A Wall of Sound

Noise abatement and enclosure technology

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Credit: iStock/allanswart
For those of us who are old enough to remember the early days of rock-and-roll, there was an audio technique and music production formula used to make the big hits of the era—the “wall of sound.” The idea was to channel the various frequencies and volumes of the various voice and instrument tracks to make for a more impressive musical experience. Something similar is being done, but in the opposite direction, by the use of sound abatement technology, covers, insulation, and enclosure structures.

This is one of the most important, yet frequently overlooked, requirements of engine equipment and generator operations. By definition, sound can travel through any elastic medium (solid, liquid, or gas), but for most applications, the operator is concerned with noise propagation through air. However, sound waves in air are generated by the vibration, impact of movement of a solid object or a liquid surface. Noise is any unwanted sound perceived by a worker hearing it (so no, if a tree falls in a forest and there is nobody to hear it, it really does not make a noise—though it does make a sound). Excessive or chronically high noise levels can damage worker hearing in the long run, while creating an unsafe work environment. Simply put, noise is a form of pollution. OSHA and other safety regulations, industry standards, and laboratory testing have established allowable noise thresholds in the workplace.

The control of noise pollution relies on three physical factors: sound, path, and receiver. The source of the noise can be redesigned to minimize noise production at the source. This is often done by mounting the generator on shock-absorbing and vibration-minimizing isolators. Paths between the noise source and the receiver can be lengthened by distance, routed through the tubes and chambers of a silencer, or blocked by insulated enclosures. The allowable noise levels for the receiver can be achieved by these methods. Personal protective equipment (PPE) for ears such as disposable ear plugs, ear muffs, and sound attenuating headsets can be used to minimize noise impact further. Sound attenuation seeks to minimize the impact of noise by artificially lengthening the path from the generator to the ear, or by absorbing and trapping the sound waves created by the operation of a power source.

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Given that sound can travel far distances and have a great deal of power behind it, sound attenuation presents a unique engineering design challenge—especially in regard to electrical power generation applications. Generator noise results from the fact that no physical operation is 100% efficient due to the three laws of thermodynamics. Combustion of the fuel driving the generator is never complete and instead of being completely applied to the creation of mechanical movement, the combustion results in wasted energy in the form of heat. The result is friction, vibration, and noise.

The impact of sound from an electrical power generator is measured by its Sound Pressure Levels (SPL), or the total sound radiated from a source with respect to a reference power of watts produced by the electrical generator or portable genset, which can differ from predicted levels of sound due to the addition of ambient background noise. So, the proposed sound attenuation system should include a built-in factor of safety to account for unknown noise sources in the field.

Sound and Noise Standards
How much noise is allowable? What is the maximum amount of sound allowable in the workplace environment? In addition to OSHA workplace standards, allowable noise levels are often governed by local codes. Violation of a local noise ordinance can result in lawsuits, fines, and expensive legal fees. And it is not just in big cities and noisy factory floors where noise limits are in force. US parks and even wilderness areas have strict noise limits imposed on generators and vehicle operations.

So, the first task is to measure the amount of noise being generated so that it can be compared to the noise limits enforced by law and by safety standards. The standard scale for measuring noise is the decibel rating (dBA). The decibel number is a logarithmic scale with each increase of 10 decibels increasing the noise level by a factor of 10 (10 decibels is therefore 10 times more than 1 decibel, and 20 decibels is 100 times more than 1, and 30 decibels is 1,000 times more than 1, etc.). Furthermore, a doubling of the noise level/sound power is represented by each increase of 3 decibels (for example, 80 dBA + 80 dBA = 83 dBA). Examples of the decibel rating of typical sounds are shown in the Sound and Noise Comparison Chart.

Power and decibel output are directly correlated. Smaller-scale, 1,000-watt generators operate at around 45 decibels. A larger 10,000-watt generator can produce 75 decibels. To go from 45 decibels to 65 decibels, a generator’s power increases 5,000 watts from 1,000 watts to 6,000 watts. And another increase from 65 decibels to 75 decibels requires a power increase of another 4,000 watts of power. Power is not the only factor affecting decibel levels. Energy and sound are measured on an “A-weighted” scale. This measurement takes into account the elevation above sea level where the sound occurs. As elevation increases, air thickness and pressure decrease, affecting the resulting decibel level.

Putting distance between the source of the noise and the hearer greatly reduces the impact of noise due to the inverse square law. This law states that the power of any energy wave (light, sound, etc.) dissipates over a distance at a rate of the inverse of the square of the distance from the source. So, a hearer located eight times the distance from a noise source than a closer receiver will experience a sixty-fourth of the energy of the sound from the same source.

Yet, the object of sound attenuation is the protection of human hearing. What matters at the end of the day is the subjective response of a human ear to the impact of chronic or repeated sharp noises. At high-pitched noise levels (designated as at least 2,000 Hz), a listener can experience pain and eventually hearing loss after prolonged exposure. All sound attenuation efforts are aimed at reducing sound energy and frequency below these danger levels.

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Credit: Shannon Enterprises
A sound-dampening insulated enclosure

Generator and Turbine Noise
As mentioned above, no engine or generator is 100% efficient. Some of the energy created by the combustion of the fuel powering the engine or generator always becomes waste heat and vibration instead of useful work. It can often be difficult to pin down the exact source of the noise. But in most cases the direct source of noise escaping the engine or generator is the exhaust vent or the vibrating body of the machine itself. Both sources of noise emission typically originate in the instability of the combustion chambers.

There are multiple causes of high-frequency noise generated by various types of power plants (cogeneration gas turbines, simple cycle turbines, gensets, etc.). These include acoustic resonance (creation of a standing sound wave which can occur when the shape, length, and diameter of the exhaust vent match the tone created by the generator and resulting in its amplification), vibration resonance (similar to acoustic resonance, the noise is the result of structural vibration), turbulence and turbulence shedding (swirling eddies of gas escaping through the exhaust vent or tube bundles), vortex shedding (oscillating flow or air passing through a tube bundle—often a coolant radiator system), and fluid elastic instability (high speed exhaust streams becoming unstable). Low-frequency noise (less than 100 Hz), on the other hand, typically results from instability within the combustion chamber. Broadband noise across the frequency spectrum is typically the cause of turbulence.

Enclosure Design and Function for Best Results
So now that the source of the noise has been identified, what are the best methods of dealing with it? The most basic solution is to put distance between the hearer and the source of the noise. While simple and inexpensive, this approach is seldom practical given the space constraints at a typical job site, facility, or factory floor. In dense urban locations, this is not a possible choice given that there will always be someone within harmful hearing distance of the noise source even if they are not directly working with it.

So, the first attenuation option is to completely enclose the genset or engine causing the noise. The enclosure is designed to completely muffle the sound created by its operation. The interior surfaces of the enclosures are usually lined with sound-absorbing materials. The efficiency and effectiveness of this system is a result of several factors: the impact angle of the incident sound wave, the overall roughness of the absorbing surface, the geometry and shapes of the individual indentations, porosity of the absorbing surface, and the softness or hardness of the absorbing materials.

Many genset and engine suppliers or manufactures often include an enclosure with the delivery of their power generator. Third-party manufacturers who provide custom-built enclosure designs are also a significant source for enclosure structures. Enclosures can simply protect the genset from adverse weather conditions or provide complete sound attenuation.

Generator enclosures are constructed with various methods depending on customer preferences, cost considerations, physical strength and durability, and effective sound attenuation. Bolted construction utilizes bolts, screws, or rivets to assemble the structural supports and panels that make up the enclosure. Welded enclosures are held together by structural elements that are welded together to form the enclosure’s framework and overlaid by panels. Prefabricated enclosures are assembled from side and roof panels designed to attach directly to each other using joint assemblies. The more advanced stressed-skin technique utilizes extruded shapes joined together, which is then integrated with a shaped skin to form a tight-fitting structure. And it is only a matter of time before someone applies the method of 3D printing to create custom-fitting enclosures.

Credit: Shannon Enterprises
A custom-made, sound-attenuating enclosure

Other Sound Reduction Methods and Advances in Sound Attenuation Technology
A method known to every car owner, mufflers and noise silencers affixed to the car’s exhaust pipe can also be used for genset exhaust vents. These are relatively complicated and compact structures that utilize either a reactive, absorptive, or combination method to reduce sound escaping with the exhausts. Reactive mufflers or silencers reduce low-frequency sounds, while the absorptive version manages high-frequency sounds. A combination system handles a broad range of frequencies. The design and operation of these silencers depends not just on the anticipated frequency but also on flow rate, humidity and temperature of the exhaust, and resultant back pressure.

Advances in material science have led to advances in noise attenuation. These advances have been largely in the field of absorptive materials. Absorption is a sound reduction method that utilizes materials with soft, pitted, or rough surfaces that trap sound waves. This is the opposite of hard and smooth surfaces that naturally reflect sound waves without much loss of energy in the form of heat and vibration of the reflective surface. By contrast, sound-absorptive materials have porous surfaces that trap sound waves within the recessed pores, converting it into heat as the waves bounce back and forth within the pores.

A preferred place to position this material is as lining on the inside walls of duct work and piping that handles airflow inlet and airflow outlet. Sound absorbing materials can be installed in the form of heavy floor rugs and carpets, absorbent ceiling tiles and panels, specially designed window blinds and curtains, and coverings panels that line the surfaces of all or part of a room’s walls. All of these materials have one thing in common—a surface with open pores that can trap sound waves instead of reflecting them. Even when structurally reinforced, the structural integrity shielding needs to be porous as well.

A sound absorption material’s ability to reduce sound is measured by its absorption coefficient. This is defined as the ratio of sound wave energy absorbed into the material’s surface compared to the energy of the initial sound wave generated by the power source. The value of the absorptive coefficient ranges from 0 to 1. A highly absorptive material with an absorptive coefficient that is rated 0.8 would absorb 90% of the incoming sound. By contrast, a highly reflective surface would have an absorptive coefficient as low as 0.05, absorbing only 5% of incident sound (note that no material or surface configuration is ever perfectly reflective or perfectly absorptive). Absorptive materials (and their resultant absorption coefficients) are designed for particular frequencies, usually at octave intervals or 1/3 octave bands. There is an inverse relationship between frequency and wavelength. The greater the sound energy, the higher the frequency, and the higher the frequency, the shorter the wavelength. Shorter wavelengths are easier to trap in small pores and so sound-absorptive materials are most efficient at higher frequencies.

Recent developments in sound absorption design have resulted in a radically different approach to dealing with low-frequency sounds (ranging from 0 to 100 Hz) which propagate on ultra-long wavelengths. This development involves the use of flexible wall panels mounted with a thin space of air between the panel and the wall; in effect, these panels act as a diaphragmatic absorber. Acting like the surface of a drum, the vibrating panels translate sound energy to the air space behind it. This allows for the absorption of low-frequency sounds by creating a sound resonating effect in the air gap. A variation of this design is the filling of the air gap with porous material to further reduce the sharpness of the tuning.

In addition to absorption, there is the method of redirection and reflection. In situations where sound cannot be effectively absorbed, it can be repeatedly reflected, resulting in diffusion of the sound away from receivers. The goal is to do this for enough cycles before the air flow that propagates the sound wave can leave through the generator exhaust system. One example of this approach is a recently developed meta-material made of a series of hollow columns. The material can be designed with variable widths of the channels between the columns (from wide open to almost closed off) and the sizes and dimensions of the hollow cavities within the columns.

Then there are active measures such as noise cancellation technology (a.k.a. active noise reduction). This approach is an example of “fighting fire with fire.” Sound waves of equal but opposite amplitudes moving in the opposite direction from each other can cancel each other out, resulting in a flattened sound wave signature. Advanced electronics, combined with counter sound generators, can be used to great effect, but tend to be more costly than other methods. As a result, this approach is often used at the large scale in extreme situations such as deadening the noise from a commercial jet engine. At a smaller scale, individual headsets can employ this technology effectively.

Credit: Shannon Enterprises
Blanket insulation for sound mitigation

Major Suppliers
Pritchard Brown is an industry leader in the art of sound attenuation technologies for standby power systems. They can analyze, design, and deliver custom engineered packaging solutions for the most critical applications. Their staff is experienced in the factors that affect sound attenuation such as acoustics, fluid dynamics, and site constraints. This knowledge allows for the in-house engineering and manufacturing of acoustic air handling devices and composite wall panels. They offer standard attenuation packages through 40 dBA, and are the only enclosure manufacturer to have achieved a 55 dBA reduction on a 1.8-MW diesel generator set. Their enclosure designs maintain a weatherproof environment and allow proper cooling and combustion air management. Their goal is the creation of customized total system solutions based on site requirements and resultant noise level requirements.

Girtz Industries manufactures the Girtz Z-GUARD product line that utilizes innovative post and panel design (which is much stronger than standard formed-panel enclosures, making this an appropriate application for extreme weather conditions and seismic zones) for stationary sound attenuated enclosures. The Z-GUARD provides several unique features such as a completely removable, fully welded roof to prevent water intrusion. The modular design allows for the placement of two or more enclosures side by side creating one large space when the adjacent interior wall is removed. This allows a scalable enclosure system for virtually any sized generator—and future expansions. In addition to side-by-side placement, these enclosures are stackable for installations on small footprints. Built on structural steel, the post and panel design comes with a secure fully welded roof as well as fixed, motorized, or sound attenuation louvers.

Robinson Custom Enclosures is a one-stop shop with the unique ability to handle all facets of their customers’ projects in-house, which allows the company to control quality, lead times, and manufacturing costs. Combining skill in intricate metal fabrication and machining with electrical engineering knowledge, the company produces integrated structural enclosure and electrical solutions. Their sound attenuation offerings include unique solutions to solve many sound reduction problems assisted by their extensive array of testing equipment and years of experience and data. This knowledge is applied to flexible and extensive in-house manufacturing capabilities. Their well-built enclosures will provide a quality working environment for workers and protection of valuable equipment.

Shannon Enterprises provides removable, reusable blanket insulation for sound attenuation that allows a customer to reduce harmful noise at the source. Their blanket insulation is a pre-engineered thermal-acoustical insulation system designed to reduce noise levels and improve the surrounding work environment. Each blanket is designed with CAD technology and can be custom-fitted for a wide variety of equipment coverage. Their LT450A-TT features an inner and outer chemical resistant Teflon fiberglass cloth, with high density fiberglass mat and barium sulfate loaded vinyl. The blankets are removable and reusable. They can be installed with minimum effort and are secured with a stainless-steel wire twist fastening system without the need of specialized tools or extensive training (plant personnel can learn how to install and remove quickly). The blankets are vibration-resistant and satisfy applicable OSHA safety requirements.

The LT450A-TT is designed to operate as a sound attenuation barrier at a maximum service temperature of 450°F. It consists of a 16.5-ounces-per-square-yard PTFE impregnated Fiberglass Cloth and 11 PCF fiberglass needled fiber that acts as a sound absorber. The blanket design conforms to the shape of the surface of the equipment in order to minimize any air voids between the equipment and the blanket while providing complete coverage. State-of-the-art CAD design ensures project accuracy for installation. Shannon guarantees that all custom manufactured blankets will accommodate all controls, fixtures, and appurtenances and will cover the cost of replacing the blanket if failure is the result of poor fitting.

Hennig Enclosures provides a wide range of enclosures, from the most simple and basic equipment shelter, to sophisticated and complex sound attenuation structures. They provide custom designs that meet each application’s unique specifications. In addition to customized designs, their diesel generator enclosures come with the following standard features: full integration of generator, enclosure, and fuel tank; powder coat paint finish; flush-mount handles with key or pad locking; full-length continuous geared hinges; caulked seams; exhaust systems; leak-proof roof design with internal gutter system; wiring in conduit or raceway (full NEC compliance); dampers/louvers; and plenums guaranteed to meet genset airflow requirements, internally mounted silencer, subbase UL142 fuel tank, 14-gauge galvannealed steel modular panel constructions, and mineral wool insulation with perforated aluminum liner on sound attenuated units.

MIRATECH is a manufacturer of exhaust silencers and components due to their recent acquisition of the well-known EM and COWL brand products. They provide a full range of standardized silencers and accessories as well as custom-engineered sound attenuation solutions. Their engineering staff can design and manufacture silencing systems to a high level of performance through computer system modeling, analysis, testing, and validation. MIRATECH’s complete line of silencers includes: cylindrical silencers for a variety of applications (industrial, residential, critical, hospital, and extreme grades), disc-style silencers for hospital installations and critical to extreme grades, oval-style silencers for hospital and super hospital grades, low pressure drop cylindrical silencers, ATEX spark-arresting silencers, spark arresters and combined spark arresting silencers, silencer/catalyst combo units, and Cowl brand compact spiral silencers for space-constrained and small engine applications. Each type of silencer comes with installation accessories such as flexes, elbows, thermal wraps, and clamps.

Maxim Silencers provide noise control across the entire audible range with a wide variety of silencers for an equally wide variety of applications. The basic design of the Maxim M Series Chamber silencers incorporates non-resonant side tube arrangements to permit passage of the exhaust gases from one chamber to another. The resultant reversal of flow develops predictable and controllable back pressure. Their chamber type silencers allow for side inlet exhaust connections which will improve the ease of installation by eliminating the need for elbows in the exhaust piping. The Maxim MSA Series Spark arresting chamber silencers are used for marine service, refineries, and other hazardous environments. These chamber type designs retain hot carbon and soot particles, which minimizes danger and helps provide a safer work area and cleaner exhaust. Maxim MT Series Low Pressure/Straight through silencers are designed with an unobstructed passage through the silencer with no flow reversal. The resultant pressure drop across the silencer is somewhat greater than the length of the pipe. Though it achieves both goals, the emphasis on this design is on low pressure drop instead of sound attenuation. Less extreme are the Maxim MD Series Low Profile “pancake” compact chamber silencers which are engineered to provide residential- to hospital-grade sound attenuation. However, their compact design allows for minimal space utilization. This makes them an obvious choice for packaged power applications with minimal available area and limited height. Custom design features such as dual inlets, heavy-duty welded construction, and temperature resistant insulation within a double wall construction are available with a standard finish of high temperature resistant silicone black. Lastly, the Maxim MR Series Thin Line chamber silencers are designed for critical/extreme grade attenuation, while operating in a compact space (suitable for marine applications). Like the MD series, the MR series comes with the same custom design features and high temperature resistant finishes. Both can be constructed of carbon steel or stainless steel.  BE_bug_web

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