Noise as a Technological and Policy Challenge

Noise can be controlled at the source, along its path, or at the location of the receiver.

The Source-Path-Receiver Model
For a systems approach to the technological issues of noise reduction, the source-path-receiver model, which has been in use for many years, can provide a helpful starting point (Bolt and Ingard, 1957). This model enables engineers to treat each part of a system separately, thus facilitating identification of the technical challenges for that part.

Noise can be controlled at any stage of the model—by reducing it at the source, interfering with its transmission paths, and/or changing the characteristics of the receiver. The descriptors most frequently used in a noise control system are the levels of sound emission from the source, attenuation level of noise along the path(s), and the sound-pressure level at the location of the receiver (i.e., imission). A source is usually characterized by the total acoustic power it radiates into the environment and, less frequently, by the directivity of the sound radiation. Because a wide range of acoustic powers can be radiated by sources (and sound pressures at receiver locations), a logarithmic scale is used for each descriptor.



The sound power level, the logarithmic ratio of the acoustic power of the source and a reference power (10^-12 W), can be conveniently expressed in bels. The acoustic power is often A-frequency-weighted to bring the level more in line with human perception (IEC, 2002). For practical reasons, most industries prefer to use the decibel (dB) for this ratio (1 bel equals 10 dB).

An A-weighted level frequently used to describe cumulative noise exposure is the day-night sound level, L_dn (the time-weighted average level over a 24-hour period, but with 10 dB added for nighttime hours [10:00 p.m. to 07:00 a.m.]). The abbreviations dBA or dB(A) are often used as the unit of an A-weighted level, even though the level, and not the decibel, is A-weighted.

At the receiver’s location, the sound-pressure level, also expressed in dB, is 20 times the logarithmic ratio of the root-mean-square sound pressure and a reference pressure (2x10^-5 N/m²). Sound-pressure levels are also often A-frequency weighted.

The Source
The acoustic power radiated by a source is generally a very small fraction of the total electrical or mechanical power of the source. Shaw (1975) estimated this fraction to be 10^-5 with a range of 10^-3 to 10^-7 for a wide variety of sources—from dishwashers to the airplanes of that era. A more recent illustration can be derived from a current European Union (EU) Directive on noise emissions from outdoor equipment (EU, 2005). The examples below are Stage II values, which became effective on January 3, 2006.

Example 1. A welding and power generator with an electrical rating of 10 kW must radiate no more than 100 dB of acoustic power. The ratio of the two powers is 10^-6. This ratio is not very different from those of a wide range of electrical ratings.

Example 2. A compressor with a net installed power of 15 kW must radiate no more than 97 dB of acoustic power. The ratio of the two powers is 0.33?10^-6. The ratio is smaller for net installed powers < 15kW, but not very different for powers > 15 kW.

Aerodynamic noise generated by turbulent flow or fluctuating lift forces is a common problem for aircraft, air-moving devices, and air-cooled machinery. Many sources generate aerodynamic noise, and, although once again the acoustic power radiated is a very small fraction of the power of the flow, aerodynamic noise creates significant problems in homes, offices, and communities.

Because the acoustic power radiated is small, even if the mechanical motion of a noise source is known, it is often very difficult to predict the level of sound radiation. The propagation of sound through a structure and subsequent radiation from the structure further complicate the situation.

The Path(s)
Theoretically, sound waves outdoors spread geometrically at a rate of 6 dB per doubling of distance. However, this seldom occurs in the real world because of sound absorption at the ground surface, ground geometry, air turbulence, and wind and temperature gradients. Although propagation models have been developed, conditions are constantly changing, and predicting noise levels at a receiver outdoors is a complex undertaking.

Silencers and mufflers are common ways of attenuating noise along a path, indoors or outdoors, and their performance can be predicted or measured. However, these devices are frequently expensive add-ons that can raise the cost of a product.

For equipment used indoors, attenuation depends on the sound-absorptive properties of room surfaces, room geometry, and the scattering of sound from objects in the room. Thus, designing machines to ensure that a given sound-pressure level reaches the receiver can be challenging indoors as well.

The Receiver
Measuring sound-pressure levels, usually A-weighted, at receiver locations is a straightforward process. However, difficulties arise in correlating the physical magnitudes of the sound to the subjective and physiological reactions of the receiver. The human ear is, perhaps, the most variable receiver in its response to physical signals, because the principal effects of noise on people are both psychological and physiological.
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Prolonged exposure to
high noise levels may have
physiological effects.
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The psychological effects of noise include annoyance, sleep disturbance, interference with communication, and adverse effects on learning, social behavior, job performance, and safety—all of which affect quality of life. However, people can adapt to many sounds. For example, they may become accustomed to sounds that were extremely disturbing when they were first heard. After hearing a sound over a long period of time, some people may find it tolerable, or even acceptable.

An example of this is someone who purchases a residential property adjacent to a busy highway. One of the motivating factors for the purchase is the reduced price of the property because of its exposure to highway noise; an equivalent property in a quiet neighborhood would be significantly more expensive. At first, the buyer may find the traffic noise annoying. But after a period of time, the negative psychological effect may fade as he or she becomes accustomed to the noise.

The ability of humans to adapt to noise, particularly when there are financial advantages in doing so, presents a major difficulty in determining metrics for community noise. It is difficult to determine physical parameters for annoyance, loudness, sleep disturbance, and other effects of noise that are subject to adaptation.

Prolonged exposure to high noise levels may have physiological effects, such as significant hearing loss caused by the atrophy of hair cells in the inner ear. Noise-induced hearing loss (NIHL) is well understood and is subject to international standardization (ISO, 1990). NIHL is frequently accompanied by tinnitus, a persistent ringing in the ears, which is sometimes severe, even for people with mild hearing impairments. Even relatively low levels of noise that are well below the threshold for causing hearing loss can have adverse effects on the auditory system—such as masking signals and alarms and interfering with speech.

Noise may also increase stress and damage the cardiovascular system. Such non-auditory effects are under investigation but have not been subject to international standardization.

Workplace Noise
Excessive noise in the workplace, which has been a problem since the industrial revolution, remains a problem despite years of effort. In a study of noise and military service in 2005, the Institute of Medicine provided an assessment of costs to the federal government for compensation for hearing loss incurred during military service. “At the end of 2004, the monthly compensation payments to veterans with hearing loss as their major form of disability represented an annualized cost of some $660 million . . . [W]ith tinnitus as the major disability . . . [the cost is] close to $190 million on an annualized basis” (IOM, 2005). Corresponding costs for civilian federal employees in 2001 were reported by the U.S. Army Center for Health Promotion and Preventive Medicine to be about $43.8 million (CHPPM, 2003). Thus the total cost for compensation to veterans and other federal employees is almost $1 billion per year. Similar figures for non-federal employees in American industry are not available.

The technological challenges to controlling noise in the workplace include (1) using available technology to solve as many of the problems of excessive noise as possible and (2) developing new technology for addressing the remaining problems. According to Bruce and Wood (2003), “The lack of clear direction at the national level is the reason this very solvable problem remains a challenge to our society . . . we lack the will to resolve this problem.”

Highway Noise
The principal tactic for reducing surface-transportation noise is the noise barrier, a palliative measure that does nothing to reduce the level of noise radiated by vehicles in motion (i.e., the sources). The Federal Highway Administration (FHWA) has reported that, by the end of 2004, more than 3,500 km of noise barriers had been constructed by 45 state departments of transportation and the Commonwealth of Puerto Rico at a cost of more than $2.6 billion ($3.4 billion in 2004 dollars) (FHWA, 2006). Some roads have barriers on both sides, others on only one side.

The cost per square meter of a noise barrier depends on its height, construction materials, and terrain (Polcak, 2003). However, the effectiveness of roadside noise barriers is marginal. They provide a modest degree of shielding from traffic noise for residences immediately behind the barrier, but their effectiveness diminishes rapidly with distance from the barrier (Daigle, 1999).

At the present time, a low level of interior noise, particularly in a passenger vehicle, is an important sales feature. Thus market forces are driving the automotive industry to invest large sums in research and development to reduce interior vehicle noise.

No comparable efforts are being made to reduce exterior vehicle noise. At highway speeds, most exterior noise is produced by the interaction between tires and the road surface. Table 1 shows crossover speeds—the speeds at which road/tire interaction noise is equal to the power train noise—for different types and vintages of vehicles. At higher speeds, tire/road-surface interaction noise dominates.

A body of evidence now indicates that significant noise reductions can be achieved by reducing tire/road-surface noise (Donavan, 2005). The technological challenge is to engineer road surfaces and tires to reduce noise levels.

TABLE 1 Crossover Speeds for Various
Types of Vehicles

Vehicle Type	Cruising	Accelerating
Cars produced from 1985 to 1995	30–35 km/h	45–50 km/h
Cars produced since 1996	15–25 km/h	30–45 km/h
Heavy vehicles produced from 1985 to 1995	40–50 km/h	50–55 km/h
Heavy vehicles produced since 1996	30–35 km/h	45–50 km/h

Source: Adapted from Sandberg and Ejsmont, 2002.

Aircraft Noise
In the 1960s, high-bypass-ratio jet engines were introduced that made passenger aircraft much quieter than their predecessors. As a result, noise generated by passenger jets sitting or taxiing on the ground and during landing, takeoff, and cruising was reduced significantly. However, aircraft operations have also increased considerably since 1960. Thus, although individual aircraft have become quieter, as required by the International Civil Aviation Organization, noise levels surrounding airports have not dropped commensurately. In a circular published by the Transportation Research Board in 2006 and in a National Research Council (NRC) report in 2002, aviation noise was still identified as a critical environmental issue.

To provide relief for residents in communities near airports, the Federal Aviation Administration (FAA) has been funding upgraded sound insulation in homes, which significantly reduces noise inside residences. The Residential Sound Insulation Program is supported by the FAA under the Airport Improvement Program and by local contributions. Near O’Hare Airport alone, 6,179 homes had been insulated by the end of 2006 at a cost of $189 million. An additional 770 homes near O’Hare will be insulated, at a total cost of $21.6 million. The sound-insulation program improves the quality of life inside residences but does nothing to reduce outdoor noise. Once again the problem is being approached reactively, at the path stage rather than the source stage.

The engineering challenge for the future is to develop new technologies for quieter aircraft. Major steps in this direction are being taken (NASA, 2007), but, given the lead time for new designs and the phase-out time for current aircraft, many years will pass before these efforts lead to significant reductions in airport noise.

Noise in Urban Areas
Noise in urban areas continues to be a problem. A hotline installed several years ago in New York City for complaints about noise logged more than 335,000 complaints from July 2004 to June 2005 (Bronzaft, personal communication).¹ Many complaints were about noise generated by people, for which there are probably no engineering solutions. But in a 2004 report that ranked complaints by type, several that did have engineering solutions ranked high on the list—highway or street traffic, motorcycles, garbage and delivery trucks, and airplanes and helicopters (Bronzaft and Van Ryzin, 2004).

The control of noise in urban areas is regulated by building codes and local ordinances. Shoddy construction practices made possible by lenient building codes can facilitate the transmission of noise in buildings, and local ordinances may not prevent unnecessary noise from everyday activities. Enforcing noise codes can be difficult if authorities have not been properly trained or do not have the necessary equipment to make physical measurements or if the judicial system does not provide sufficient support for enforcement authorities.

In addition, requirements related to noise may differ greatly from one administrative area to the next. This lack of uniformity presents problems that must be addressed on a regional or, perhaps, national basis. A new noise ordinance for New York City that became effective on July 1, 2007, might very well serve as a model for noise control in other major urban areas (NYC, 2007).
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U.S. companies face
competition from foreign
manufacturers that produce
quieter products.
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One serious adverse effect of excessive noise is interference with the education of children. Noise may enter the classroom from outside or inside (e.g., from air conditioners and other equipment). A summary of noise issues related to hearing-impaired children published in the Federal Register in 1998 (FR, 1998) led to American National Standard S12.60 on acoustical conditions in classrooms (ANSI, 2002). This problem has also been addressed in an NRC report, Review and Assessment of the Health and Productivity Benefits of Green Schools: An Interim Report (NRC, 2006).

The challenge for engineers is to provide technological guidance to officials and legislators who are drafting codes, guidelines, standards, and ordinances for controlling noise levels in homes and public buildings.

Product Noise and U.S. Competitiveness
In recent years, the demand for quieter products has increased significantly, particularly in parts of the world to which the United States exports consumer goods and industrial equipment. U.S. companies are facing growing competition from foreign manufacturers that produce goods with lower noise levels. In many countries in Europe and Asia, people purchasing white goods are provided with information on noise levels.

For U.S. products to compete in those markets, they must be designed to meet the criteria of overseas buyers. Exported, low-noise products are becoming increasingly important to our economic future. At present, no international agreement on a labeling system has been negotiated to provide noise-level information to consumers, but U.S. companies would be wise to label their products in ways that can be easily understood by consumers abroad.

U.S. producers also face regulatory challenges, particularly in the EU where noise-emission requirements have been adopted for many products used outdoors (EU, 2005). American manufacturers must now meet these requirements for their equipment to be sold in the EU. In addition, noise-imission requirements in manufacturing facilities are more stringent in the EU than in the United States (EU, 2003). Thus a piece of equipment that can be installed in an American facility may not be acceptable for a facility in the EU.

The challenge for engineers is to assist U.S. manufacturers in acquiring the technical expertise to ensure that U.S. products are designed to meet low-noise requirements in overseas markets.

Educational Challenges
Courses in noise control engineering are being taught in about a dozen U.S. universities. However, there are some indications that the demand for trained professionals in the field exceeds the supply and that industry and government are hiring people in other fields, such as aerodynamicists, crash-worthiness engineers, physicists, mechanical engineers, vibration specialists, electrical engineers, and others, to perform noise control engineering work (Bernhard, 2005). Many educators argue that engineers need graduate training and a degree in the field to be fully effective.

The challenge today is to ensure that the U.S. educational system continues to produce well trained engineers with leadership skills in the development of new technologies necessary for a quieter world.

Noise Policy
The United States does not have a cohesive, coordinated policy for controlling noise. Nor does any federal agency have oversight responsibility. With the exception of aircraft and some aspects of railway noise, product-emission regulations are not enforced for major sources of noise. Current federal, state, and local policies are largely uncoordinated and often obsolete, and enforcement of noise regulations is uneven at best.
Meanwhile, government authorities outside the United States, particularly in the EU, are moving rapidly to protect their own interests by adopting national and international noise control policies (Beranek and Lang, 2003). The challenge to the United States is to develop an effective national noise control policy that covers products from conception to implementation.

Political Challenges
Up to now, practicing engineers and others with backgrounds in engineering have played a limited role in the development of noise control policies in the United States, even though they have the best understanding of the technological challenges involved. The challenge for engineers is to play a decisive role in the future.

As Senator John H. Sununu, an NAE member, warned almost two decades ago (Sununu, 1989):

It is clear to engineers and scientists . . . that science and technology—engineering—not only continue to play a role in improving quality of life . . . but also are critical to developing and implementing policy at the national and international level . . . . I stress this point because I am concerned that engineers in general have been negligent in their direct participation in the process of shaping and implementing public policy.

Engineers can, and must, play an active role in defining the requirements for a national noise control policy. Engineers can clarify for legislators the challenges and limits of noise control technology. They can provide policy makers with technical knowledge of the major sources of noise, the noise emissions of competitive products in the world, the relative importance of each noise source to properly balanced noise reduction, the techniques available for reducing noise from various sources, and the research and development programs necessary to find ways to reduce noise from sources for which no satisfactory techniques have been developed (Beranek and Lang, 2003).

Conclusions
The United States urgently needs a new national policy based on technological innovation with the emphasis on citizen/industry/government cooperation and with funding for new research projects, as needed. A national noise control policy would improve the quality of life in America. It would conserve the hearing of workers in our industries, enhance their productivity, and reduce the number of accidents. It would reduce noise in our cities and preserve quiet in our wilderness areas. Our children would benefit from a quiet classroom atmosphere that enhances their learning and social skills.

A carefully considered, coordinated policy would encourage American manufacturers to produce low-noise products and put the United States in a position to work with other nations on the development of a global noise control policy that could lead to a quieter world for people everywhere.

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