Sound is what we hear. The sensation of hearing results from a reaction to rapid pressure fluctuations in the air that cause the eardrum to move tiny bones, which transmit the movement to a fluid-filled sac that contains hair-like nerves. When those nerves are stimulated by motion, they send signals to the brain. Different lengths of these nerves respond to different frequencies.
Noise, on the other hand, is unwanted or annoying sound. While some of the difference is subjective, excessive or prolonged noise can cause harm to humans, animals, and structures. It can also violate local codes.
OSHA established the Noise Control Act of 1972 to promote an environment free from noise that jeopardizes the health and welfare of citizens. The Act established coordination of federal research in noise control and authorized federal noise emission standards of commercial products.
This Act was the basis for the National Building Code, says Chris Murray, commercial director for Soundown, which he summarizes as a regulation that restricts the amount of noise and vibration at the property line. “‘Good neighbor control’ is more of an issue.”
Such regulations or specifications are typically defined as an A-weighted sound level at a defined point or points, explains John Hawkins, engineering manager, Powertherm. The location is commonly the nearest property line or receptor.
The regulations are intended to eliminate or minimize disturbing noise, complaints, and legal action from neighbors, but compliance doesn’t guarantee the eradication of noise complaints, even if enforcement of noise ordinances is increasing.
In the late 1970s, Hawkins says the Chicago noise code addressing the factory and business noise level at the curb was rarely enforced. Now, however, he says local officials are “pulling out the meter again” to register decibel levels.
Noise ordinances are seeing more enforcement, not new laws, states Jeremy Off, engineering manager, Robinson Custom Enclosures. “The sound quality issue is being brought to the forefront as companies are forced to adhere to strict regulations.”
However, there are many problems with both the regulations and the enforcement of them. First, the ordinances are not always clear. Some of the complexity arises from frequency. Lower frequencies don’t always translate to measurements. Engines and turbochargers emit a high frequency, Off explains, but many generator fans emit a low, pulsing frequency. Both can be annoying. Both can cause damage.
Hawkins believes that it’s important to better define noise and to treat high and low frequency noise differently. “We must define the problematic noise—define what’s creating the problem: frequency range.”
Sound is the result of rapid fluctuations of pressure that reach the receiver or receptor. The frequency of a sound is the number of times in a given period (usually one second) that the power or pressure changes from zero to maximum to minimum back to zero, completing one cycle. Frequency is generally determined by the source and usually expressed in Hertz or cycles per second.
Frequency can be altered by physical variables in the path. For example, if the medium changes or the medium’s temperature changes, the frequency can be altered. If the length of the path between the source and the receptor is changing, the frequency in the path and at the receptor will change. A well-known example of this Doppler Effect is the change in pitch of the whistle or horn on a rapidly approaching train or vehicle. As the source approaches the receptor, the time between the arrival of the pressure variations at a point is a function of the time they are emitted by the source less the difference in time required for each to travel the decreasing distance. As the vehicle approaches, the pressure changes arrive at the receptor more rapidly; therefore, the frequency is higher than at the source. As it passes the receptor, the distance is suddenly increasing; the time from source to receptor is increasing and, therefore, the frequency is lower than at the source. There is a sudden decrease in frequency at the instant of passing. If the speed of sound in the path changes due to a change in the medium, a similar “frequency shift” will be noted between the source and receptor.
“Knowledge of the frequency or dominant frequency range of an offending noise allows us to identify the probable source or sources,” indicates Hawkins. “For example, if the noise has a pure or single dominant frequency, we start looking for a rotary machine such as a centrifugal compressor or fan as the probable source. If we detect a strong 400-hertz tone in the noise, we might quickly track it to an eight-blade fan turning 3,000 rpm. A noise with dominant frequency of 90 hertz might come from a four-stroke cycle, six-cylinder reciprocating engine running 1,800 rpm.”
Unfortunately, he adds, noise is usually made up of hundreds, if not thousands, of frequencies coming from many complex sources at a particular installation. Determination of particular frequencies or frequency ranges, along with knowledge of the operating parameters of potential sources, is an important first step in identifying sources of particular noise problems.
Most regulations specify noise level in decibels, and most sound mitigation solutions adjust the sound level due to pulsations to what the human ear responds to. Hawkins says the human ear responds most to 1,000 hertz—high-frequency noise. Walls are absorptive for high-frequency noise. Air absorption is another effective technique because high-frequency dies off faster with distance.
But low-frequency noise is different. “Low-frequency penetrates barriers,” states Hawkins. “And it travels more distance.” He explains that low-frequency noise is also an earthborn noise, which is why it rattles the windows.
Weighting the Issue
Increased sensitivity to both low- and high-frequency noise is new, Hawkins says. “There’s a lot more sensitivity to noise sources…and a lot more enforced regulations.” For example, he says, Colorado recently passed legislation to limit noise from oil and gas production equipment releasing high-pressure gases. Vent silencers helped restrict the release to a certain time period.
A vent silencer is a device used to reduce the noise levels created when high-pressure gas or steam is expended into the atmosphere. Because most of the noise occurs at the expansion across the valve and is emitted at the stack opening, the silencer is installed at the exit to muffle this noise.
Sensitivity to noise isn’t just a matter of personal intolerance. Sound can impact the receptor, which could be a human, animal, or a structure. “The ears of any animal respond differently to equal sound pressure levels at different frequencies,” elaborates Hawkins. Human hearing response is the basis for the A-weighted correction to sound pressure levels, which converts them to an A-weighted sound level, or A-SL. This A-weighted sound level is not a sound pressure level. It is used frequently in legislation and standards and specifications because it represents the average or typical response of the human ear.
The human ear is most sensitive to sounds that contain dominant frequencies in the 1,000, 2,000, and 4,000 Hz octave bands. In the same way an “out of balance” tire on your automobile will begin to vibrate noticeably, even violently at certain speeds, the eardrum, connecting bones, and fluid filled sac respond more to certain frequencies than others. At higher than 4,000 Hz, the response begins to drop off. As the frequencies decrease from the 1,000 Hz octave, the response falls off at an ever-increasing rate.
Beyond irritation, increased sensitivity to high-frequency noise can lead to actual damage to our hearing in a much shorter time than exposure to lower frequency sounds at the same sound pressure level. As a result, the lower frequency sounds will receive a much lower A-Weighted sound level than sounds in the 1,000 to 4,000 Hz octaves with equal SPL.
In addition to potential hearing loss, Hawkins says that excessive exposure to high A-SLs has been shown to cause other physiological effects, such as increased blood pressure. Animals may be even more sensitive to higher or lower frequencies than the average human is. For example, dogs respond to a whistle which is at such a high frequency that most people cannot detect the sound.
The less frequently seen weighting system is the C-weighted level. This weighting actually applies much lower correction values to the lower and the highest octaves, and no correction to the mid-range octaves relative to the actual linear level. While the A-weighting provides an indication of human hearing response and is valuable in setting limits to avoid hearing damage in the workplace as well as interference with speech and other daily human endeavors, C-weighting is indicative of the annoyance potential of the noise. The C-weighted sound level recognizes the potential for low frequencies to travel further than high frequencies, due to the negligible effect of air absorption.
For example, Hawkins explains, if you stand near a running engine, you can hear air movement-induced noises, mechanical noises, and other noises at the upper mid- and high- frequency ranges. However, at 100 yards away, air absorption tends to reduce the level of those high-frequency sources. While that lower frequency is not likely to damage your hearing, it can be very annoying, particularly at night. And to compound that issue, the low frequencies are much more likely to pass through the walls and windows of common structures. “As a result,” he concludes, “we are beginning to see the emergence of legislated regulations [that] address both A-weighted and C-weighted sound levels.”
In Fairmont, MN, when a company added generator sets, the neighbors complained—not just about the noise level, but about the frequency ranges. “A few years ago, generators were stand-by and only turned on occasionally to test,” reflects Hawkins. “Now, with more onsite power generation taking place, they run 24/7. There’s a lot more noise.” The Fairmont company used resonators to solve the problem. A resonator is often used as an effective silencer in air duct noise control applications to reduce low-frequency noise at its resonance frequency with narrow attenuation band.
It’s difficult to completely eliminate noise due to concern about restricting the flow of air or exhaust, and for other reasons. There’s flow-born noise on a generator set, Hawkins says—noise in the exhaust system, the air intake, and the ventilation system. The Ritz blower on a Detroit Diesel genset features two shafts with a spiral gear to push air on the outer edge of the impeller. “It makes a lot of noise.”
There are three types of silencers: reactive (which has a chamber), absorptive, and resonator. “In reactive silencers, the diameter relative to the diameter of the internal tube is a ratio that impacts the amount of noise reduction,” explains Hawkins. It is always a compromise between restricting air flow and noise, budget, and space. A reactive-type silencer used at Siemens in Corpus Christi, TX, cost $192,000. With a 25-foot-long silencer that is 15 feet in diameter, it requires a lot of space.
Although Hawkins says there hasn’t been a great deal of new development in sound attenuation—just “refining and combining” of current technology—he does believe new technology is beginning to emerge, and he believes that customizing equipment for the application is the direction the industry is taking.
Closing In On Sound
Custom solutions to meet requirements and restrictions is the future of sound attenuation, believes Off (Robinson Custom Enclosures). Manufacturing custom enclosures for sound attenuation for the power generation industry is currently a niche market, but he says business has increased and he sees the market expanding.
Sound dampening solutions can be classified in two categories, he says. One is mass dampening—balancing the structural base to handle the vibrational load so that the structure translates vibrations to acoustics in order to allow people to hear versus feel sound; it incorporates attachments such as vibration isolators and absorption.
To mitigate vibrational noise, Robinson custom designs weather-proof standby enclosures that are mounted on rooftops. A lot of factors are considered in determining the right product for the application. “Some of the customer questions we ask include: how quiet do you want it to be? At the property line or are you working toward an ordinance or code?” says Off. Based on that, Robinson builds a solution for distance, taking into consideration assorted options, such as wall materials (composites, thickness) and weather.
Depending on a site’s geography, the specifications may require a particular thickness for thermal energy regulations. “They may require thicker [material] than the sound requirements do,” explains Off, citing maintenance issues. Thicker material helps to maintain internal temperature when the generator is not running.
The manufacturer is currently exploring different types of insulation models to suit various environmental situations, Off indicates. “Different sounds warrant different materials.” For acoustical silencers, batts, foam, and wool is used.
Off discusses the density of different materials and how they help balance the acoustical specs. Fiberglass is a standard material that has undergone extensive ISO and ANSI testing for fire ratings and high temperature. Foam board, both closed- and open-cell, is also a commonly used material. A new ground-based mineral used to make a batt is different in how it makes air and noise move.
Additional customer questions relate to how to quiet the noise from the radiator and engine. “We design to certain decibel level at a prescribed distance, such as a property line or a building,” elaborates Off. He says that Robinson Custom Enclosures is often able to achieve a 45–50 decibel noise reduction, using a combination of tactics specifically tailored to the application.
“We try to balance the radiator and the engine cooling required to bring air in,” says Off. Robinson Custom Enclosures accomplishes that by changing the path of airflow with a hood to bring air in from the bottom. However, if you’re located in a cold region, don’t bring air from too low down.
“When you turn air, it’s harder for sound to escape.” Lengthening enclosure ducts and adding baffle silencers also absorbs noise. The trade-off is real estate, because the more sound reduction that’s required, the more space (including height as well as length) for insulation and equipment that’s needed. “Equipment with an electrical system or power generation requires clearances,” says Off, noting that there can be landmark regulations on height that can affect a design.
One method to improve noise reduction is to minimize the number of doors in a structure. “Get creative in the design to avoid doors,” advises Off. “Door seals and latches can short circuits to the sound attenuation path. Use fewer doors; use smaller doors.” However, he adds, with a larger, more complex system with more components, electrical clearance, and serviceability, access becomes a challenge. Eliminating doors may contribute to that challenge.
To keep a facility quiet and to avoid neighbor issues, some companies embed generators in their basements. This is common where there’s a need for peak shaving or when a facility—like a hospital—needs to generate its own power. Vibration isolation is a critical component of any noise reduction program, but perhaps more so in these cases.
Isolation (resilient) mounts protect high-tolerance machining equipment or electronics from vibration in their environment. Traditional mounting systems such as rubber isolation and spring isolation mounting (for both for indoor and outdoor applications) involved complicated installation. These systems, based on springs and rubber blocks, required the casting of a slab in place and jacking it up to place it. “There’s no way to fix it if there’s a failure in jacking it up,” says the director of Soundown, which offers a range of product lines for noise and vibration control. “The categories for generators include vibration, isolation, and airborne noise control. We offer five product lines in each of those.”
One of those is a high-performance solution to mitigate vibrations in buildings and to neighbors that involves pouring concrete into a 30-feet-wide by 40-feet-long by 7- to 8-feet-diameter inertia pit, upon which an isolated elastomeric slab is placed, and on this slab, the generator. Murray calls the elastomeric slab “the gold standard” and explains that it’s lined with a specialized product made of a proprietary sheet good formula consisting of a cellular polyurethane elastomer. For interior applications, a spring mount is inserted between the slab and the unit.
When this system is used outdoors, he says they “just isolate the generator from the slab with rubber or spring mounts,” because the enclosure is more critical for airborne noise. Soundown carries products to line the enclosures for airborne noise.
The benefits are numerous, from a significant weight reduction compared with previous methods, to crucial time savings. “Our system takes one guy one day,” says Murray. “Treat the pit, dump the concrete. It’s time- and cost-efficient.”
Soundown’s anti-vibration mounts are designed to meet isolation requirements. The BRB mounts using rubber in shear and compression feature a ratio of height to width that is optimized for high deflections, low natural frequencies, and superior vibration isolation results.
Acoustic Absorption materials reduce reverberant noise in enclosures, reducing overall sound pressure levels. Current materials commonly used include foam, fiberglass, and mineral wool, all of which can be finished with a range of durable vapor barrier facings.
Soundown’s Acoustic Absorption Foam is an open-cell acoustical-grade polyether designed to have excellent acoustic properties. It’s as thick as 3 inches, with options for different facings, as well as pressure-sensitive adhesive.
Vibration Isolation Pads are based on Soundown’s Sylomer product line. This load-bearing, foamed urethane product is effective in reducing structure-born noise and vibration in finished surfaces, such as floors and joiner panels. It’s available in six different densities and thicknesses up to 2 inches.
With a growing population expanding into new areas, and with the increase of onsite power generation, sound attenuation will continue to be an issue. New codes may be written, and all regulations face tougher enforcement, making effective sound dampening crucial. New materials and combinations of methods will continue to be developed to keep sound from becoming noise.