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Sound Attenuation Basics

Sound.jpgWhen dealing with large engine driven equipment on a job site one of the most overlooked issues is sound level. In the truest sense of the word a sound wave can be defined as any disturbance that is propagated in an elastic medium, which may be a solid, liquid or gas. Noise can be defined as any unwanted sound perceived by the hearing sense of human beings. Worker exposure to excessive or repetitive noise over a long period of time can result in hearing loss. The creation of an excessive source of noise into an environment can be potentially hazardous, as well as unpleasant to nearby commercial tenants and residents. Excessive noise control measures have been enacted at the national and local levels to maintain both safety and peace of mind. Typically, regulations are bundled together based upon the land use characteristics and the proximity to residential or other delicate areas.

Noise control containment involves a system composing of three basic fundamentals: sound, path and receiver. Before a solution to a complex noise problem can be designed, the dominant source of noise pollution must be known, the characteristics of the transmission path must be discovered and an acceptable level of noise permitted must be established.

During the installation of an electrical generating system, many field factors can add to deviations of actual Sound Power Levels (SPL: Total sound radiated from a source with respect to a reference power of Watts) versus predicted levels of sound. The noise that is present in the natural environment of the genset prior to installation is referred to as ambient noise. The ambient, or background noise, should be measured and calculated before the installation of equipment. Therefore, a margin of safety should be applied to calculated values if all field conditions are not fully studied. For example, buildings, walls, signs and auxiliary equipment commonly change the sound field. Obstacles within the sound path will partially reflect, absorb and transmit sound. It is important to study field conditions and know the local decibel laws before embarking on an electrical generation project.

Sound waves need to be not only accounted for in the air but in solid and liquid forms as well. Airborne sound is typically created by the vibration in solids or turbulence in fluids. It is important to note that sound waves in solids and liquids can travel great distances before producing audible sound in the air. An example of vibrational noise would be the ability to hear a train through the rails at a long distance prior to the airborne transmission of sound waves. It is this type of sound transmission that often times makes it difficult to acoustically isolate generator sets. Without adequate vibration isolation of the skid base on a generator, vibrations will travel unopposed through the skid, so that much of the noise is not subjected to acoustic dampening systems designed into the sound attenuated enclosure.

Ideally, used generators should be mounted on isolators, or on a concrete pad with the sound attenuated enclosure completely surrounding the base of the unit. Even small leakages in the system can contribute greatly to overall sound levels. Gaskets should be considered to prevent any noise leaks though gaps in an uneven concrete surface or around enclosure obtrusions. Fluid flowing through pipes can also produce radiated sound that can be transmitted through a building or enclosed structure. Sound attenuation can be applied through ridged couplings for pipes and electrical connectors. Where needed, connections should be flexible or isolated enough to prevent transmission or vibration to the sound attenuated enclosure walls.


Sound2.jpgCommonly Used Sound Absorption Materials

When sound waves are produced they are naturally reflected when they hit a hard surface. Installing an absorption surface over hard surfaces in most generators can reduce some of the reflected sound. In a room with hard surfaces, soft materials such as absorbent ceiling panels, floor rugs or carpeting, and blinds or special absorbent wall coverings, will reduce noise by reflecting sound. Only reflected sound can be muffled as described, while directed sound will not be directly affected in anyway.

The vast majority of sound absorption composites are comprised of porous materials of varying density which convert sound energy into heat within the open pores of the material. When researching sound muffling insulators it is best to look for materials with air channels that are open to the surface so that sound waves can propagate into the material. If pores are sealed, as in closed cell foam, the material is generally a poor absorber. Any pores should not be sealed by paint, coverings or any protective coverings. Structural integrity shielding for sound absorption material should always be perforated if applied.

When beginning the material evaluation process prior to a project a few key factors must be considered. The main metric when measuring for sound absorption boils down to a material’s ability to absorb energy defined as the absorption coefficient. The absorption coefficient is mathematically defined as the ratio of sound energy waves absorbed by a given surface in relation to the sound energy incident upon the surface. Absorption coefficient can vary between 0 and 1. For example, a = 0.8 then 80% of the sound energy will be absorbed. Another way to view sound coefficient levels is by looking at an open door or window. Sound waves are absorbed through the opening of the window 100% a = 1 versus being reflected back into the room. The absorption coefficient is wholly dependent on frequency, and is usually printed for either an octave or 1/3 octave bands. Porous engineered sound absorbers are most efficient at higher frequencies, while improving the materials thickness, or mass, can increase low frequency absorption.

When low frequency sound absorption is needed, panel sound absorption materials often are the solution. Thin, flexible panels are mounted away from the wall, creating a shallow air cavity between the material of the two. This air pocket between the panel and wall creates a means for sound absorption at commonly tuned low frequencies. Sound waves, at the frequency of interest, produce a resonating effect within the air pocket which causes the panel to vibrate. By simply filling the cavity with a secondary porous material one can reduce the sharpness of the tuning. This type of sound attenuation solution can be inhibitive and is usually used to treat a specific tone or narrow band from the offending source of sound.

Sound Attenuation vs. dB Level Graph


The traditional approach to sound attenuation utilizes a sound absorbing material sandwiched between a perforated lining and the external structure. The perforated lining typically consists of various patterns with small evenly spaced holes that can effectively absorb sound at common “tuned” frequencies. Medium and larger perforations are used for lower pitched frequencies but are not as commonly used. The perforated facing is constructed on the top of the porous sound absorbing material. Depending on the thickness, spacing and hole size the facing can also increase absorption to the overall structure at certain frequencies. Most high frequency sounds are reduced significantly using this system because of reflections from the solid areas of the facing. A perforated facing design where the open cavity is at least 20% of the overall material will not significantly degrade the absorption of high frequency sound over the typical range. Anything over 20% will have an impact on the overall absorption of sound.


Sound Attenuated Generator Set Structures

If one of the enclosure’s requirements is sound attenuation then mechanical and combustion air exhaust data must be ascertained prior to making a purchasing decision. This information is typically available from the manufacturer expressed in decibels at a predetermined distance from a noise source. That data might also include a full noise spectrum analysis, which any qualified enclosure manufacturer can interpret for the customer. It is important to also include the noise rating of the radiator from the manufacturer; this includes remote radiators as well.

It is important to note that dimensions, noise and airflow requirements can vary greatly from manufacturer to manufacturer for a given kW rating. For example, a 2013 800 kW generator will not produce the exact same noise pattern as a 2004 800 kW generator. Each unit is a little bit different. When sizing multiple enclosures for more than one generator set, whether it is a sound attenuated or weatherproof enclosure, it is advisable to make a decision based upon the worst case data so that the sound attenuated enclosures will work with all generator sets considered for the project.

Basics of Mass Law

Mass law refers to the transmission loss of particles in solid panels, and states that within a limited frequency range, the magnitude of the loss is controlled entirely by the mass per unit area of the wall. The mass law’s basic principal states that transmission loss increases 6 decibels for each doubling of frequency, or each doubling of the wall mass per area, up to a plateau for a given frequency. For example, a lead sheet has a transmission loss of 13 dB at 63 Hz, 19 dB at 125 Hz, 25 dB at 250 Hz etc… If you were to double the thickness of the lead sheet 1/16 in. to 1/8 in. thick, the residual transmission loss at 63 Hz becomes 13 + 6, or 19 dB. The combination of lightweight materials and mass layers are typically used in combination of one another to provide a composite layer that is adequate to achieve the desired sound attenuation required.

Graph Showing Mass Law


Noise Resonance

All materials, whether man made or natural, have a natural mode of vibration known as resonance frequency. Resonance frequency formulation is based upon many characteristics, including mass. Lightweight skid mounting structures under generators can sometimes result in higher overall noise levels due to vibration of the base by the forcing frequency waves of the engine, thereby resulting in amplification of the sound pressure level at that given frequency. Resonance can be occasionally seen in paneled buildings, or base structures where a “drumming” affect is dominant and will actually amplify the sound source. When selecting a generator it is important to take into consideration the proper vibration isolators. Good isolators, installed on generators, become very critical in dampening the forcing frequency of the engine and isolating it from the rest of the structure.

Coverings

Most machine components and interconnecting piping systems come standard with thermal insulating wrappings to both provide protection to operating personnel from burns as well as prevent excessive heat loss. Large equipment like generators or turbines typically requires connecting service piping, which can also be the source of intense noise. In most cases, it is frequently possible to obtain both acoustical and thermal insulation through a single composite treatment to the exterior of the pipe or metal component. The most commonly used insulation wrapping materials on generators are foam composites or a chemical spray that is painted over the pipe.

Mufflers/Noise Silencers

The use of noise silencers is a necessary way to control noise coming from engine combustion, fans and blowers. Noise silencers are normally divided into 3 types: reactive, absorptive and a combination of reactive/absorptive. Reactive silencers work best at absorbing lower level frequencies while absorptive silencers achieve much greater sound attenuation at higher frequencies. A more effective and overall encompassing design to meet a wide variety of acoustical challenges would include a system of both reactive and absorption elements. The proper selection of a silencer depends on a number of different factors including flow rate, noise spectrum, temperature, humidity, allowable backpressure etc…


Sound3.jpgMost Common Sound Attenuated Enclosures

The most common device used to muffle noise from generators is acoustical enclosures. Typical sound attenuated generator enclosures consist of panels that are multi-layered composite treatments comprising of an impervious exterior layer as well as a layer of porous sound absorption material facing towards the inside of the equipment. The main absorption layer is impervious which blocks the passage of sound energy radiated by the enclosed source of sound from the generator. The porous sound absorbing lining will dissipate the retained sound energy, and also provide heat-insulating properties. In a typical skin-tight sound attenuated enclosure, maintenance is performed from outside hinged doors and the air intake louvers.

Weather-Proof vs. Weather -Protective Enclosures

In addition to sound attenuated enclosures one must determine if weatherproof enclosures are needed as well. The main decision boils down to whether or not the customer needs a weather-protective or weather-proof enclosure. When the customer only needs to keep rain and or snow off of the generator set under normal anticipated weather conditions, the solution is a simple weather-protective enclosure. Options include a weather-protective enclosure that is typically skintight usually supplied by the generator set manufacturer or a drip-proof weather resistant enclosure from a specific generator set enclosure manufacturer.

Under more extreme circumstances a full weather-proof enclosure might be needed. Weather-proof enclosure should be used when the generator set needs to withstand extreme conditions such as wind, precipitation, seismic activity or temperature. Weatherproof dictates that the entrance of rain, snow, sleet or hail shall not damage the alternator or engine inside the enclosure. Parameters such as effective wind loading (in miles per hour) or roof loading (in pounds per square foot) in snow or ice-prone regions and rain penetration resistance (in ounces of water per square foot of inlet opening per hour) can be used compare potential manufactures and ensure a reliable system of protection.

A truly weatherproof enclosure for a generator will withstand hurricane force winds up to 150 mph and a substantial amount of snow (greater than 30 lbs/ft2 without permanent deformation of the structure. As always it is important to note that truly the term “weatherproof” is subjective from manufacturer to manufacturer. UL 2200 deals specifically with genset codes and standards. UL2200 defines to levels of protection for the installed genset. The first definition is in regards to the term “Rainproof” which is defined as allowing neither wetting of a live part nor entrance of water above the lowest live part. “Raintight” is defined as no entrance of water into the enclosure whatsoever. These UL2200 updates replace more loosely based terms such as weatherproof, drip-proof and weather resistant. It is always best to research specific specifications prior to purchasing.

Generator Enclosure Construction

Generator Enclosures are manufactured with various preferences to strength, sound attenuation and cost in mind. The most popular construction designs include:

Bolted – A basic generator enclosure which is constructed with metal panels that are bolted, riveted or screwed together to form a finished enclosure. The overall depth of the section of the formed panel combined with the material thickness serves as the structural elements of the enclosure.

Welded – The basic welded framework of fabricated or structural metal elements is overlaid with sheet metal attached with welds, bolts rivets or screws.

Pre-fabricated Panels – Panels come pre-manufactured and are joined together to form the roof and side panels of the enclosure. Pre-hung door assemblies are utilized and the wall and roof panels generally include thermal or acoustic insulation with sheet metal interior lining on the genset.

Stressed Skin / Semi-Monocoque – This construction method is a combination of extrude and/or fabricated shapes are attached together forming a basic structure, with is then integrated with an external skin which is then finally attached to the structure with hard rivets. The end result is a lightweight, durable generator enclosure where both the skin and the structural lining/skeleton combine to become load-bearing elements.


Generator Enclosure Materials

When considering a new generator enclosure for a genset it important to take into consideration the best materials for both short term and long-term usage. It is critical to factor in the balance between initial costs and long-term considerations associated with maintenance and the geographical location of the generator. See the chart below for most commonly used enclosure materials:

Generator Enclosure Materials


Sound Attenuation Recommendations

One of the key decisions when considering the purchase of a generator is whether or how to much sound attenuation is required. It is important to note that this decision should be made early on in the research stage as it will often dictate the enclosure size, air handling options, and even the materials of choice for construction. It is useful to know when doing sound requirement testing that the fundamental unit of sound pressure measurement (the decibel dB) is a logarithmic ratio. This basic principal can be correlated to the amount of sound attenuation in an enclosure. The larger the relative size, weight and air handling complexity, as more sound attenuation is required, the more the overall cost goes up.

In lieu of overspending too much on sound attenuation it is important to chart the true site noise requirements from the start of the generator project. Most cities have ordinances regarding the maximum permissible sound levels at the property line, 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 town prior to making any generator enclosure purchasing decision.

Most generator applications are standby rated and surprisingly many municipalities have laxed restrictions for strictly standby units. Most laws are tougher when dealing with prime or continuous power or cogeneration applications because of their extended use of operation. If a particular noise level at the property line must be achieved, then the sound attenuated enclosure manufacturer should be told beforehand what the requirement is and how far the enclosure will be from the property line. Additionally, the generator enclosure manufacturer should be made aware of the layout of the surrounding buildings, equipment, ancillary structures and the topography. For example, a large structure nearby the genset, a grass-covered berm or heavy foliage around the site, or a hard surfaced parking lot can greatly influence the propagation of sound and therefore, the enclosure design needed for the project.

Noise guidelines are often enforced through the Environmental Protection Act (EPA). As stated above various authorities at the national, regional and municipal levels publish noise control guidelines and limits. A wide variation of standards exists 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. A basic sound meter can calculate an instantaneous SPL, give a weighting only reading, and unusually has an arbitrary time constant (the rate the meter responds to sound). Optional features on more expensive meters include different weighting networks (A, B, C, D or linear), different time constraints (fast 1/8 second to slow 1 second, impulse control fast-rise, slow decay) integrating capability with average sound levels over a given time, statistical/histogram results presentation, logging or memory features, and octave band filtering (which can process the sound in one band at a time.)

A real-time sound analyzer is an all-purpose sound measuring device that uses multiple processors to measure various sound levels at the same time. By using a real-time sound analyzer the user can observe sound properties over the entire spectrum of interest in real time, without loss of any data. A real-time analyzer can perform the work of many sound level meters by simultaneously measuring all octave or 1/3 octave bands instead of just one octave band at one time. Other options features of a real-time sound analyzer can include Fast Fourier Transform (FFT) measurements for discrete frequency analysis, and sound intensity measurements using an sound intensity probe. A sound intensity probe essentially acts as a device with two microphones seprated by a spacer and estimates the instantaneous sound intensity by simultaneously measuring pressures at both microphones. It is generally a good idea to conduct a sound intensity analysis because it determines the contributions of individual source elements to an overall sound level in a multi-source environmenDecible Metert.

A word of caution for potential customers: Because sound is a wave phenomenon, there is a mathematical rule, the inverse square law, which is often applied as a rule-of-thumb to determine the effects of distance on sound level. Without exploring all of the underlying theory, the inverse square law simply states that for a point source of sound under free field conditions that the sound level will decrease by 6 dB each time the distance from the source is doubled. For example if one measured 100 dB (A) at 50 feet, then we would measure 94 dB (A) if we moved to 100 feet. It is also important to note that the term “free field” does not begin until 30-50 feet away from the generator.


Classifying Sound Attenuation

Decibel MeterWhen there is no specific decibel level to be met at a given distance then it is common to provide a performance specification; that is to specify only the amount of attenuation required by the enclosure itself. Most manufacturers of generator enclosures, therefore, standardize on certain levels of sound attenuation at some predetermined distance. For example, a manufacture might specify that 25 dB (A) reduction is obtainable at 10 feet, a 10 dB(A) reduction at 1 meter for so. These broad data opinions usually represent an average rating for the enclosure measured at several different points around the generator enclosure.

When looking at generator enclosure designs it is best to insist the enclosure structure is engineered in such a way that there are no points at which noise measured is more than 3-5 db(A) higher than the promised average. For example, if the radiator discharged air is not treated properly, the resultant sound level measured adjacent to the enclosure might be unacceptably high, even though the average attenuation meets the design criteria.

Any amount of sound attenuation applied to a generator set will alter the noise produced by the engine and yield a more tolerable deadened sound characterized by a rush of air rather than a mechanical or fan noise of a high-speed diesel or gas engine. By defining a performance specification it is then up to the materials and methodology produced by the generator enclosure manufacturer.


Sound Attenuation Costs

The most overlooked aspect of choosing a sound attenuation enclosure system is that the as the amount of sound attenuation increases, ie the quitter the unit becomes, the larger the generator enclosure will be. The effect is most directly correlated with kW rating and air flow needed for the generator.

Once the sound attenuation approaches 40 dB(A) of reduction (40 dB(A) is considered the maximum economically viable reduction by a standard prefabricated enclosure) it is not unusual to see more enclosure space devoted to air handing as to the muffling of the engine itself.

It is extremely important to work closely with the generator enclosure manufacturer to determine what is the correct level of attenuation available given site requirements, how much room will be required to achieve a particular db(A) level and of course the customer’s budget.


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