One person’s sound, also described as a “disturbance of mechanical energy,” is another person’s noise, which is particularly disturbing when it’s unwanted or reaches intolerable levels. But how much is too much and how is it measured?

By Lori Lovely

Sound consists of vibrations, characterized by the the properties of waves (frequency, wavelength, period, amplitude, speed, and direction), that travel through the air (or other matter) and are audible to humans. A variation in pressure above and below the atmospheric pressure is known as sound pressure, measured in units of pascal (Pa). The number of pressure variations per second is the frequency, measured in cycles per second called hertz (Hz).

The normal audible frequency range for humans is between 20 and 20,000 Hz, although this range varies significantly with age, gender, and hearing damage. Few people can still hear 20,000 Hz by the time they are teenagers; they progressively lose the ability to hear both higher frequencies and low-level sounds as they age. The human ear is most sensitive to frequencies of 1000 to 3,500 Hz.

Sound levels are measured on a logarithmic scale in units called decibels (dB). A decibel represents a ratio—a dimensionless unit, not an absolute value. Technically, a decibel is one-tenth of a bel, or transmission unit—a unit of measure created by Bell Telephone Laboratory engineers to quantify the reduction in audio level over a 1-mile length of phone cable. Because a bel is cumbersome, the decibel is more commonly used.

Excessive noise can be hazardous, affecting the human body by impairing hearing and putting stress on the heart and circulatory system. Extensive exposure to excessive noise can result in hearing loss. Prolonged exposure to sound pressure levels exceeding 85 dB can permanently damage the ear, resulting in tinnitus or hearing loss. Sound levels in excess of 130 dB are more than the human ear can safely withstand and can result in serious pain and permanent damage.

In an effort to prevent problems, governments at all levels have enacted laws and regulations to limit noise, typically based on zoning. Noise guidelines are often enforced through such legislation as the Environmental Protection Act.

Shhh!
Larry Hansen, founder and principal engineer of Engineered Aero-Acoustics in New Hope, MN, believes the scale is getting “much tougher,” down to as low as 45 dB at one meter in some places. “With the expansion of oil and gas products, there are more wells—more noise closer to urban areas. The result is tighter criteria for noise. We do tremendous business in California, even in private residences.” He notes that some California communities instituted noise laws with day/night levels, where night restrictions are up to an additional 10 dB lower.

California isn’t the only restrictive state when it comes to noise. Hansen mentions a downtown Chicago condominium homeowners’ association that requires a Dolby 5.1 (home theater) standard. Historical sites across the country often demand noise reduction in the name of ambiance. Engineered Aero-Acoustics has experience with projects at the William Penn Hotel in Colonial Williamsburg and on historic battlefields.

“Sound is becoming an issue everywhere,” agrees Bob Bullock, product manager for Commercial Acoustics in Phoenix, AZ, acknowledging that specific regions are exceptionally sound-conscious. “Anywhere that’s growing—Los Angeles, Vegas, Phoenix—is going to be more sound-conscious, even if the noisemaker was there before the residents.” He recalls a two-week Christmas shutdown of a plant that typically ran 24 hours a day. When it started up again, there were a lot of calls complaining about the noise that was suddenly more perceptible.

Dino Perin, industrial original equipment manufacturer (OEM) engineering manager at E-A-R Specialty Composites in Indianapolis, IN, agrees that sound is becoming more of an issue and points out that California legislation is particularly tough on rules governing property line abatement. However, according to Mike Witkowski, vice president of sales with Pritchard Brown of Baltimore, MD, more often it’s not new community noise ordinances that are causing changes in the market; it’s the fact that local authorities are paying more attention to them now. “The phenomenon occurred in the late 1990s,” he elaborates. “People bypassed permitting, installing megawatt generators that are loud. There’s a lot of equipment. As a result, authorities are paying attention to ordinances already in place.”

Sound attenuation became a big business with the use of outdoor standby power generators.

He points out that since the decibel is a logarithmic ratio, as the amount of required attenuation increases, so too do the size, weight, air-handling complexity, and cost of the system. Therefore, he advises, it’s important to know the site noise requirements. “Most communities have ordinances regarding maximum permissible sound levels at the property line,” he writes, “but it is sometimes unclear how a standby generator set, which runs one hour a week for exercise or during the occasional utility power outage, is defined as a noise source.” And, he adds, because the majority are standby appliances, they’re held to a different level of scrutiny. Typically, they’re tested weekly during the day, when the noise level is already high.

Witkowski says Pritchard Brown is an established leader in regard to sound-attenuation jobs, with a good record of meeting requirements. One thing that helps is getting an acoustician involved early in the project. “It’s easer to design a system up front than to field-modify it later.” That’s important for more reasons than mere cost, because “people and state and local governments pay more attention to noise now. Congestion is now an issue here. The Europeans were decades ahead of the US in the importance placed on sound attenuation: They’ve been out of room for thousands of years. It’s just hitting us, but it’s hitting hard. This is a litigious society with a noticeable lack of tolerance.”

Stop the Noise
The initial work involved in sound attenuation includes due diligence on state, municipal, local and special interest codes, Hansen insists. “Not many companies go to that extent, but the average customer says, ‘Get me into compliance.’ They want a turnkey operation to meet all criteria, and if you don’t meet the criteria, you’ve got problems.”

“Typically, sound requirements are at some distance, like the property line. It’s the client’s job to tell us the requirements,” Bullock reasons. Similarly at E-A-R, Perin says, customers are responsible for informing the company of the noise requirements so it can develop an appropriate solution for the application.

Starting with the basic requirements and limitations for each application is the most logical course of action. “If that’s easy, the work is easy,” says Brad Fennell, marketing manager for Chillicothe Metal Co. Inc. in Chillicothe, IL., “but municipalities are more sensitive these days and have enacted 35 to 40 decibel reductions.” He considers it best to get sound data from the engine manufacturers and do three-party testing. “That’s the easy part. The difficult part is the sizing restrictions. You have to look at all the aspects, such as the airflow requirements. To avoid heat buildup, you must size the housing and openings appropriately.”

Due to unprecedented growth in the use of outdoor enclosures for standby power generators and increasing noise limitations, sound attenuation has become a big business. According to an article written by Witkowski in 1996, published in Powerline, sound attenuation “reduces source noise by a minimum of 3 dB,” with levels of reduction reaching 40 dB, depending on site conditions and prevailing local codes. Thanks to improved sound-attenuation capabilities in the enclosures themselves, one of the first critical decisions is if sound attenuation is needed, and if so, how much. That information will dictate the enclosure size, air-handling choices, and materials selection.

David Gries, acoustical engineer for E-A-R, references an article he wrote titled “Noise Control Solutions for Standby Power Generators,” in which he details considerations during enclosure design. Beyond noise levels and site requirements, the sound path must be managed. He recommends minimizing openings. Where that’s not possible, he suggests torturous paths to reduce noise radiating from the structure. Eliminating or severely minimizing openings can adversely affect the cooling, particularly with smaller profile gensets that incorporate noise control materials. “The resulting heat issues can make thermal management one of the dominant design considerations in the ultimate solution package.” As Hansen says, coolers/radiators have to be treated carefully because the cooling system is the blood flow of the generator.

For those and other reasons, Fennell likes to get an acoustical engineer involved on the front end of the project. “In Chicago, 55 decibels are required at the property line,” he gives as an example. “If the engineer doesn’t do a sound study to find the limits, maybe a 70-decibel unit is sufficient.” Because there’s a diminishing return on sound attenuation, the difference between 55 and 70 can cut the price in half because the corresponding product is much cheaper. If the decibel level makes no sense for an area, he suggests going to the jurisdiction to get a variance. “It’s like building a house; it happens frequently.”

Averaging 200–300 large (1 MW and up), custom-designed units per year, most of the Pritchard Brown’s work is in bid-spec environments where the best price that meets the specifications gets the job. “By nature, the tendency of the manufacturer is to propose a solution under ideal conditions,” Witkowski elaborates. “Usually, it will not meet specs, but 95% of the time, it’s not challenged.”

Silent Running
Challenged or not, specifications are a critical aspect of the job. Once specifications and zoning requirements have been collected, the real work begins: matching the right materials and setup to the application. Each company approaches it slightly differently. “Each application is highly individual,” Hansen repeats. “Any generator is different in a different environment, and efficiency of airflow varies with altitude and air density.”

Traditionally, Hansen explains, it was best to visit the site prior to designing a sound attenuation system. Today, computers simulate installation on a 3D basis using topographical maps and other information. Engineered Aero-Acoustics has engine specs from all major the manufacturers built into the computer, but all the topographical and aerodynamic parameters outside engineering make each application different.

A statistical energy analysis is performed using software to run a simulation. “We take the geometry from the generator and apply our materials in various configurations,” Perin says. It’s easier said than done, however; a lot of work is involved and each change in the configuration nets different results, allowing the customer to balance the cost of the product with individual economic goals.

Factors taken into consideration when configuring a solution include available space, the size of the unit, the initial noise spectrum of the genset without an enclosure, engine noise (the number of cylinders), and the type of engine, because different engines have different “signatures.”

The product has to be tested in the lab, Bullock insists. Acousticians need to know the performance of the product in order to make accurate predictions with a software program that models the acoustics of the generator with a multipath analysis. Modifications are then based on the sound profile of a particular generator. Quieter generators need less acoustical treatment. “The lack of sound equates to dollars—it’s more efficient.”

Engine manufacturer tests are done in a semi-free field to develop an ISO standard, the sound-power rating that is used in calculations. Engineered Aero-Acoustics acousticians take basic data from the sound-power rating and look at each location chosen for installation when configuring a system. “A decibel is not always a decibel; you have to dig deeper,” Hansen advises.

Since most people want their genset tucked into a secure architectural structure (no more open green-field distance) for security reasons, free-field data is not right for generators in architectural buildings, Hansen says. Engineered Aero-Acoustics has a lot of experience with secure facilities. “People want their generator to be protected from a variety of threats: terrorism concerns for banks, hospitals, and any critical-use application; terrorism; weather, including intense storms, high winds, and tides; and copper theft for just about everyone.”

Engineered Aero-Acoustics also considers what will be done to the site afterwards: installation of fences, decorative facades, obstructions and potential future expansion of the site including upgrades, additional generators, and plans for growth. “It’s part of our checklist,” Hansen reveals. “We use a two-page list to bring up potential issues in the design phase.” Future planning can be as easy as just leaving space, he notes.

Next, a computational fluid dynamics (CFD) module tracks air into the generator, between buildings, around structures, etc. It determines flow patterns—the route of the air—in order to track pressure losses. “It tells us how much silencing can be added without exceeding the aerodynamic limits of the engine,” Hansen explains.

Material World
The computer models allow acousticians to formulate opinions about troubling frequencies, Perin explains, and select the materials best suited for them. According to Fennell, there’s “nothing new on the market materials-wise. We’ve had the same materials for 10 years. It’s how you use them—with more sophistication.” For example, Chillicothe Metal Co. layers a double wall with an air gap, then uses two to three engine silencers in a series that works with the engine and another one inside the pipes to create more than one current of air flow.

“The products don’t change much,” Bullock echoes. “It’s how you apply them that does.” Gensets are a small portion of business for Commercial Acoustics, a division of Metal Form Manufacturing Co., which designs and manufactures HVAC, security, and sound-reduction solutions. The company’s products, which include acoustical louvers, plenums, and enclosures, as well as transfer silencers, incorporate a double-skin acoustical panel and sound attenuators. “Designs haven’t changed since the ’60s,” he says. “Galvanized steel is galvanized steel—there’s not much new in cutting. It all depends on the performance requirements. You use different liners and fill material and heavier gauges to reduce noise. With silencers, it’s about how much you restrict the airflow and how much space you have. Radiator fans can only push against so much restriction; you have to be careful with silencers. If you reduce the volume of airflow, you reduce the cooling.”

Similarly, Witkowski contends that there are “no technological breakthroughs. It’s old school, simple. There are no new materials. We still use fiberglass insulation, sheets of steel, and rock wool. You trap noise in a box and absorb as much as possible. Gensets need openings for cool air.”

But Perin contends that materials do make a difference. “We’re geared to home in to eliminate noises humans are sensitive to. A couple thousand hertz make a fan sound more pleasing.” That’s why E-A-R uses a continuous-cast process for its polyeurethane foam and custom formulations for its heat-reflective laminated surface film.

“We classify foam for sound absorption.” There are seven parameters to manipulate, including porosity, density, and tortuosity—the path sound takes within the foam. E-A-R’s “trade secret” chemical process creates an open-cell structure of foam through which sound is manipulated. Available in three market-specific formulations, it’s proprietary Tufcote acoustical absorbing foams reduce noise levels within enclosed spaces. Protective facings are available in thicknesses from 0.25-inch to 2 inches.

When noise from a turbocompressor disturbed adjacent offices and the louvered filter-baffle placed at the air-intake opening didn’t sufficiently reduce noise levels, E-A-R added strips of Tufcote E-100SF foam, a self-faced, 1-inch acoustical absorbing foam. It reduced the noise to acceptable levels. For additional treatment, strips of an E-A-R foam-barrier-foam composite faced with aluminized polyester were applied to the louvers.

E-A-R materials consist of 1) sound absorbers that absorb sound by allowing sound to penetrate; 2) limp mass barriers that prevent sound from escaping; 3) and combinations of layered barriers and sound absorbers in a double-wall system. Issues with moving air can arise, because open areas must be retained. If the open area is too large, the barrier is less effective.

The issue is heat. Because E-A-R uses polymer materials, it incorporates films to reflect radiant heat. Radiant heat barriers reflect infrared light. Among the company’s proprietary techniques are a polyester with aluminum coating and a thin aluminum. “There’s a lot to it,” Perin hints, adding that the company’s cadre of about 20 engineers continues ongoing R&D for new materials and techniques for applying them.

E-A-R materials are supplied in sheets or die-cut or diamond-cut goods. With in-house molding capabilities for parts and outside suppliers, Perin says the company covers everything “from sheet goods to the finished product.” Perin notes that the company also makes damping materials for sheet metal panels and isolaters that fit under equipment to reduce vibration.

Can You Hear Me Now?
Sound attenuation goes beyond merely materials. A little science is necessary for the equation if the application is to successfully squelch the noise. “It’s not just putting up a barrier,” Hansen emphasizes. “That interrupts airflow to the generator. Generators must breathe, or they get high temperatures and shut down.” In addition, he adds, barriers are less affective at night or if they’re downwind because they bend sound. “Natural and design phenomenon have to be integrated. Adding aerodynamics to acoustics adds a third dimension to the design.”

So, too, do emissions and noise restrictions. Tier 1 and 2 requirements demand more airflow, Hansen believes. “We remediated a water treatment plant in northern California. The old generators—around 20 years or more—had airflow requirements less than today’s Tier 1 gens. We had to convert skylights to air inflow and evenly distribute air.”

He relates another example of the importance of airflow. One customer called with a concern about overheating. Upon investigation, it was determined that the cause was the customer’s installation of bug screens that cut airflow by 50%. “We guarantee our designs will meet the criteria (as long as no changes have been made) or we’ll fix it at no additional cost. So far, we’ve never had to do it.”

There’s a lot of risk to guarantee a specific sound level, Fennell admits. “It’s a combination of calculations: silencer manufacturer, panels, space, target decibels, size of generator … We’re putting them in enclosures—it’s much quieter than the ambient noise.”

“If generators didn’t need space for air to cool, they’d be easier to quiet,” exclaims Witkowski. “But they need big openings to move air, and air generates noise in addition to the engine noise.”

Witkowski explains that there are three parts to noise: mechanical noise, the fan, and moving air. “Every situation in noise control involves a system composed of three basic elements: source, path, and receiver,” he postulates. “Before a solution to a complex noise problem can be designed, the dominant source of the noise must be known, the characteristics of the significant transmission path must be understood, and a criterion for the level of noise considered permissible or desirable in this situation must be available.”

Gries listed noise sources for power generators as engine, exhaust, airflow associated with cooling fans, and alternator. The spectrum of each “is dependent on geometry, output power, and load conditions.”

The first step in developing an effective noise control program is to identify the noise sources and quantify the level of noise. Typical tests, Gries explains, involve sound-pressure measurements at eight locations at a distance of 7 meters from the generator at full load.

But there is an additional source of noise that must be added into the equation: ambient noise already present in situ. “Sound is additive,” Fennell states. “You must take that into account in the design.” Two like noises operating in the same field create a 3-dB “bump.” Therefore, if the engine block, airflow/fan and engine exhaust are all set at 55, the actual reading will be 58 dB.

And, he adds, “the specs haven’t addressed the ambient noise contribution.” Ambient, or background, noise present in the environment before the introduction of additional noise sources should be measured and taken into account during the design phase, due to its influence on the accuracy of measurements of the targeted noise source.

As Witkowski explains, in some industrial environments, ambient noise is higher than or close to the noise limits of the community. “Acousticians can estimate, but there’s no way to tell if the genset meets the conditions because there’s too much background noise.” For example, if ambient noise is at a reasonably quiet 60 dB and the noise source is 60 dB, when the engine is started, it registers a 63 dB measurement. “It would have to be 10 dB below ambient not to hear the engine.” Similarly, ambient noise at 110 dB and a diesel engine at 110 dB equal 113 dB. “Doubling the amount of energy adds 3 decibles to the overall sound level.”

Sound is logarithmic over a large range, and decibels are a ratio of one number to another. “It’s misleading,” he says. “People don’t understand that to change one level by 10 decibels takes away 90% of the sound energy. It’s not simple math, not an exact science.” That’s why Pritchard Brown stresses working with an engineer up front.

As part of a premium package option, Pritchard Brown performs testing at the factory if ambient noise is high. “We advocate having the manufacturer back up test results because 5 decibles can change the cost 30% to 50%. Many manufacturers make assumptions about free-field patterns,” Witkowski says, “but it’s difficult to predict. There are so many variables that affect noise.”

In the same way, Chillicothe Metal goes one step further in quiet applications, insisting on testing the genset in the facility to see if it operates per published sound level. “It could affect wall construction if it’s not correct in design,” Fennell explains. “Small variances are common. With 125 or 250 hertz, if you’re 3 to 4 decibels off, it’s significant in attenuation.”  

If the generator is loud and needs a lot of sound taken out, you may need more resistance than the radiator fan can push against, Bullock cautions. In that instance, you can use a scoop or upduct with a silencer. “You take sound out in the silencer and redirect it. That way you get additional sound reduction and create a block in the line of sight from the source to the receiver and take advantage of natural attenuation through the air.”

Silencers control the noise generated by engine combustion, blowers, and fans. They’re available in three types: reactive, absorptive, and a combination reactive/absorptive. Reactive silencers generally provide better noise attenuation at lower frequencies, while absorptive silencers can achieve much greater noise attenuation at higher frequencies. Incorporating both elements achieves overall higher noise attenuation. Selection of the appropriate silencer depends upon several factors, such as noise spectrum, flow rate, temperature, and allowable backpressure.

“There are a lot of things we can do, but we’re limited by physics,” Bullock continues. “It’s a very iterative process: This is what I want; this is what you can do. If this, then that.”

Static pressure (resistance) takes out sound. If you can’t afford much pressure, you use size, length, model, or a combination to get the sound out. Even so, it’s not always easy. Noisy generators in super-quiet areas are very difficult. Bullock has used an enclosure around an enclosure around the generator. “It’s good for sound coming through the enclosure: engine exhaust, cooling air.”

Although Bullock believes it’s best to reduce sound at the point of generation, he has used a remote radiator on extreme cases.  “We move it somewhere else, plumb it, and install a separate fan. We work with each customer on individual projects. Last week, a project had no close neighbors but a higher ambient level. This week, in downtown LA, there are very strict requirements to meet.”

There’s a Kind of Hush All Over the World
The industry has changed, Hansen reflects. “Twenty years ago, if we did two or three projects a year, it was something to notice. Now, we do projects daily.” The amount of jobs isn’t the only thing that’s different. The technology has changed considerably. “Today, with computers, we get more. We used to take one frequency and track it, but with 3D, we look at 180 degrees. We can see hemispherical dispersion of sound and can plot sound contours and simulate climatic changes: wet snow, dry snow, water …” The computer program also has the ability to learn: It makes suggestions of combinations based on the parameters. “All are engineering trade-offs; we have to consider the side-effects.”

Urban growth and emergencies create demand, Fennell observes, especially in hospitals, wastewater treatment plants, water pumping stations, and lift stations. “Cities are beefing up for emergencies.”

Cities aren’t the only ones beefing up. “Packages are getting bigger,” Witkowski notices. “The demand for power is greater, particularly as data centers move from Tier 3 to Tier 4 reliability. The size of standby power sources has increased, and there are more packages per location.” That means noise increases exponentially, so making them quiet is an issue.

It also means that understanding of job-site constraints and creative application of existing technology are the major hurdles in designing custom solutions for critical applications where limited space at the job site and multiple systems are the norm.

Hansen believes that new buildings size generators to the smallest load possible. The space given to mechanical systems is small because it decreases income-generating square footage, says Bullock. However, a smaller footprint and less cost means you add less attenuation. There are ways around a small footprint. Very expensive applications are easy to spread lengthwise, Fennell says, “but you can build them tall and bring in air from above if space is limited.”

As Witkowski wrote more than 10 years ago, the most overlooked aspect in sound attenuation is that as the amount of attenuation increases, so does the size of the enclosure. However, that’s no longer acceptable. As Gries wrote, “the growing popularity of standby generators has coincided with stricter demands for lower environmental impact and physical profiles in ever more powerful units.” Combined with increasingly limiting noise ordinances, it results in a market demand for more compact units with yet more powerful attenuation capabilities. That, in turn, has fueled the need for innovative engineered solutions that outperform traditional designs.

“It’s a shrinking world,” Perin reflects. He sees the coming challenge as making a product with a small footprint to fit limited space—but one that is able to produce less noise. “It takes a lot of work to reduce the size of enclosures and the noise coming from them. Most work on the principle of ‘more is better.’” However, if necessity drives invention, the current surge of generator sales (especially for residential use) and the expected growth of the market will surely forge solutions for producing less noise in less space.

Lori Lovely writes on topics related to transportation and technology.

DE - January/February 2008