Introduction to The Sound of Silence
We live in a noisy world. From trucks rumbling down the highway to earsplitting jackhammers attacking concrete sidewalks, it is difficult to escape noise. During the course of an average day, people are subjected to a wide variety of annoying, and even maddening, sounds: wailing sirens, barking dogs, the ringing of cell phones, and the roar of jetliners passing overhead. Noise is such a constant part of modern life that when a moment of pure silence arrives, it seems positively alien.
The barrage of unwanted sound assaulting people's ears in our society is often called noise pollution. It is an apt term, because not only does noise degrade one's environment, but it can also have harmful effects on health. Scientists have come to understand that reducing the amount of noise in the environment is important to people's enjoyment of life and to their physical and mental well-being.
It is probably impossible to return to the peace and quiet that existed before the Industrial Revolution of the 1700's and 1800's, when loud machines first came onto the scene. However, scientists and engineers think the world does not have to be as noisy as it is. Researchers are seeking new ways to stifle noise with innovative sound-absorbing materials, new designs for noise-reducing devices such as car mufflers, and high-tech electronic noise-cancellation systems that pit sound waves against each other. With these innovations and advances in other areas, such as architectural design and construction engineering, scientists and engineers are helping to at least create islands of quiet.
Noise As A Health Risk
Physicians might say it's none too soon. The health effects of inescapable noise have long been known to the medical profession. The most obvious effect is hearing loss. According to the National Institute for Occupational Safety and Health, a federal agency that investigates workplace hazards, more Americans suffer from work-related hearing loss than from any other occupational impairment. In addition, the National Campaign for Hearing Health, a public-service effort sponsored by the Deafness Research Foundation, reported in August 2000 that one-third of all cases of hearing loss in the United States are caused by noise in the workplace.
But noise doesn't have to be ear-shattering to be harmful. Any unwanted sound that continues indefinitely and is beyond a person's control can have effects on health. The U.S. Environmental Protection Agency has reported that constant noise may contribute to high blood pressure and heart disease. Noise can also hamper the immune system's ability to fight infection and disease, and it can cause psychological problems such as fatigue and irritability.
These reactions to noise are a biological inheritance from our prehistoric ancestors. To survive, early humans had to respond quickly to threats in the environment, such as dangerous animals. Often, the first warning of such a threat was the sound it made. For example, a person who heard the roar of a dangerous animal had to be instantly ready to engage in a fight to the death or run for safety. To spur this “fight or flight” response, the human body evolved to suddenly produce large amounts of adrenalin, a substance that increases heart rate and elevates blood pressure to provide muscles with additional oxygen. In our modern society, noises are usually more annoying than threatening, but our physical responses have not changed.
Because noise can be stressful—or at the very least, distracting—reducing noise has long been important in places such as libraries and art museums. But the quest for quiet is spreading, in part because people put a value on peaceful surroundings. For example, houses located close to busy highways are usually worth substantially less than ones in quiet neighborhoods. Prospective buyers are usually willing to pay more to avoid being subjected to the constant drone of traffic.
The Characteristics of Sound
All sound, from the harsh clanging of a bell to the soothing notes of a violin, is the result of vibration. When a surface oscillates (moves back and forth), it emits waves into the surrounding air (or into whatever medium, such as water, that surrounds it). These waves are called “compression waves.” By way of analogy, they can be likened to the waves created when you toss a pebble into a pond. However, unlike water waves on the surface of a pond, which spread outward in two dimensions, compression waves travel in three dimensions. If you could see sound waves, they would look like expanding spheres moving away from the vibrating object.
One important characteristic of sound waves is their frequency, the number of waves produced by a vibrating source per second. Scientists measure frequency in units called hertz. One hertz equals one cycle (vibration, or sound wave) per second. Humans can hear sounds that are between about 20 and 20,000 hertz. If an object is vibrating at frequencies outside of this range, our ears cannot detect the sound.
Whether a sound is high-pitched like a whistle or low-pitched like a cello depends on its frequency. The frequency of sound waves is related to their wavelength, the distance between the peaks of the waves. The higher the frequency of sound waves, the shorter their wavelength, and the higher their pitch. Conversely, the lower the frequency of sound waves, the longer their wavelength, and the lower their pitch.
Two other important characteristics of sounds are their intensity and loudness. The intensity of a sound is a measure of how forcefully the object vibrates. Scientists measure the intensity of sounds in units called decibels. On the decibel scale, every increase of 10 decibels represents a tenfold increase in wave power. For example, the sound of an ordinary conversation is typically about 60 decibels; heavy city traffic can generate noises as loud as 85 decibels; the sound level in a crowded nightclub can reach 105 decibels; a typical rock concert is about 110 decibels; and a jet taking off emits about 135 decibels.
Approaches to Stopping Noise
Loudness is a measure of how we perceive a sound. Equally intense sounds of different frequencies may not sound equally loud. The reason is that the human ear has a low sensitivity to sounds near the upper and lower ends of the audible-frequency range.
People have been trying to reduce noise for centuries. In ancient Rome, there were laws restricting people from driving chariots on the stone streets at night because of the racket they created. Cars have been equipped with mufflers to stifle engine noise since the early 1900's. Many building features, such as carpeting and drapes, serve not only as decorations but also to absorb sound. People in schools and offices often use other materials to stifle sound. In some classrooms, for example, tennis balls cut in half and affixed to the bottoms of chair legs are used to eliminate the screeching of chairs against tile floors.
Some types of sounds are more difficult to block than others. For example, it is easier to block high-pitched noise than low-frequency rumbles. That is because the sound waves of low-frequency sounds are farther apart than those of high-frequency sounds, and they displace larger amounts of air. These larger amounts of air being pushed toward a sound-deadening structure make it difficult for the structure to absorb all of the sound. As a result, some of the sound waves pass through. About five times as much thickness of material is needed to block a sound with a frequency of 200 hertz, with a wavelength of about 1.7 meters (5.5 feet)—roughly the pitch of a church bell—than is needed to block the shrill 1,000-hertz sound of a police whistle, which has a wavelength of about 0.3 meters (1 foot).
There are two approaches to stopping noise: passive and active. In passive noise control, sound-absorbing materials and specially designed structures are used to muffle or block sound waves and structural vibrations. Active noise cancellation involves the generation of precisely tuned sound waves designed to cancel or greatly diminish bothersome sound waves.
Passive Noise Control For Buildings
Passive noise control can be as simple as covering floors with thick carpets or as elaborate as designing a building from the ground up to shut out unwanted sound. One noteworthy example of the latter approach was the renovation of a 1920's-era building into a new home for the Goodman Theatre in Chicago, a project that was completed in December 2000. Architects for the project faced a challenging noise problem. Downtown traffic and the clatter of trains on elevated tracks outside the building created sound levels exceeding 100 decibels at the theater's front entrance. In addition, subway trains roared through a tunnel just 6 meters (20 feet) below the floor, creating vibrations that would ruin the delicate silence that a theater demands.
To isolate the theater from the subway, the structure had to be placed atop a novel layer-cake of sound-absorbing materials. Engineers first poured a slab of concrete made to be especially stiff. Atop that, they installed a layer of steel-reinforced rubber slabs 25 centimeters (10 inches) thick. The builders then poured another layer of concrete on top of the rubber isolators to support the building itself. This sequence of materials sets up what acoustic engineers call an “impedance mismatch.” The difference in the characteristics of the materials reflects sound waves, rather than permitting them to move through the materials to reach the interior of the structure.
To shield the interior of the theater from the traffic and elevated-train noise, the architects designed two sets of walls separated by an air gap 5 centimeters (2 inches) wide. Each wall rests on its own foundation, so there is no connection between the walls. The air space between the walls prevents sound from moving directly into the building. These construction techniques dramatically reduce the amount of noise that enters the theater, cutting the 100-decibel din outside the entrance to less than 20 decibels inside, about the level of a whisper.
Advances In Shielding Against Sound
While large projects such as the new Goodman Theatre cost millions of dollars, researchers have been investigating less expensive ways to carve out smaller zones of quiet. In 1999, for example, engineers at the Georgia Institute of Technology (Georgia Tech) in Atlanta, reported that they had developed special curtains designed to act as a shield against sound. The curtains consist of two pieces of fabric with pockets between them. The pockets can be filled with various noise-absorbing materials, such as cardboard. The researchers found that a prototype of these “Quiet Curtains” reduced noise by about 7 decibels. If extensions are added to the top and bottom of the curtains so that the curtains reach from the ceiling to the floor, they cut noise by 12 decibels. The engineers said that Quiet Curtains could be used to reduce noise in many places, such as nursing homes, offices, libraries, and factories.
In 2001, researchers at the Hong Kong University of Science and Technology were developing a new class of materials called “resonant sonic materials” that served as a passive barrier against low-frequency noise. The material the researchers created consists of small lead balls about 1 centimeter (0.4 inch) in diameter, coated with soft silicone rubber and glued together with epoxy to form blocks about 13 centimeters (5 inches) on a side. A layer of these embedded spheres almost perfectly reflects sound waves of two specific frequencies—400 and 1,100 hertz—making the material potentially a highly effective noise shield against some types of sounds, such as the loud hum of a motor. The researchers expected that the materials would be ready for commercial use by 2003.
Scientists have also been investigating the sound-blocking potential of another new material, called polyamide foam. The foam, which was originally developed for use as insulation in submarines, could replace the fiberglass insulation that is wrapped around heating and air conditioning ducts in many buildings. As well as creating a barrier against the movement of heat into or out of the ducts, insulating materials cut down on the noise of the air moving through the ducts. Fiberglass insulation, however, has several drawbacks, including the fact that the glass fibers can damage the skin and that moisture often accumulates on the material, encouraging the growth of bacteria. Polyamide foam is a good insulator and absorbs the sound of rushing air as well as fiberglass, but without the drawbacks.
Splitting Concrete Without Splitting Eardrums
While some scientists focused on making buildings quieter, other researchers were seeking ways to reduce the amount of noise in the outside environment. One group of scientists, at the Brookhaven National Laboratory in Brookhaven, New York, was developing a new version of what is perhaps the premier emblem of excessive urban noise: the jackhammer. They called their new, quieter pavement-buster the Rapid Cutter of Concrete, or “Raptor.”
A conventional jackhammer, which bombards peoples' eardrums with sound levels of about 120 decibels, uses compressed air to drive a steel cutting head relentlessly against a hard surface, such as concrete, until it crumbles. The Raptor uses bursts of compressed air to propel steel nails into pavement at very high speed to break it apart. Although the sound of each shot is about as loud as a conventional jackhammer, the Raptor does not operate continuously, firing a nail about every 6 to 10 seconds. In 2001, the Brookhaven engineers had plans to make the tool even kinder to people's ears by outfitting it with a silencing device similar to those often used on firearms. They expected that this modification would reduce the Raptor's noise level to only about 85 decibels.
Reducing Highway Noise
Traffic sounds from highways are another major form of modern-day noise pollution that scientists are seeking to control. At the Institute for Safe, Quiet, and Durable Highways, a U.S. agency created in 2000 at Purdue University in West Lafayette, Indiana, researchers are attempting to improve on the wooden barriers that are used along many highways to cut down on noise. Such barriers cost as much as $1 million per mile to construct, but they block only a fraction of the noise generated by cars and trucks.
Highway noise is caused largely by the vehicles' tires. Air becomes trapped between a tire and the road, then bursts from the confines of the treads, producing a variety of pops and whistles. Also, the blocklike shapes in the tread design strike against the road surface like hammers. Adding to the din, the tire treads and the steel belts inside the tires vibrate as the tire rotates, creating even more noise.
One technique for quieting highway noise is to use a coarser, more porous pavement surface. Compared with conventional pavement, the porous variety—made with larger chunks of rock—is less smooth. The irregular surface actually dampens sound waves by trapping them in the pavement. Porous road surfaces, already in wide use in parts of Europe, have been shown to reduce traffic noise by about 6 to 10 decibels, compared with typical pavement.
The sound-deadening qualities of coarse pavement can be destroyed, however, by the dirt, sand, and other debris that inevitably accumulate on a road surface. These drifting materials fill in the gaps of the irregular surface, making it as smooth—and as noisy—as conventional pavement. To counteract this problem, highway engineers in the United States have been developing pavement consisting of a layer of fine-grained porous pavement 2.5 centimeters (1 inch) deep on top of a thicker layer of coarser-grained porous material. The sound waves penetrate the fine layer and are trapped in the coarse pavement below, preventing them from reflecting up from the road. Since the coarse layer would not be exposed to weather, the engineers expected that it would retain its noise-reducing properties longer.
Rubberized asphalt is also being studied for its noise-reducing properties. This type of pavement is made of asphalt mixed with rubber from recycled tires. Initial test results show that rubberized asphalt is about 50 percent quieter than conventional asphalt. Each year about 190 million tires are discarded in the United States and are piled up in huge mounds in landfills. According to the Rubberized Asphalt Concrete Technology Center, an industry trade group, each kilometer (0.6 mile) of a four-lane highway with a rubberized-asphalt surface 5 centimeters (2 inches) thick would require an amount of rubber equal to more than 5,000 waste tires.
Active Noise Cancellation
While highway engineers looked for new ways to soften the noise of passing vehicles, automakers concentrated on making life quieter for the drivers. A quiet ride has traditionally been a symbol of luxury. But as vehicles more and more become rolling technology centers, with cellular telephones, televisions, and computer equipment, silence is becoming a necessity for everyone. Many electronic communication systems, such as hands-free cellular phones using voice-recognition technology, work better if background noise is kept low.
Much of the noise in a vehicle comes from the engine. To combat engine noise, engineers are working to develop more effective mufflers, including ones using active noise cancellation. This technology takes advantage of a property of sound waves known as destructive interference, in which the peak of one sound wave collides with the trough of another wave. When this happens, the two waves cancel each other out, thereby eliminating the sound. Active noise-cancellation systems use electronics to analyze the wave of an unwanted sound and generate an opposite “antinoise” wave to eliminate it.
One of the first mufflers using active noise cancellation was developed in the late 1980's by engineers at the NCT Group, Inc., of Westport, Connecticut. The NCT Group's device is quite different from a traditional muffler, which routes exhaust gases from the engine through a series of perforated tubes so they can expand and cool before exiting the tailpipe. In contrast, the electronic muffler uses a microphone to continuously monitor the vibrations of the exhaust pipe and feeds the information to an electronic signal processor. The processor then computes the appropriate vibration-canceling waves and causes a transducer (vibration-producing device) to generate the waves and send them toward the engine. There, the sound waves and antinoise waves collide and cancel each other out.
Practical Applications of Active Noice Cancellation
While active noise cancellation was not in wide use in 2001, the technique was becoming a practical noise-fighting solution for some specialized applications. For example, several companies, such as the Bose Corporation of Framingham, Massachusetts, were marketing headphones with active noise cancellation for use by professionals working in loud environments, such as airline pilots.
Active noise cancellation works best when the relative positions of the listener, the noise source, and the antinoise source stay the same. That is because effective noise cancellation demands extremely precise positioning. The antinoise wave must be almost perfectly lined up with the noise wave in order to nullify it. If the two waves are not aligned, the system magnifies the unwanted sound rather than eliminating it. A sound wave with a frequency of 1,000 hertz is about 0.3 meter (1 foot) long. For active noise cancellation to work, the antinoise wave must overlap the noise wave to a precision of about half a wavelength, or about 15 centimeters (6 inches). For this reason, it would be difficult to devise an active noise-cancellation system that is effective when the sources of noise and the listener are moving relative to one another.
However, active noise cancellation does appear promising for applications such as quieting the interiors of cars and aircraft. Here the position of the noise source—the engine (or engines)—is well-known and the passengers are generally sitting in fixed locations and facing the same direction. Engineers strategically place scores of microphones and antinoise sources throughout the passenger areas. In aircraft systems, engineers have found that they can muffle aircraft engine noise by as much as 10 decibels.
In December 2000, Honda Motor Co., Ltd., of Tokyo, announced that it was fitting some of its vehicles sold in Japan with an active noise-cancellation system. The system was designed to reduce low-frequency sound below 100 hertz. The system reduces road noise in vehicles by 10 decibels. To prevent the system from reducing the volume of the car stereo, Honda's noise-control system was engineered to recognize and ignore music from the car stereo and cancel only unwanted sounds.
An active noise-cancellation system that stops unwanted sounds closer to their source was introduced in 2001 by Bombardier Aerospace, an aircraft manufacturer with headquarters in Dorval, Quebec. Rather than broadcasting antinoise waves within the cabin to negate the unwanted thrum of the aircraft's engines, the Bombardier system cancels the vibrations that transmit the noise from the engines to the interior of the plane. The system uses microphones to pick up the vibrations in the cabin walls. It then analyzes the signals and generates countervibrations in the walls to produce a net result of zero vibrations.
If You Can't Block A Sound, Mask It
In some cases the best strategy for dealing with unwanted noise is not to block it or cancel it, but to obscure it. In well-designed restaurants, it is possible to sit a short distance away from another table and not be able to make out what the neighboring diners are saying. That is because the whole room is filled with the soft babble of many people talking, drowning out particular conversations. Engineers have learned that this is an excellent way to obscure the sound of a human voice.
This technique, called “sound masking,” is used widely in offices, where most employees now labor in cubicles that are not fully enclosed by walls. Some offices are even designed without interior dividing walls. These kinds of environments can be filled with noisy distractions that disrupt employees' concentration and make them less productive.
To address this problem, some noise-control companies sell sound-masking systems that make work areas seem quieter. The simplest approach, which was first developed in the 1960's, entails generating a type of unobtrusive sound called “white noise,” which has roughly equal amounts of energy in each frequency across the entire range of human hearing (20 to 20,000 hertz). White noise is essentially a background hum that people are barely aware of, similar to radio static. This type of noise is called “white” because it contains all audio frequencies, just as white light contains all optical frequencies.
Although white noise is effective at masking sounds, acoustic experts are finding that “pink noise” can be an even less intrusive sound-masking tool. Pink noise contains roughly the same amount of sound energy in every octave, the range between a sound of a particular frequency and a sound with twice that frequency. Compared with white noise, pink noise has more energy at lower frequencies, making it sound more like a rumble than the “hiss” of white noise.
Noise Prevention
Despite the innovative means scientists have devised to combat noise in the environment, no technology has been developed that can effectively silence all noise. So, public policymakers have turned to noise-abatement laws to reduce the amount of noise in the environment. For example, strict rules dictating that planes maintain a certain altitude over populated areas have greatly reduced the number of people affected by the roar of airplanes taking off and landing at airports. Some experts consider such planning at least as important as technological advances in noise reduction.
In the future, regulations limiting the sound levels that can be emitted by automobiles and other products could do even more to reduce the amount of noise in the environment. Thus far, governments in Europe have been more vigorous about such regulations than has the United States. For example, in France, appliances such as washing machines cannot be sold unless they meet strict noise requirements.
While people still endure a great deal of noise in their daily lives, scientists and governments are making advances toward a quieter world. Although the world will probably never be as quiet and peaceful as before the days of heavy machines, it may be well to remember that even the ancient Romans had to cope with noise.