Noise Control
Noise control refers to the methods and strategies employed to reduce unwanted sound, which can be disruptive or harmful to human health and well-being. Noise is scientifically defined as random oscillations of air that can be perceived as irritating or damaging, particularly in high decibel levels. The evolution of noise control techniques has been significant since the 20th century, including the use of sound-absorbing materials, traffic noise barriers, and innovations in machinery design to minimize noise production. Sources of noise can be airborne or structure-borne, necessitating varied approaches for mitigation, such as enclosing noisy equipment or employing shock absorbers.
Legislation addressing noise pollution has developed over centuries, culminating in significant acts like the Noise Control Act of 1972 in the U.S., which established standards for industrial machinery and mandated environmental noise research. Chronic exposure to noise can have serious health implications, including sleep disruption, increased stress, and permanent hearing loss. In residential areas, effective noise management often incorporates design principles that reduce sound transmission, while personal protective equipment like earplugs and earmuffs plays a crucial role in safeguarding individuals in noisy environments. As noise control technology continues to advance, the focus remains on improving quality of life and protecting public health in increasingly urbanized settings.
Noise Control
Summary
Scientifically, noise is defined as an intermittent random oscillation of the air that may be perceived by humans. Because the effects on humans are usually unwelcome, often quite disturbing, or even dangerous both physically and psychologically, noise may be defined as any unwanted sound. During the twentieth century, as humans became increasingly aware of the dangers of noise, many techniques for mitigating the problem evolved. These included sound-absorbing materials for buildings, barriers to reduce traffic noise near residential areas, reducing noise at its source, preventing noise from accumulating in the ears, and active noise control by sound cancellation. Improvements on these techniques and methods, as well as research into new ones, continued into the first decades of the twenty-first century.
Definition and Basic Principles
A noise problem can arise from one of three sources: It may be airborne, structure-borne, or a combination of the two. Airborne noise control is achieved either by reducing noise in the environment of a listener or reducing noise at its source, for example, by designing quieter machinery or enclosing loud machines. Enclosing a noise source to reduce propagated sound presents the somewhat daunting challenge of balancing contradictory factors; the enclosure must be sufficiently airtight to contain noise while allowing for ventilation, power cable access, and material flow. Structure-borne noise can be suppressed by shock absorbers to isolate the machinery from the enclosure. Ductwork that penetrates the isolation walls must be resiliently supported on flexible hangers. When extremely quiet environments are mandatory, such as for a recording studio, it is easier to isolate the room from its environment rather than attempt to suppress all possible sources of impinging noise. This is best achieved by forming an acoustically and vibrationally isolated room within the building. This requires soundproof walls and ceilings, as well as a resiliently supported floor with isolated walls and ceiling to attenuate structure-borne vibration. Protecting individual listeners from dangerous noise levels is best achieved by using earplugs or tight-fitting acoustical earmuffs.
Background and History
Attempts to control noise by legislation have occurred for centuries. Bern, Switzerland, for example, has, over the centuries, imposed dozens of noise ordinances, including regulations against singing and shouting in the streets (1628), noisy conduct at night (1763), industrial woodworking noise (1886), and loud automobile sounds (1913). With increasing technology has come increasing noise. By the 1960s, it was realized that exposure to loud industrial sounds would eventually cause employee hearing loss. The Walsh-Healey Act of 1960 attempted to control this noise, but standards were not specified until 1966 after the Bureau of Labor Standards accumulated data indicating that daily exposure to sounds above 85 dB would eventually cause permanent hearing impairment and that at higher sound levels, the detrimental effects occurred sooner and more severely.
By the 1970s, a heightened environmental awareness recognized noise as an insidiously dangerous and pervasive epidemic pollutant not confined to the workplace. Even the moderate levels experienced in the nonwork environment will have a cumulative effect, eventually causing hearing loss. Noise occurring during sleep, even though it may not awaken the sleeper, can result in a less soporific slumber by interfering with sleep stages and adversely affecting physical and mental health. In the first concerted effort to control environmental noise, Congress enacted the Noise Control Act in 1972. New content was also added to the 1990 Clean Air Act in regard to noise pollution. In February 2024, Congresswoman Nikema Williams reintroduced the Negating Neighborhood Noise Act, which signaled the continuing problem of noise control, especially in urban areas. In the twenty-first century, many local ordinances concerning noise pollution exist.
How It Works
Effects of Noise on People. Noise can impair performance and adversely affect human health by increasing stress, interfering with sleep, producing anxiety, and raising blood pressure. Chronic exposure may also cause increased hormone excretion and psychological depression. Continuous exposure to sounds 90 dB or greater will cause permanent hearing loss, particularly in the higher frequencies. Sick people are particularly susceptible to excessive environmental noise; it can lengthen convalescence and necessitate additional medical treatments. It is also known that long-term exposure to moderate sound levels (85 dB or less) is the leading cause of presbycusis in older adults.
Legislation for Noise Control. Pressured by environmental organizations and concerned citizens, Congress enacted the Noise Control Act (1972), which set noise emission standards for industrial machinery, commercial products, major appliances, aircraft, and motor vehicles. The act also required the US Environmental Protection Agency (EPA) to coordinate all existing federal programs relating to noise research, control, and regulation. For the first time, environmental noise-impact studies would be required for new industrial sites and interstate highways. The EPA was also mandated to provide technical assistance to state and local governments planning to address noise mitigation and hazards. Since 1983, the Occupational Safety and Health Administration (OSHA) has regulated industrial noise exposure for workers; this limits the permissible exposure to 90 dB over an eight-hour workday and mandates regular audiometric testing. Further federal legislation was added to the 1990 Clean Air Act.
Subsequent to federal legislation, municipalities plan zoning to keep housing developments isolated from industrial regions. Noise ordinances, for day and night, have been enacted for most residential regions as well as industrial sites. Highway noise is abated in residential areas by concrete noise-reducing barriers installed along interstate highways.
Design for Quiet. It is considerably easier to design noise control into new buildings than to attempt a retrofit when unanticipated problems appear. Incorporated noise control does not substantially increase construction costs, while later corrections can be quite expensive.
In apartment buildings, noise transmitted through walls is always problematic; the porous insulation typically used to retard heat flow does not effectively absorb transmitted sounds. Effective absorption of transmitted noise is best achieved with massive materials (such as brick), but this is at variance with modern construction practices using lightweight materials. Living spaces located near busy freeways should be designed to reduce the transmitted noise to no more than 60 dB.
Concert halls designed for metropolitan areas present unique challenges. Automotive traffic noise is ubiquitous, and low-jet flyovers are not uncommon. The interior must be isolated from conversation in the foyer as well as from ventilation and plumbing sounds if one is to hear the nuances of a performance. Shielding from external noise is best achieved by constructing a box-within-a-box—an outer shell enclosing a spacious foyer with several well-isolated performance halls located in the interior area. Ventilation noise is controlled by the careful design of ducts to minimize turbulence. This design was successfully employed for the Kennedy Center in Washington, DC, where the interior background noise never exceeds 30 dB despite the extremely noisy location.
Applications and Products
There are three places where noise can be reduced: at its source, during transmission, and at the receiving end.
Controlling Noise at Its Source. Traffic noise is best reduced at its source by improved engine enclosures and better tire design. When interstate highways pass near residential areas, the next best means of controlling traffic noise is to block it by lining the highway with concrete barriers or sound-absorbing vegetation. Internal automotive noise is best decreased by reducing air turbulence with more aerodynamically efficient shapes, employing additional sound isolation, and suppressing vibration.
In the vicinity of airports, jet noise is increasingly problematic because ever-more powerful engines create more sound. To reduce the radiated noise, two methods are employed: surrounding the engine's moving parts with acoustic linings to attenuate high-frequency jet whine and reducing the number of rotor blades and stator vanes, which may diminish power.
Construction equipment produces notoriously high decibel levels, both in the operator's cab and outside it. If the cab is open, the operator has no recourse but to use earplugs. Equipment manufacturers, however, have begun producing machines with enclosed cabins carefully designed to attenuate the interior noise level, but reducing external noise has been considerably less successful. On occasion, rubber tractor treads have been employed instead of metal; they produce less noise, but they are considerably less durable.
If noise cannot be suppressed at its origin, it can often be contained in a sound-attenuating structure. Inert and relatively massive walls reduce sound transmission, the actual decibel loss being dependent on frequency, sound absorption, and the tightness of the enclosure. An effective enclosure must also minimize structure-borne noise. This is best achieved by mounting vibrating machinery on resilient supports and supporting penetrating ducts on spring hangers. If the duct connects to the machine, as in the case of ventilation fans, the duct must be vibrationally isolated from the fan housing by a flexible hose.
Controlling Transmitted Noise. Propagated noise can be either airborne or structure-borne. Airborne noise is generated by vibration, impact, or airflow turbulence. In closed rooms, sound is both reflected from the surfaces and transmitted through the enclosures, the material determining the proportion reflected and transmitted. Absorbent walls reflect less energy, reducing the interior sound level, but the transmitted sound may be increased. Hard surfaces reflect most of the sound, creating the phenomenon termed reverberation. A long reverberation time is detrimental in any room where speech intelligibility is important; reverberation is best controlled by applying acoustical absorbing materials to interior surfaces. Transmitted structure-borne noise is extremely difficult to control and is best managed at the source.
Airborne transmitted noise may arise from two different sources: external noise penetrating the structure or internal noise transmitted through walls, ducts, or openings. For external noise, barriers or massive walls must be employed to attenuate the sound. If the noise is produced in the same building and transmitted through walls or ducts, specific procedures must be implemented. Relatively massive walls best prevent noise transmitted through apartment walls; if this is not possible, another method of control is to use staggered studs. The framing studs on one side of the wall are staggered, with those constituting the other side. The two interior walls do not share the same supporting studs, and there is airspace between the walls, reducing transmitted sound. Noise transmitted through air ducts is a pervasive problem. The first line of defense is the source; ventilation equipment should be designed to be as quiet as possible while still performing its desired function. Turbulence in the ducts, heard as a hissing sound, is best controlled by keeping the airflow slow and eliminating sharp turns in the ducts. When turns are necessary, turning vanes consisting of curved surfaces mounted inside the duct smoothly direct the airflow around corners. There is no excuse for sound-leak openings around conduit and pipe penetrations, and such are typical of shoddy workmanship. The optimum noise reduction acceptable for a room depends on the room's intended purpose. The requirements for music rehearsal space are much more stringent than the requirements for many other spaces.
Controlling Noise at the Receiver. When noise cannot be adequately controlled at its source or reduced during transmission, the last resort is to attenuate it at the ears of the receiver. Hearing protection was first instituted for outdoor airport workers exposed to the extremely high levels of noise generated by jet aircraft. Gradually, hearing protection became mandatory for noisy industrial environments, with the type of protection dependent on the noise spectrum. It is generally less expensive for industry to reduce noise at the source or isolate workers from noisy environments than to provide adequate protective devices. The cost of these devices usually exceeds the original capital outlay. Employees must be trained in their correct usage, monitored to ensure they are being worn, and possibly compensated when hearing loss occurs because of noncompliance.
There are three primary types of ear-protection devices: earplugs, earmuffs, and ear caps. They must form an acoustic seal across a broad frequency range to be effective. Earplugs achieve this by being inserted into the ear canal. Typically, they are made from malleable dense foam that tightly fills the ear canal when inserted, attenuating noise by about 20 dB across the frequency spectrum, but they are unable to prevent sound from reaching the inner ear through bone conduction by the skull. Earmuffs cover the ear, forming an acoustic seal around the ear. Being external to the ear, they also reduce bone-conduction sound. The attenuation provided by the muff depends on its construction and how well it seals the ear. Ear caps fit over the entrance to the ear canal and protrude only slightly into the interior. They are held in position by a tension band going over the head or under the chin.
Electronic noise-reduction earmuffs use the principle of adding two out-of-phase signals to reduce the sound level. A small microphone on the earmuff surface records the surrounding noise, inverts the signal phase, and adds it, through small speakers in the earphones, to the original signal, effectively canceling the sound. These active noise suppression systems are quite effective and are widely accessible to consumers.
Careers and Course Work
Noise control, like other environmental problems, requires people with broad scientific, technical, and creative skills. Because the global soundscape is not becoming less noisy, despite heightened awareness, public protests, and new legislation, there is an ever-increasing need for qualified people in noise control engineering and audiology. Vibration and means of suppressing it are best learned by obtaining a Bachelor's degree in mechanical engineering or physics. If one wishes to work in the field of noise reduction by means of signal modification, this is best achieved by a degree in electrical engineering. Audiologists assess and diagnose hearing problems, as well as design hearing-conservation programs for industry and initiate basic research on hearing loss and its prevention by noise control. Becoming a certified audiologist requires a Bachelor's degree in communication sciences and a doctoral degree in audiology.
Social Context and Future Prospects
Engineers working in the industry can provide valuable assistance in developing quieter products and designing sound-insulating areas to separate industrial workers from noisy machines. In noise control engineering, there is a need for innovative ideas and new technologies for noise and vibration measurement and control in industrial and commercial applications. Sound meter applications downloaded on smartphones allow individuals to better monitor the noise level of any environment.
As noise-induced hearing loss becomes more problematic with an aging population subjected to a lifetime of excessively loud sounds, audiologists need to assist those with hearing loss by fitting hearing aids. Manufacturers of hearing aids are continuously researching how to produce smaller, more accurate, and higher-quality devices for those with hearing loss. Considerable research effort is also being extended on ear-covering devices to attenuate incoming sound before it reaches the eardrum. Active noise reduction technologies have become popular additions to various products, including cars and headphones. A wide variety of noise-canceling headphones and earbuds is available to consumers, and some hearing aids have wireless smartphone connectivity built in to allow users to easily adjust noise levels and reduce background noise in different environments.
As the twenty-first century progressed, more advances in noise control technology emerged. New sound control materials were developed with increased efficiency and were also more environmentally friendly. Innovations in the design of noise barriers and the idea of incorporating noise control into architectural design were explored. The automotive and personal electronics industries also made advances in noise control technology. Managing noise and its adverse effects remained an important social and economic concept in addition to being tied to personal health and wellness.
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