living area. In addition, the cabin producer must have knowledge of the plan of the surrounding buildings, auxiliary structures and topography. For example, a large structure near the generator, wooded area around the site, or hard surface parking will greatly affect the transmission of sound, so a cabinet design is required for the project.
A sound level meter (decibel meter) is a common device used to measure sound level. The sound level meter works through the microphone that measures the sound pressure and the electronic circuit that converts this pressure to SPL reading. A basic sound meter can calculate a sudden SPL, providing the possibility to read the values.
The real-time sound analyzer is a multi-purpose sound measuring device that can simultaneously measure various sound levels using multiple processors. Audio features can be observed without loss of data over the entire real-time spectrum of interest to the real-time audio analyzer. This analyzer can fulfill the function of multiple sound level meters by measuring all octave or 1/3 octave bands simultaneously instead of one octave band at a time. Other optional features of this analyzer include Fast Fourier Transform (FFT) measurements for discrete frequency analysis and sound intensity measurements using the sound intensity probe.
A caveat to potential customers: Since sound is a wave phenomenon, there is an inverse square law where the rule of thumb applies to determine the effects of distance on sound level. The inverse square law simply states that for a sound source in free space conditions, the sound level will decrease by 6 dB for every two times the distance from the source. E.g; if the sound level is measured at 100 dB(A) in 50 steps, 94 dB(A) is measured in 100 steps. It is also important to know that the term “free space” does not start until the generator is 30-50 steps away.
Noise Reduction Classification
If there is no specific decibel level to be reached at a given distance, it is common to specify the performance specification, which is achieved by determining the amount of reduction required in the cabinet itself. That’s why cabinet manufacturers standardize sound reduction amounts for certain distances. E.g; a manufacturer should be able to specify that a 25 dB(A) reduction in 10 steps is a 10 dB(A) reduction in 1 meter. These broad data usually represent average values measured at different points around the generator cabinet.
When examining generator cabinet designs, it should be noted whether the promised value is designed so that there is no noise point above 3-5 db(A). For example, if the radiator discharge air cannot be treated properly, the noise level around the cabin will be unacceptably high even if it meets the design criteria.