NOISE AND YOUR HEALTH

SOUND AND ACOUSTICS

The term sound refers both to waves of compression and expansion that travel through a physical medium such as air.
Acoustics is the general term for the scientific study of sound waves and for the art and engineering of handling them. Acoustics deals with all sound waves, not only those whose vibrations that fall within the range of human hearing.
Sound waves range from the very slow vibrations of the Earth to very rapid vibrations.
Acoustics is important in the fields of speech and hearing, music production, theater design, the control of environmental noise, and medical diagnosis and therapy.

Every kind of sound is produced by vibration. The sound source may be a violin, an automobile horn, or a barking dog. Whatever it is, some part of it is vibrating while it is producing sound.
The vibrations from the source disturb the air in such a way that sound waves are produced. These waves travel out in all directions, expanding in balloon like fashion from the source of the sound. If the waves happen to reach someone's ear, they set up vibrations that are perceived as sound

Sound, then, depends on three things:

• There must be a vibrating source to set up sound waves.
• A medium (such as air) to carry the waves.
• A receiver to detect them. Sound waves cannot travel through a vacuum.

The Pitch of Sounds

Some sounds are high and others are low; some are loud and others barely audible; some are pleasant and others harsh.

The three basic properties of any pure sound are: 

• Pitch. 
• Intensity.
• Quality.

Pitch is simply the rate at which vibrations are produced. This is usually expressed as the number of Hz (hertz, or cycles per second). One cycle is a complete vibration back and forth. The number of Hz is the frequency of the tone. The higher the frequency of a tone, the higher its pitch.

Intensity and Tone Quality

The intensity of a sound has nothing to do with its pitch. A high tone can be either loud or soft, and so can a low tone. Intensity depends upon the strength, or amplitude, of the vibrations producing the sound. A piano string, for example, vibrates gently if the key is struck softly. The string swings back and forth in a narrow arc, and the tone it sends out is soft. If the key is struck forcefully, however, the string swings back and forth in a wider arc. The stronger vibration then produces a louder tone. The explanation of this is that a vibration of greater amplitude compresses the molecules of the air more forcefully and gives them greater energy. When a series of such strong compression waves enters the ear, the brain interprets it as a loud tone.

Decibel, is one tenth of a bel; unit of measure of loudness of sounds to normal human ears; because the power of the ear to distinguish differences  loudness decreases as volume increases.
The bel scale is made logarithmic; each unit is 10 times the preceding one; thus a barely audible whisper measures one bel (10 decibels) and a speeding express train about 10 bels (100 decibels), though the train generates 10 billion times as much sound energy. Measurements are made with a special sound meter (acoustimeter) containing numerous electrical circuits whose aggregate sensitivity to pitch and loudness corresponds to that of the human ear .

As we said, the loudness of sounds is measured in decibels (dB). On the scale used, 0 indicates the softest audible sound.
The rustle of leaves is rated as 20 dB, average street noise as 70, and nearby thunder as 120. Above this level sound begins to be painful, and prolonged exposure to sound at such levels may damage hearing.

The quality, or timbre, of a sound is more complicated than pitch or intensity. The tone of a flute has a pleasant quality while the screech of a blue jay has an unpleasant one. Neither sound is a simple tone, however, the flute is producing higher frequencies as well. These softer and higher tones are called overtones. In the example of the flute, the main overtones heard are the octave and the 12th. These overtones harmonize well with the principal note (or fundamental) and account for the sweet tone of the flute.

Other instruments sound different combinations of overtones, which give them their special tone quality. The human voice and stringed instruments such as the violin and piano are very rich in overtones. Overtones that harmonize better than others are notes of the same scale.

NOISES

Are sounds thought of as harsh are combinations of tones that do not harmonize. If the raucous call of a blue jay were analyzed, it would be found to be a combination of extremely discordant notes.
Noises are miscellaneous combinations of tones, unpleasant because they are unrelated.
Most noises are found within urban environments from cars, trucks, buses, airplanes, industrial operations, construction projects, street repair, air conditioning equipment, power tools, lawn mowers, radios, television sets, voices, explosions. There is an active and growing interest in developing a means to identify and measure these noisemakers. This requires a variety of measurement processes to account for their different natures.
There are primarily two ways to assess noise. One is concerned with measuring the sounds produced by a single type of noise maker, such as an airplane. This procedure is largely standardized and can be used to set public noise abatement policy. The second means is designed to assess the overall noise exposure in a given place. This description of a noise environment helps in determining the suitability of a place for various purposes

NOISE FEATURES.

Regardless of the type of noise, there are five elements to be considered in describing it: loudness, frequency distribution, directional distribution, time distribution, and operating conditions.

The loudness, or magnitude, of noise determines its intensity, which is measured in decibels. Ordinary conversation has an intensity of 40 decibels at a distance of a few feet. Traffic at a busy intersection produces an average of 75 decibels, while a rock music concert or a boiler factory can reach more than 130 decibels.

The frequency of sound that is, whether it is high pitched or low pitched is measured in Hertz. The human ear is not sensitive to all sound frequencies, so to assess the annoyance factor of noise, it is necessary to know its frequency range as well as loudness.

Some noises are distributed more in one direction than another. Outside sounds are, of course, carried in the direction of the wind, and they may be blocked by buildings, but urban sounds generally radiate in all directions. The time distribution of sound sources are categorized as steady state, fluctuating, or impulsive. Steady state noises, such as air conditioners, are from fixed locations and maintain a constant magnitude over a period of hours. Fluctuating sounds include motor traffic and airplane landings or takeoffs. Impulsive noises come from sources such as jackhammers, explosions, backfiring autos, or sonic booms. Impulsive noises are more annoying than fluctuating noises because of their unexpectedness.

Quality of noise varies with the operating conditions of its mechanical source, if it has one. A car engine that is racing is louder than one that is idling, and aircraft noise varies with the thrust, jet velocity, fan or compressor speeds, aircraft position, and speed.

Most noise sources are measured in terms of intensity, or strength of the sound field. The standard unit, one decibel (dB), is the amount of sound that is just audible to the average human. The decibel scale is somewhat misleading because it is logarithmic rather than linear; for example, a noise source measuring 70 dB is 10 times as loud as a source measuring 60 dB and 100 times as loud as a source reading 50 dB.

Noise may be generally associated with industrial society, where heavy machinery, motor vehicles, and aircraft have become everyday items. Noise pollution is more intense in the work environment than in the general environment, although ambient noise increased an average of one dB per year during the 1980s.
The average background noise in a typical home today is between 40 and 50 decibels.
Some examples of high level sources in the environment are heavy trucks (90 dB at 15 m/50 ft), freight trains (75 dB at 15 m/50 ft), and air conditioning (60 dB at 6 m/20 ft).

RECEIVERS OF SOUND

The most important, and technically the most impressive, of all sound receivers is the EAR. The ear is capable of detecting sound for which the fluctuations of density of the air are less than one ten millionth of 1 percent. This figure corresponds to a particle displacement of less than one atomic diameter. Because the range of acoustic amplitudes that the ear can detect is so large, it has been convenient to define a compressed scale to describe acoustic intensities. In this decibel scale a 10 decibel (dB) difference between two sounds is perceived as a loudness difference of a factor of two.

HUMAN HEARING SYSTEM

The Ability to Hear Sounds

The human ear cannot hear all possible frequencies. Very few people can hear any fewer than 16 Hz or any more than about 20 kHz (kilohertz 1 kHz equals 1,000 Hz). Music rarely makes use of this whole range of audible frequencies. The lowest note on a piano has a frequency of 27 Hz and the highest note a little more than 4 kHz. Frequency modulation (FM) radio stations broadcast notes up to 15 kHz. These can be heard through hi-fi receivers.

Frequencies greater than the human ear can hear are called supersonic or ultrasonic waves. A silent dog whistle is pitched at supersonic frequency. A dog hears these waves as sound though a human being does not.

EAR

The ear is the organ of hearing and equilibrium (balance) in vertebrates. The ear converts sound waves in the air to nerve impulses that are relayed to the brain, where they are interpreted as sound rather than as mere vibrations. The innermost portion of the ear maintains BIOLOGICAL EQUILIBRIUM through the so-called vestibular apparatus, which includes the semicircular canals. Any change in the position of the head or body causes the apparatus to transmit nerve impulses to the brain, evoking muscular reflexes that tend to restore the normal position.

STRUCTURE OF THE EAR

The ear in humans and most other mammals consists of three parts: the outer, middle, and inner portions. The outer ear, or pinna, is the structure commonly called the ear. It is a skin-covered flap of elastic cartilage projecting from the side of the head and funneling sound into the middle ear. The middle ear is an air-filled chamber containing the eardrum, or tympanic membrane, and connected to the pharynx by the eustachian tube, thus equalizing the pressure on the two sides of the eardrum. The inner ear alone contains the sensory receptors for hearing, which are enclosed in a fluid-filled chamber called the cochlea. The middle and outer ears serve only to receive and amplify sound waves and occur only in amphibians and mammals, whereas the inner ear is present in all vertebrates.

HEARING

The characteristics of sound that can be detected by the human ear include volume, pitch, and tone. In general, sound volume depends on the amplitude, or intensity, of the sound wave; the greater the amplitude, the louder the sound. Pitch is related to the frequency of the sound wave, or the number of waves per unit time passing a point of reference; the greater the frequency, the higher the pitch. The tone, or quality, of a sound is a more complex property than volume or pitch. Variations in quality, such as are produced when an oboe and a violin play the same note, depend on the number and kind of overtones or harmonics (combinations of frequencies). Humans can hear frequencies between about 30 and 20,000 waves, or cycles, per second (cps, or Hertz, abbreviated Hz). A whistle producing 30,000 Hz is audible to dogs. Bats can produce and hear sounds of approximately 100,000 Hz, in the ultrasonic range, and use this ability in their highly evolved systems of navigation.

Basically, the ear is adapted for transmitting vibrations from air to the fluid medium of the cochlea. Sounds travel down the auditory canal and cause the eardrum to vibrate. The vibrations are transmitted through the middle ear by a sequence of three tiny bones, the auditory ossicles, called, because of their shapes, the hammer, anvil, and stirrup. The last of the bones, the stirrup, rests on a membrane-covered opening (the oval window) in the bony wall of the snail-shaped cochlea, and carries the vibrations to fluids inside the cochlea. The vibrations create waves on a membrane running along the length of the cochlea (the basilar membrane).

The true sound receptors are thousands of specialized hair cells, in the organ of Corti, spread across the basilar membrane. The deformation of the hairs causes them to initiate electrical impulses that are relayed by the auditory nerve to the brain. The ability to recognize pitch is based on the fact that cells stimulated by low frequencies occur at the apex of the cochlea, whereas those stimulated by high frequencies occur at the base. Nerve impulses from each region along the basilar membrane are relayed to slightly different regions of the brain, and the sensation of pitch depends on which area of the brain is stimulated.

Loud sounds cause more intense stimulation of hair cells and result in the transmission of more impulses per unit time to the brain

HAZARDOUS OF NOISE

It is generally accepted that a person's perception of noise depends on the characteristics of the sound: its loudness, frequency, and whether it is customary or unusual. To some extent, an individual's age, emotional makeup, tastes, beliefs, and other factors determine the degree of annoyance with noise. For city dwellers who are used to many sounds all day long, the silence of the countryside can be as unsettling as city noises are to a farmer. Individuals who work in constantly noisy environments such as factories or airports may adapt to the noises they hear, but may also eventually become deafened by them.

The most readily measurable physiological effect of noise pollution is damage to hearing, which may be either temporary or permanent and may cause disruption of normal activities or just general annoyance. The effect is variable, depending upon individual susceptibility, duration of exposure, nature of noise (loudness), and time distribution of exposure (such as steady or intermittent). On the average an individual will experience a threshold shift (a shift in an individual's upper limit of sound detectability) when exposed to noise levels of 75 to 80 dB for several hours. This shift will last only several hours once the source of noise pollution is removed. A second physiologically important level is the threshold of pain, at which even short-term exposure will cause physical pain (130 to 140 dB). Any noise sustained at this level will cause a permanent threshold shift or permanent partial hearing loss. At the uppermost level of noise (greater than 150 dB), even a single short-term blast may cause traumatic hearing loss and physical damage inside the ear.

Exposure to various degrees of noise may cause temporary or permanent hearing damage. A single exposure to such an extremely intense sound as an explosion may produce a severe and permanent loss of hearing. Repeated exposures to sounds that reach more than 80 to 90 decibels may cause gradual loss of hearing. This happens because the hair cells of the inner ear, and sometimes even the nerve fibers, may be destroyed.

Although little hard information is available on the psychological side effects of increased noise levels, many researchers attribute increased irritability, lower productivity, decreased tolerance levels, increased incidence of ulcers, migraine headaches, fatigue, and allergic responses to continued exposures to high-level noises in the workplace and the general environment.

PROTECTING YOUR HEARING

• Avoid exposure to high dB noises .
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• Wear earplugs or earmuffs as a form of protection when exposure to noises is necessary.
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• Check your home's appliances for noise level .
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• Replace high noise appliances by low noise designed apparatus.
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• Avoid listen music at high levels.
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• Be careful with the use of head sets during long periods.
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• Do not allow children near sources of noises.
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• Avoid exposure to places where music is played at high volumes.

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