Noise

Noise

Noise denotes unwanted or unmusical sounds, especially those that are random or irregular. The attitude that noise is not conducive to the well-being of sentient creatures is as old as history. The Babylonian story of the Flood, as recorded at about the time of Hammurabi (2000 BC), attributes the event to the anger of the gods over the noise of men. In his ethical directives to doctors, Hippocrates (400 BC) stated, "Noise must be avoided and kept well removed from the sick." Some of the sources of noise in those times may be gleaned from the history of Sybaris, a Greek colony in southern Italy (600 BC), where noisy trades, such as blacksmithing and stonemasonry, were prohibited within city walls. Impact noises were evidently the Sybarites' main concern.

This was not the case in Imperial Rome, where the main noise came from traffic. In that walled city of one million people, traffic congestion was so great that goods vehicles were allowed on the streets only at night. The continual movement of steel-rimmed wheels on stone-paved streets created a clatter that caused Marcus Martial (80 AD) to complain: "I have Rome right beside my bed at night."

In England in the days of Henry VIII, the method of maintaining "the peace, quiet, rest and tranquility of the citizens" was primarily by prohibiting certain noisy activities at least during resting hours (9 PM to 4 AM).

The Measuring of Noise

One problem that inhibited noise control before the 20th century was the inability to measure noise levels. The ear is an unreliable measuring instrument, a problem further complicated by the ability to become habituated to frequent or regular background sounds. The invention of electronic amplifiers made sound-level meters convenient and reasonably standardized, allowing correlations between exposure to noise levels and quantitatively measured effects on people. These instruments are calibrated in decibels (db) and have weighting networks discriminating against certain frequencies. For example, the A-weighting network, most widely used in dealing with human response, gives less weight to low frequencies because the ear does not respond to low frequencies as well as it does to high.

"Boilermaker's ears," a HEARING LOSS resulting from long-term exposure to extreme noise, had long been recognized, but in the middle of the 20th century a more quantitative relationship was established between noise exposure and hearing loss. With improved methods of measurement it became possible to relate subjective reactions (eg, annoyance, estimation of loudness) to a detailed analysis of the sound field.

Noise levels may be divided into 3 ranges according to their effects on people. Zero decibels is the approximate threshold of hearing. Levels of 0-40 db are generally judged as quiet and annoy only if they contain strong tone components. Those between 40 and 80 db may be a nuisance, depending on their nature and the conditions under which they occur. For example, the level of conversational speech at a distance of 30 cm averages about 60 db. A full orchestra may play for extended periods at 80 db and well above that for brief periods, yet we willingly pay for exposure to it. Modern rock concerts can generate noise levels much higher than that. Noise levels of 80-120 db are uncommon in our environment, but are common in some industries and at airports.

Nuisance Noise

Nuisance noises generally occur outdoors in urban and suburban environments and are the subject of noise bylaws. Significant causes include car traffic, which is all-pervasive, truck and public transportation traffic (road and rail), which is more restricted, and air traffic, which is usually more localized but can have adverse effects even far from urban areas. Nuisance noise may come from air conditioners, lawnmowers, snow blowers, etc. These noises are annoying because they may startle and distract, and interfere with understanding speech or with rest and sleep.

Distance can be a very effective protection from environmental noise: in a uniform atmosphere, each time the distance doubles, the noise from a single source is reduced by 6 db. For example, most passenger cars travelling at 60 km per hour produce a noise level of 65 db at 15 m, 59 db at 30 m and 53 db at 60 m. Unfortunately, distance often cannot be controlled at will. The atmosphere is not always uniform and noise can be significantly greater or less than expected because of temperature or wind gradients. Buildings, rock formations and forests act as noise barriers, most effectively at short distances. Solid barriers are most effective when close to the source or the receiver; forests are fairly effective when large (a row of bushes or trees a few metres thick provides no significant attenuation). Restricting the number of flights during the night, the flight paths, and the type of buildings allowed at certain distances is the primary means of keeping air-traffic noise down in residential areas near an airport.

House design affects noise levels indoors: walls with good sound attenuation, double windows with large air spaces between the panes, mechanical ventilation, good weather-stripping around doors and windows and sound-absorbing materials inside the house all help reduce noise. Some jurisdictions require special constructions to ensure certain levels of noise inside the home.

Noise generated electronically and broadcast from loudspeakers may be used in open-plan offices to mask speech from adjacent workstations, thus reducing distraction and providing a small increase in privacy. This kind of noise is called masking sound. It has a sound similar to that of distant traffic. White noise, which has nearly uniform energy distribution over the audible frequency range, is not used for this purpose. It has an irritating hissing sound that most people will not tolerate.

Humans are not the only animals to suffer from excessive noise. For example, in wild bird populations, noises in the 40-80 db range may interfere with nesting success. Even normally acceptable sounds may be a source of debilitating stress, for example, birds may desert their breeding territories in response to excessive playbacks of territorial songs.

Damaging Noise

Noise may cause physiological damage, especially to the inner ear; duration of exposure is fundamentally important. Noise levels above 120 db are rare, except near powerful sources such as jet or rocket engines. Sound may be felt at 120 db; it will cause pain in the ears at 130 db. At 160 db or higher, combustible sound-absorbing materials may be ignited by the heat into which the sound energy is transformed.

Canadian Noise Abatement Legislation
In comparison with other countries, Canada has little noise abatement legislation and the legislation that exists is aimed primarily at private individuals. Such laws as do exist to regulate various kinds of noise pollution are controlled by different levels of government. Noise from traffic, industry, etc, is regulated by municipal governments. Ottawa was the first Canadian city to pass a quantitative noise bylaw (1970). The limit set by bylaws for environmental noise may vary from a low of 40 db in suburban, purely residential areas at night, to a high of 75 db in a purely industrial area during the day.

The Canadian government (like most other Western governments) has set a limit of 90 db to the noise level to which unprotected industrial workers may be constantly exposed during a working day. Many governments set this limit at 85 db. For higher levels, permissible exposure time is lower. Exposure without protection to levels above 115 db is usually not permitted. WORKERS' COMPENSATION boards have authority to investigate industrial noise.

Aircraft noise is controlled by Transport Canada. Canadian airports that have noise abatement policies for aircraft takeoffs and landings include Montréal International, Toronto's Lester B. Pearson International and Winnipeg International. No regulations exist to control small plane or helicopter noise. Partial curfews (affecting flights from midnight to 7 AM) are in place at Montreal and Toronto airports.

The National Building Code of Canada is a model code that has been adopted by most provinces and municipalities. It contains requirements for sound insulation between dwellings sharing walls or floors. The requirements are stated in terms of a rating called the sound transmission class (STC). The minimum STC required is 50; higher values give more satisfaction. The STC is calculated from sound transmission values at frequencies ranging from 125 to 4000 Hz.

Canadian Noise Abatement Research

The Institute for Research in Construction of the NATIONAL RESEARCH COUNCIL OF CANADA (NRC) conducts research on the factors important in determining the sound insulation of a wall, floor or other building element. A great deal of other research has been done at the NRC on reducing noise at the source; for example, noise in suction rolls (such as in paper mills) may be reduced by designing quiet drilling patterns. Attempts to block sound once it is produced resulted in the development of hearing protectors with liquid-filled cushions, which are used at airports around the world.

Controlling noise in the community by road and community design and by legislation has received a great deal of attention, as has the transmission of sound under the complex conditions prevailing in the real world. The effects of various levels of noise on sleep characteristics have been studied. The University of Toronto's Institute of Aerospace Research has been involved in examining AERODYNAMIC noise, both at boundary layers and in jets. A group at the University of Sherbrooke studies primarily the basic physics of noise radiation from machinery and through enclosures.