Building Green ... Quietly: Noise Pollution and What to Do About It
In 1984, renowned “sound tracker” Gordon Hempton ferreted out 21 locations in Washington State that were free of all human-generated sound for 15 consecutive minutes. Last year, he found that just three remained. In Minneapolis, $164 million has been spent since 1992 soundproofing homes in the vicinity of the airport, with retrofitting costs this year of $44,000 per home. Nearly one-quarter of the cost of the $11 billion “Big Dig” tunnel project in Boston, Massachusetts is being spent on environmental remediation—much of it on noise management. A recent study by the American Society of Interior Designers (ASID) reveals that 70% of office workers feel their productivity is being reduced by workspace noise. As you read this issue of
EBN, take stock of the acoustical environment in your office, your neighborhood, and your home. Noise at some level is probably a 24-hour companion, potentially affecting your productivity, your well-being, even your health. This article takes a look at (a listen to?) noise and its management in and around the built environment. At every level, there are basic principles to consider and actions we can take to build with less “bother.”
Random House Unabridged Dictionary defines noise as “a nonharmonious or discordant group of sounds”—its origin is the Latin word
nausea, meaning seasickness. This is revealing because noise is an inherently subjective phenomenon that has long been associated with health and well-being. But noise is made up of sound, and understanding the quantifiable aspects of noise requires a little basic physics.
Sound is energy expressed as pressure variations in air. What we hear—speech, music, noise—is actually an infinite arrangement of changes in frequency (pitch) and intensity (volume) of these air pressure waves. Our ears have the ability to detect these variations over given ranges—typically from 20 to 20,000 cycles per second (hertz or Hz) for frequency and from 10
-12 to 10 watts per square meter (W/m
2) for intensity. To ground these numbers a bit—if you strike middle C on a piano, the string vibrates at 260 Hz; someone shouting at you at about a meter’s distance is registered at your ear at about .001 W/m
Physicists find measuring sound intensity difficult, lay persons find the units difficult to grasp, and everyone finds the huge range unwieldy. Sound
pressure levels (directly related to sound intensity) on the other hand, are relatively easy to measure and can be expressed in a familiar unit, the decibel (dB). The decibel is a logarithmic ratio that mathematically compresses sound pressure levels, as shown in Table 1 (page 10). An increase in sound level of 10 dB means a perceived doubling—50 dB sounds twice as loud as 40 dB. In a final link between sound levels and frequency, decibel ratings are often adjusted to give greater weight to certain frequencies. For example, dBA stands for A-weighted decibels that reflect the human ear’s sensitivity bias to frequencies from 500 to 3000 Hz, the band that includes human speech.
Noise, a largely subjective evaluation of sound, is related to auditory perception and can significantly affect human health and well-being. The impacts of noise on people include damage to hearing, impaired growth and development of babies and children, overall stress and sleep disturbance, and reduced productivity/safety in the workplace. Physical damage to the ear can involve rupture of the eardrum (typically at levels of about 150 dB) or trauma to the inner ear caused by prolonged exposure to sounds louder than 85 dBA. (There is a lot of variability in individual tolerance to sounds in and around this sound level.) Little research has been done on the impacts of high noise levels on babies and children; some research, however, has linked premature birth, intrauterine growth retardation, and high-frequency hearing loss in children to excessive noise during pregnancy. Field studies with school-age children have linked high noise levels with poor performance on reading tests and auditory discrimination problems. (A field study of rural and suburban school-age children in Ohio found routine 24-hour equivalent noise levels of 77 to 84 dBA, well above the EPA-recommended upper limit of 70 dBA.)
The effects of noise on daily activities and sleep are highly variable. The Federal Aviation Administration (FAA) uses a “high annoyance” level of 65 DNL (outdoor day-night sound level measured in dBA, averaged over 24 hours and weighted to reflect greater sensitivity to sound levels at night), but noise pollution activists criticize just about everything about this metric—the technology used, the level, and the units. Research reported in the December 1998
Journal of the Acoustical Society of America arrived at a “high annoyance” level of 57 dBA, and the World Health Organization recommends an even lower level of 50 to 55 dB. Activists also want the “A” weighted scale replaced by the “C” weighted scale that is more commonly used for outdoor noise measurements, because the “C” weighted scale does not discount low-frequency sound associated with aircraft.
In recent decades, the impacts of noise in the workplace have increased. While today’s office and other work equipment may be less noisy, we are packing more activity into the same workspace. The ASID study cited earlier revealed that 70% of U.S. office workers today work in an open-floor-plan office. (Workplace acoustics are discussed in detail later.)
Noise and Ecosystems
Noise can have ecological impacts as well. Measured impacts of low-flying aircraft, off-road motorcycles, Jet skis, snowmobiles, chain saws, and vehicle traffic are wide-ranging. These include: songbirds that cannot locate or attract mates, hearing-impaired desert rats, disoriented and beached whales, and aircraft-harried caribou calves unable to nurse or trampled by fleeing parents. While it can be difficult to attribute the singular impact of noise on wildlife, noise pollution is believed by many ecologists to play a major role in disrupting, changing, or even eliminating wildlife habitat. Sound tracker Gordon Hempton notes: “We have an endangered species list; quiet [in natural soundscapes] is even more endangered.”
As the impacts of noise in the modern world have broadened and deepened, our understanding of its effects on people, wildlife, and ecosystems has not kept pace. The EPA Office of Noise Abatement and Control, established in the early 1970s, was closed during the early 1980s as part of federal budget cuts. Little coordinated research on noise pollution has been carried out in the U.S. since, although noise remains a high priority in terms of research and regulation in many European countries.
Five basic strategies can be employed to control noise: source reduction, sound blocking, sound absorption, sound masking, and sound cancellation. Each strategy is based on the following principles of sound:
• Airborne sound energy dissipates rapidly over distance—airborne sound levels are inversely proportional to distance squared. In a space with no sound reflection, this means that sound or noise decreases by 6 dB for every doubling of the distance between the source and the receiver. (Structure-borne sound transmission is highly dependent on the materials involved but can carry, with little loss in level, for great distances.)
• Sound energy striking a surface can be reflected, absorbed, or transmitted. Any sound energy not reflected is either absorbed or transmitted.
• Sounds that are forms of communication—speech, music, signals—can be special forms of noise with elevated levels of distraction for a given level of sound energy.
• Pure, single tones of sound—such as a whistle, a note on the piano, or the hum of a motor—move as a wave of single (or nearly so) amplitude and frequency. Most noise is a complicated jumble of waves with many amplitudes and frequencies.
Source Reduction: For the control of any pollutant, including noise, this is the first and most effective strategy—the fewer or smaller the noise sources, the lower the impact. Although increasing the distance between sources of noise and “receivers” does not reduce sound, it is commonly used in noise reduction. Locating airports remotely and regulating the direction and altitude of aircraft activity reduces noise impacts in populated areas without reducing sound levels. A decrease in the decibel level is the manner in which noise reduction is typically expressed.
Sound Blocking: Barriers work by reflecting and/or absorbing sound. The term acoustical shadow is often used to describe the effect of sound blocking, particularly for outdoor sound barriers.
Sound Absorption: Sound energy can be broken up into lots of small, disconnected vibrations. The highly variegated surfaces of good sound absorbers allow the sound to penetrate the material, becoming “trapped” and dissipating (absorption).
Sound Masking: This is basically the acoustical equivalent of “if you can’t beat them, join them.” Within a certain range, if the background sound level is raised with sound of particular quality—intensity, frequency, and lack of pattern—the initial sound source becomes “hidden” and the masking sound can remain unobtrusive. Masking systems are most often employed in office buildings to deal with the special issue of speech intelligibility.
Sound Cancellation: Since sound is an energy wave, if one sound source is superimposed on another that is exactly 180 degrees out of phase, the original sound field is essentially cancelled. According to Mohan Barman of Aercoustics Engineering, Ltd., this technique has to date proven feasible in the field only for relatively steady (or slow to change), low-frequency noise sources (<200 Hz), such as ductwork, large cooling system fans, and transformers. Barman characterized the technique in this way: “The sound cancellation system must be placed as close to the noise source as possible, and while such systems tend to be very expensive, they are effective.”
How and when each of these strategies—or a combination of them—is employed in controlling noise depends on the specific environment and the needs and sensitivities of the “receivers”—either people, wildlife, or both. For those of us involved in designing and constructing buildings and communities, it’s useful to separate the issues of outdoor noise from those associated with noise generated inside the built environment.
Controlling noise outdoors
Only the first noise management strategy—noise reduction—can be effectively employed in the Great Outdoors. Noise reduction in natural soundscapes has a lot to do with public policy. Rules and regulations can restrict the time and place of activity—from air travel to heavy equipment operation. In our national parks, there has been growing awareness of protecting our natural soundscapes. Recent National Park Service (NPS) surveys of visitors conducted for Congress reveal that just about as many visitors come to enjoy the natural quiet (91%) as the scenery (93%). Under a recent NPS Director’s Order, all park managers will use a new reference manual for “…the protection, maintenance, or restoration of the natural soundscape resource in a condition unimpaired by inappropriate or excessive noise sources….” However, regarding one of the most contentious elements of park noise management—aircraft overflights—the NPS must work cooperatively with the Federal Aviation Administration. The US-Citizens Aviation Watch Association views this arrangement as inherently flawed because the FAA is “…serving both as regulator and promoter of aviation.” Bill Schmidt, an NPS natural resource representative, says, “The issue [of aircraft overflight sovereignty] is still up in the air, but it is the NPS that should ultimately be making the call in our National Parks.”
Managing outdoor noise in developed areas—neighborhoods, cities, transportation centers—adds opportunities for blocking and (to a limited extent) absorbing sound to the toolkit of noise control. The Noise Pollution Clearinghouse has a Law Library replete with dozens of examples of municipal noise control ordinances from around the country. A noteworthy example of local regulations effecting source reduction involves portable, combustion-engine leafblowers—over 300 hundred municipalities nationwide have banned their use! One way to reduce noise is to move the noise sources—major roads and airports, for example—further away from populated areas. This practice, however, can encourage sprawl, creating an apparent environmental conflict. Noise pollution activists point to the ready green link between reduced road and air travel and greater use of high-speed rail and mass transit: less air, water,
and noise pollution.
Source reduction can have a lot to do with technology, particularly the muffling capacity of aircraft, highway vehicles, and equipment. “Stage 3” jet engines are about 10 dB quieter (half as loud) than “Stage 2” engines, but the commercial airline industry has been slow to make the switch to Stage 3 technology because of the added cost. Interestingly, there is currently little effort being made to further reduce engine noise from cars and trucks because tire and road noise dominate at highway speeds. Motorcycles are a different item altogether—a generally greater focus on outdoor noise in Europe forced American motorcycle manufacturer Harley-Davidson to quiet its European models. The result is a motorcycle that is half as loud as its American counterpart but features the same distinctive Harley sound
and higher horsepower.
Dealing with noise generated by our buildings invariably focuses on HVAC equipment. It can be difficult and complicated to boil down the noise generated by equipment to a single number for purposes of comparison, but information is becoming more available. The American Refrigeration Institute (ARI) has voluntary standards for the noise ratings of both large and small outdoor units as well as nonducted indoor units.
Consumer Reports magazine routinely includes noise ratings for many consumer and office products. And a standard for product noise labeling is currently under development by the American National Standards Institute for all equipment and appliances.
The most common example of sound blocking outdoors is the sound barrier system—solid walls, earth berms, and vegetation—associated with roads and highways. In their new guide,
Sustainable Landscape Construction (see
Vol. 9, No. 11), authors J. William Thompson and Kim Sorvig describe the problems with sound barriers: they are all expensive (at least $1 to $2 million per linear mile), result in—at the very best—only a 10 dB reduction in noise, can block sunlight and prevailing breezes, only provide relief along the acoustical shadow they cast, and represent a very difficult aesthetic challenge. Earth berms and vegetative barriers are easier on the eye and do combine some sound absorption with reflection but take up much more space—a vegetative barrier must be at least 100 feet (30.5 m) wide and involve densely spaced evergreen shrubs and trees of variable mature height just to achieve a 3 to 5 dB reduction.
While the U.S. tends to build relatively tall, solid sound barriers, the Canadians take a different approach. Michael Rock of the Canadian sound barrier manufacturer Durisol, Inc. explained that Canadian sound barriers are usually much shorter, generally no more than 5 meters (16 ft). “At that height,” he said, “highly reflective materials like concrete, particularly with opposing parallel barriers, could result in an
increase in sound levels.” The Canadian government requires barriers with an NRC rating of at least 0.7 and a Sound Transmission Level of 30, so their barriers primarily absorb noise, rather than just reflect it. The Durisol sound panels (see
Vol. 7, No. 3 for a review of Durisol building blocks) have air voids that absorb sound and a concrete core to block sound that penetrates and is not absorbed. Sound panels like Durisol are also commonly used for enclosures to block noise from outdoor equipment (see photo, page 11).
Controlling noise inside the built environment
Good architectural acoustics require a coordinated and balanced consideration of all noise control principles tailored for the specific space. Our understanding of architectural acoustics has certainly become more sophisticated over time, sometimes through science and technology and sometimes through old-fashioned trial-and-error. The architectural classic text
American Building, by Fitch and Bobenhausen, documents the numerous acoustical tinkerings and eventual total reconstruction of Lincoln Center’s Philharmonic Hall in New York City from 1962 through 1976. The authors describe both the failure to observe fundamental acoustical principles and the use of evolving science and technology in the more than seven renovations that took place to achieve—finally—excellent acoustical performance.
The acoustical considerations illustrated on page 13 in an open-floor-plan office show many different principles of sound control. This example also demonstrates how dependent acoustical performance can be on early design considerations, material specifications, and professional acoustical guidance. Bear in mind that current trends for open floor plans in homes can lead to many of the same considerations in residential design—one of the primary ones being speech privacy (or conversely, speech intelligibility).
1.Windows – In terms of outdoor noise, windows are the acoustical weak link in the building envelope. In settings where outdoor noise is an issue, their placement, number, configuration, and operability must all be considered for noise management. (See the sidebar on sound-insulating windows, on page 14.)
2. Ducts – Most ducts transmit sound very effectively, so their layout—particularly of return ducts—can affect speech privacy. Sound attenuation boxes can be placed behind return grilles, reducing sound transmission without affecting airflow. Acoustic consultants like to use mineral fiberboard for both ducts and sound attenuation boxes because of their sound-absorbing properties, but this preference must be balanced against indoor-air-quality considerations of these materials.
3. Walls – Full-height walls for private offices need to
block sound, so the Sound Transmission Class rating of the wall is paramount. Bear in mind that the real sound-blocking ability of a wall assembly will only be as good as the weakest component. The contribution to sound transmission of even small gaps is significant, and the airtightness of an enclosed room for sound control must often be balanced against air-quality concerns.
4. Flanking Paths – Staggered electrical outlets, depicted here, represent just one of many room details that deal with flanking paths in sound transmission. The greater the need for sound control or privacy, the more attention must be paid to all pathways of sound transmission. Another common flanking path is controlled in this room with ceiling gypsum wallboard mounted on steel resilient channel (in lieu of a ceiling tile system in common with the rest of the office).
5. Office Machinery – The location, degree of enclosure, and noise rating of the equipment all affect office noise levels. Relying on office equipment and machinery to mask speech is problematic because of the inexact match of frequencies and the equipment’s intermittent operation (particularly Energy Star™ equipment that partially shuts down).
6. Layout – The location of specific work activities, direction of an occupant’s activity in relation to barriers, distance between work stations, and even location of a water cooler can affect ambient noise levels, the intelligibility of conversations, and ultimately office productivity. In large buildings with several floors and widely varying uses or levels of noise production, structure-borne sound transmission must be added to airborne considerations.
7. Floorcoverings – Commercial floorcoverings do little to control airborne sound but are effective in reducing structure-borne sound. A carpet pad further reduces structure-borne and impact noise but has almost no effect on airborne sound. Note that structure-borne sound transmission could be reduced by the raised floor system depicted here if an appropriate mounting system is utilized. (See
8. Ceilings – Sound absorption is the most important acoustical property of ceiling materials, as represented by the Noise Reduction Coefficient (NRC) rating. Ceiling tiles or other ceiling surfaces with an NRC of 0.9 or higher represent highly absorptive material.
9. Sound-Masking System – In an open office floor plan, a Room Criteria of between 35 and 45 dBA will render most speech unintelligible. Active sound-masking systems achieve this level of unobtrusive background noise with just the right combination of frequencies and lack of pattern. Active sound-masking systems typically consist of an array of speakers mounted just above the ceiling grid facing up so that sound is reflected off the surface above.
10. Panel Partitions – This is typically the least important component of good office acoustics but the one most often discussed. A good panel (one with adequate NRC and STC ratings) can at best fine-tune overall office performance, but only when used in the right configuration—at least 5 feet (1.5 m) in height and no more than 3 inches (76 mm) off the floor. Bear in mind that materials
covering the panel’s surface significantly affect its acoustical performance and that other design and material features of the space are almost certain to dominate the space’s acoustic performance.
Noise annoys—it can affect the quality of life and our productivity. While we (and wildlife) use sound for one of our higher functions—communication—noise represents just the opposite. As citizens and environmentalists, there is a lot we can do to protect natural soundscapes. A list of the best resources for understanding and action is included at the end of this article. In and around our buildings, designing for acoustic control is anything but simple! There is, however, much we can do as building professionals to make our buildings and communities more pleasant, more productive, and more quietly green.
Bring noise management into the design phase of building. Today’s homes and office buildings include lots of open-floor-plan design, making functional acoustics even more challenging. Managing noise can be done most easily and cost-effectively during the design phase of a building project.
Engage a qualified acoustic consultant. Match specific experience and skills to your technical needs, and ask for references. No degree or background in architecture, engineering or physics can substitute for know-how and job-specific experience, but membership in one or more of the professional associations listed below is a good indicator of professional qualification.
Integrate all four basic principles of sound control: source reduction, sound blocking, sound absorption, and sound masking. Acoustical management requires systems thinking. Neglecting any one of these can result in little or no benefit from your efforts/investments. Listed below is an excellent white paper from Herman Miller, Inc., employing a systems approach to understanding open floor plan acoustics, as well as a useful text.
Employ the motto of the Noise Pollution Clearinghouse (NPC) in the design of buildings and communities: “Good neighbors keep their noise to themselves.” NPC has plenty of resources to help green architects and builders.
– Peter Yost
For more information:
Noise Pollution Clearinghouse
P.O. Box 1137
Montpelier, VT 05601-1137
US-Citizens Aviation Watch
P.O. Box 1702
Arlington Heights, IL 60006-1702
Rutgers Noise Control Technical Assistance Center
(the only surviving research facility established in cooperation with the defunct EPA Office of Noise Control and Abatement)snowfall.envsci.rutgers.edu/estc/rntac/
The Sound Tracker
National Council of Acoustical Consultants
66 Morris Avenue, Suite 1A
Springfield, NJ 07081-1409
Institute of Noise Control Engineering
P.O. Box 3206
Poughkeepsie, NY 12603
The Acoustical Society of America
“It’s a Matter of Balance: New Understanding of Open Plan Acoustics”
Architectural Acoustics: Principles and Design by Madan Mehta, Jim Johnson, and Jorge Rocafort (Prentice-Hall, 1999)