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Putting a lid on noise pollution.

Putting a Lid On Noise Pollution

Within the past decade noise levels in the environment and the workplace have been gradually increasing while government funding for noise abatement programs has declined. In the early 1970s, the introduction of noise control regulations by federal agencies such as the Environmental Protection Agency (EPA) and the Occupational Safety and Health Administration (OSHA) seemed to signal an era of awareness of noise as a pollutant. But as the '80s dawned, government priorities began to change, the number of studies on the effects of noise on society decreased, and, according to audiologists, people had to learn to live with noise levels high enough to cause hearing damage and increased stress.

Extensive research into noise cancellation technology has been conducted over the past 10 years, though most of the work has remained in the laboratory. The concept of active noise control, whereby an interference sound wave is used to reduce the noise level at its source, has been around for over a century. Not until recently, however, with the advent of faster and cheaper microprocessors, has it been economically viable for companies to develop active noise control systems. "Computational techniques in acoustics, including advancements in control theory, invention of adaptive filtering, and the availability of fast digital electronics at affordable prices, provided both the knowledge and hardware to achieve progress in implementing this technology," said Jiri Tichy, director of the acoustics department at Pennsylvania State University. In active noise cancellation, a sound picked up by microphones is analyzed by a microprocessor, converted into a signal that is 180 degrees out of phase with the original sound, and reproduced by loudspeakers located at the source.

Several companies are developing products that control noise in the environment and the workplace. Noise Cancellation Technologies (Stamford, Conn.), under a contract with the Walker Manufacturing Co. (Grass Lake, Mich.), is developing an electronic muffler for cars that could be available by 1994. In addition, Noise Cancellation Technologies is currently producing a number of noise reduction products, including an electronic muffler that quiets compressors and headsets that dead-en unwanted sounds in the workplace. The firm is also developing active systems that will reduce noise in the interior of vehicles and in aircraft passenger compartments. Bose Corp. (Framingham, Mass.) is producing a headset for use in small passenger airplanes and helicopters. And Digisonix (Stoughton, Wis.) is producing a variety of industrial sound cancellation systems including applications for HVAC ducts, material handling fans, and vacuum pumps.

Quieter Cars

The Noise Cancellation Technologies exhaust muffler employs a feedback control design to generate an antinoise signal at the end of an automobile exhaust pipe. "A loudspeaker enclosure is placed at the end of the pipe with the outlet port of the speaker surrounding the exhaust," said vice president of research and development Jeffrey Denenberg.

The system picks up a synchronous signal from the engine and relays it to a digital signal processor. Using data derived from the harmonics of the engine noise cycle, the processor calculates the acoustical waveform of the exhaust noise and creates an antinoise that is 180 degrees out of phase with the offending sound. The antinoise has a mirror-image waveform that is released, via the loudspeaker, in exact opposition to the sound wave created by the exhaust. It results in a noise reduction of between 6 and 10 A-weighted decibels (dBA). (The A-weighting is an international standard devised to communicate a decibel level that is weighted for the sensitivity of the ear.) A reduction of 10 dBA is equivalent to a 50 percent decrease in perceived loudness.

The electronic exhaust system also uses the computing power of the digital signal processor to compensate for changes in exhaust noise, due to varying engine operating conditions. "A sensor picks up what's left of the residual noise," said Denenberg. "It correlates that with the frequencies and harmonics that correspond to the rotational rate of the engine. This tells the controller how much to change the parameters that synthesize the antinoise."

In August 1990, Noise Cancellation Technologies installed its first stationary engine muffler on a Detroit Diesel Corp. (Detroit) model 6V-92-450-horsepower diesel engine. To get it to produce the loudest possible sound, the engine was attached to a chassis dynamometer that induced a load on the flywheel. At 1800 rpm, the turbocharged 6V-92 engine created a noise level of 123 dBA, measured 2 meters from the exhaust outlet. The most effective passive muffler mounted on the engine reduced the noise level to 89 dBA and created an exhaust back-pressure of 1.5 inches of mercury; fuel consumption was 166 pounds per hour. Fitted with the Noise Cancellation Technologies electronic muffler, the engine produced 81 dBA of sound with limited back-pressure and consumed 163 pounds of fuel per hour. In addition, the electronic muffler took up 20 percent less space than the passive muffler.

Noise Cancellation Technologies has also installed an active industrial muffler on the CSX Transportation Corp.'s (Baltimore) railcar vacuums. The MasterVac sucks industrial products such as sugar, flour, and plastics from railcars through a steel tube. However, it was equipped with a passive baffled muffler. With Noise Cancellation Technologies's active muffler in place, the vacuum's noise was reduced by 20 dBA.

Driver's Seat

Efforts to achieve noise control in automobile design have resulted in developments like the silent chair prototype, which employs two microphones and two loudspeakers to create a zone of silence around the head of a person sitting in the seat. Positioning of the zone depends on the location of the speakers, which cancel out a selected intruding noise. Noise in a large enclosed zone is picked up by the microphones, and a digital signal processor produces a 180-degree out-of-phase antinoise that is played through loudspeakers to cancel out an offending sound. Because of the system's selectivity, desired noises such as speech, warning sirens, and music are not affected.

To quiet sounds in an actual automobile, six microphones and six speakers are required. "In the case of a commuter aircraft you may need 30 microphones and speakers," Denenberg said.

Noise Cancellation Technologies said it has signed an agreement with an anonymous European aerospace manufacturer to develop a zonal cancellation system that would abate undesirable noise in aircraft passenger compartments.

Additional work in the active control field has resulted in an active engine mount prototype, developed by Noise Cancellation Technologies under contract with Chrysler Corp. (Highland Park, Mich.) and an anonymous European car manufacturer. Expected to come on line in 1955, the device eliminates vibration between a car's engine and its chassis. An actuator is used to compress and expand an engine mount to create a free floating effect between the engine and the chassis. As the engine tries to push against the mount during vibration, the mount is compressed and moves with the vibration of the engine. As it vibrates the other way, the mount expands, again moving with the engine, so the vibration doesn't transfer through to the chassis. Noise Cancellation Technologies is also working with Delco Products Inc. (Dayton, Ohio), a division of General Motors Corp., to apply vibration reduction technology to Delco's engine mounts.

Easy Listening

Noise Cancellation Technologies has also developed an open-backed headset that company representatives claim is a significant improvement on conventional noise muffling headsets used in industry. The headset allows desired sounds to reach the ear through a cutaway section via the ear-cup. Unwanted noise is cancelled via a combination of miniature microphones that sense noise in the ear canal, an earphone diaphragm activator that cancels synchronous periodic noise, and a central control unit.

Noise Cancellation Technologies is selling a variation of this headset for use with the Magnetom Magnetic Resonance Imaging (MRI) system, developed by Siemens Medical Systems Inc. (Iselin, N.J.). The design employs a closed-back headset, which reduces perceived noise levels created by powerful electromagnets in the MRI system by about 70 percent. Patients may listen to a built-in radio, cassette, or compact disc player while the MRI machine takes cross-sectional images of the human body. The search for higher-resolution images in the medical profession has resulted in powerful electromagnets. Denenberg said. But the noise level and cost of an MRI system increases in proportion to the power of the electromagnets. "It's an interesting trade-off," he said. "between the cost of an MRI, the quality of the image it can provide, and how much noise it makes."

Bose Corp. is also producing an active noise cancelling headset. It is used primarily by professional pilots who fly small passenger aircraft or helicopters, and by private aircraft owners. Unlike Noise Cancellation Technologies's open-back headset, the Bose product presents a 100 percent seal around the ear with a gel-foam cushion for high-frequency noise attenuation. In addition, low-frequency noise is actively cancelled within the ear-cup itself. A noise sensor picks up the unwanted sound and a speaker produces the antinoise that cancels it out. Desired audio, such as radio communications, are passed through. A pressure servo built into the ear-cup determines what signal to apply to the speaker based on what the microphone is picking up and the noise that is coming from the radio.

For industrial applications, a variety of active noise cancellation systems have been developed by Digisonix. One such system has been installed at a plastics engineering plant in Sheboygan, Wis. The manufacturing process required in-plant ventilation for dust control. But the radial-blade centrifugal fans, which were used to draw air, also produced a 100- to 150-Hz noise. Such low-frequency sound can carry for over half a mile and cannot be easily attenuated by bulky passive silencers manufactured from stainless steel or fiberglass.

Like other active systems, the Digisonix design uses loudspeakers, a microprocessor, and microphones to reduce noise with an opposing sound wave. In the duct application, the system is fitted with two microphones, one placed downstream from the noise and the other placed near the noise source. A loudspeaker is mounted outside the duct between the two microphones. It is attached to the duct via a proprietary elastic polymer that protects the speaker from corrosion. By using adaptive filtering to pick up residual noise as well as the primary at-source noise, the system can reduce both random and discrete sounds, according to Steve Wise, president of Digisonix.

Efficient Coupling

Although simple in principle, advances in active noise control have not come without difficulty. Since Harry Olson, an inventor of acoustic transducers, designed one of the earliest active hearing protection devices in 1953, very little progress had been achieved until recently. Olson's design sensed an incident noise wave and reradiated it with reversed phase to achieve minimum sound pressure.

The use of adaptive filters to capture the residual noise escaping an active control system has facilitated a more effective means of coupling the primary noise source and its cancelling wave. "Adaptive filtering applications are complex," said Penn State's Tichy, "due to the feedback from the secondary (antinoise) sources to the input signal sensor, the time delay between the secondary source output and the error signal input, and the sound wave reflections from the environment."

Further advances in computing sound signals and determining the optimal position for outputting the antinoise, in addition to more robust design of microphones and loudspeakers, have enhanced the overall process. But, as is the case with many active mufflers, system components often are required to withstand high temperatures and adverse weather conditions such as wind and rain. "Due to both the slow and instantaneous changes of the source sound output and the environmental changes affecting the sound propagation speed and wavefront shape, the choice of controlling criterion and its implementability by electronics is critical for system performance," Tichy said.

Because active control applications have been largely untried and, until recently, existed mainly in theory, industry in general has been hesitant to invest in developments within the field, Tichy noted. "The future of active control should be seen with the hope that within the next 10 years active control installations will grow as rapidly as the publications on active noise control theories have grown in the 1980s," he said.

Environmentalists and audiologists assert that new applications could help to reduce noise pollution and hearing impairment rates. There exists a serious need to enforce current regulations and implement effective education and hearing conservation programs, not only concentrating on hearing in industry but also in the environment.

"The issue is to achieve a balance between controls that are affordable, hearing protection that can be effectively used, and setting the allowable noise exposure limit at a value that will protect approximately 90 percent of the population," said William Clark, a senior research scientist at the Central Institute for the Deaf (St. Louis).

Generally, noise levels above 140 dB may induce acoustic trauma, stretching inner ear sensory tissue beyond its elastic limit and incurring some permanent and immediate hearing loss. According to Clark, the mechanism by which the ear is damaged by noise depends upon the sound's intensity. At chronic noise levels under dB, hearing loss generally results from restricted oxygen supply to the sensory cells of the inner ear. Following a buildup of metabolites and waste products the cells degenerate. "The damage is insidious," Clark said. "It's not usually noticed by the individual who's sustaining the loss until it becomes quite significant. About 30 percent of the 16,000 receptor cells in the inner ear can be killed before there's any detectable impairment in the ability to hear." It has been established that at 84 dBA about five percent of the population will sustain a material impairment in hearing, he said, but hearing damage may begin to occur in some people at levels as low as 70 dBA.

Over 25 percent of the American population suffers enough noise-induced hearing loss by the age of 65 to be materially impaired in the ability to communicate under everyday listening circumstances, Clark said.

Clark identified three problem areas effecting hearing conservation: current federal regulations do not cover everybody working with hazardous occupational noise and are not well enforced; the hearing conservation programs that exist in industry could be more effectively administered; and more education programs on noise could be introduced and implemented in schools and the workplace. "I would suggest that there are many companies out there that are covered by the OSHA but are not in compliance with their regulations," said Clark. "In addition, there is no significant penalty or cost to these companies."

According to Alice H. Suter, a consultant on the effects of community and industrial noise and a former OSHA employee, concern for noise pollution has taken a back seat in terms of importance where federal regulators are concerned. She cites the closing of the EPA Office of Noise Abatement and Control in 1982 and a resulting lack of enforcing regulations at the federal level as symbols of the government's neglect of noise control in the past decade.

Congress established the EPA Office of Noise Abatement and Control in 1970, and two years later federal legislators passed the Noise Control Act, which gave the EPA permission to regulate environmental noise emission standards while delegating some power to the states. In its 1974 noise levels document, the EPA recommended a yearly average equivalent sound level of 70 dBA as a limit for the 24-hour day. Currently, the Department of Transportation through the Federal Highway Administration and the Federal Railroad Administration has jurisdiction over federal highway and railroad systems. The Department of Housing and Urban Development sets noise standards for new residential housing developments.

The stated federal aim for deregulation of environmental noise standards was that it allowed state and municipal regulators a greater degree of flexibility to implement their own rules tailored to suit individual community needs, Suter said. However, instead of providing support to communities, EPA regulations that remain on the books but are not now implemented tend to tie the hands of state and municipal regulators, Suter said. They preempt attempts by local officials to introduce new rules, she said, adding that the closing of the noise abatement office has stymied federal funding of technical support that was previously offered to communities.

The 1970 William-Steiger Occupational Safety and Health Act granted the Department of Labor permission to implement noise regulations for manufacturing. The Labor department set the noise standard at 90 dBA for eight hours of continuous sound. (It allowed 5 dBA to be added to this level for every halving of time exposure under eight hours, up to a noise level of 115 dBA, e.g., 95 dBA for four hours of continuous noise.) Employees were required to implement engineering controls where feasible when the noise exceeded these limits. In 1983, the regulations were amended to require that hearing conservation programs be implemented for noise exceeding the equivalent of 85 dBA for eight hours. Suter said employers generally complained that implementing engineering noise controls was often either economically or technically infeasible.

Defining Limits

Aviation within the United States has seen some significant advances in noise control regulations since the first noise limits were imposed in 1969. Currently, Part 36 of the federal aviation regulations establishes three stages of noise regulations for large transport and turbojet planes. (Stage 1 is the noisiest, Stage 3 the quietest.) Early generation aircraft certified as Stage 1, such as the Boeing 707 and McDonnell Douglas DC-8, were phased out of operation within the United States in 1985. Recent legislation mandated a phaseout of Stage 2-type certified aircraft, such as the Boeing 727 and the McDonnell Douglas DC-9, by the year 2000. Stage 3 regulations, which apply to aircraft that are currently coming off the production lines like the Boeing 757 and 767, will then go into effect.

According to a 1987 study conducted by Kenneth M. Eldred, standards director of the Acoustical Society of America (New York), the ban on Stage 2 aircraft would greatly reduce the noise exposure levels created by aircraft. He estimated that the number of people exposed to an average sound level of 65 dBA in the year 2000 would be reduced by 70 percent as compared to 1985.

Member states of the European Community (EC) follow guidelines set by the International Civil Aviation Organization (Montreal). In 1990, the EC adopted a recommendation that it would phase out Stage 2 aircraft by the year 2002.

In a move that would mainly affect people in the workplace, a 1989 EC directive required the adoption in 1992 of noise regulations by individual member states relating to the design and construction of machinery. According to William Lang, chairman of the International Institute of Noise Control Engineering, the eventual implementation of the 1989 directives is in doubt, because of the uncertainty as to whether the EC will successfully achieve a common market free of all trade barriers.

Most voluntary noise control standards in the United States are coordinated by the American National Standards Institute (ANSI). Individual organizations within ANSI responsible for acoustic standards include the American National Standards Committee S-I, Acoustics, on physical measurement and instrumentation; the American National Standards Committee S-3, Bioacoustics, on methods and instrumentation to evaluate the effects of noise on people; ASTM for measurement of noise reduction of materials and building structural systems; and the Society of Automotive Engineers on measurements of noise radiated from vehicles.

PHOTO : Silence is golden. To muffle sounds in an office ventilation duct, Digisonix devised an active cancellation setup consisting of two microphones (purple), a loudspeaker (blue), and a controller (yellow). The controller analyzes noise picked up by the microphones, producing a 180-degree out-of-phase antinoise that is played through loudspeakers to cancel out offending sounds.

PHOTO : Cutting the clatter. The Digisonix noise cancellation system has been installed at a plastics engineering plant in Sheboygan, Wis., to muffle a dust-control ventilator fan that produced an annoying 100- to 150-Hz noise. Such low frequencies can carry for over half a mile and are not easily attenuated by passive silencers.

PHOTO : Nobody beats this muffler. Noise Cancellation Technologies is developing an electronic muffler for cars that could be available by 1994. The system employs a feedback control design to generate an antinoise signal at the end of an automobile exhaust pipe.

PHOTO : Clear communications. Bose Corp. makes an active noise cancelling headset for pilots. A tight seal around the ear attenuates high-frequency noise, while low frequencies are actively cancelled via a sensor and antinoise generator.
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Title Annotation:includes related article
Author:O'Connor, Leo
Publication:Mechanical Engineering-CIME
Date:Jun 1, 1991
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