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EFFECTS OF NOISE
(summarised from WHO report on community noise)

Noise can disturb our work, rest, sleep and communication, It can damage our hearing and evoke other psychological physiological, and possibly pathological reactions. However, because of complexity, variability and the interaction of noise with other environmental factors, the adverse health effects of noise do not lend themselves to a straight forward analysis.

1 Hearing Impairment

Hearing Mechanism: The ear is made up of three parts: the outer, middle and inner ear as shown in figure 3,

The visible portion of the outer ear is called the pinna (or auricle), The pinna forms the entrance to the ear canal which conducts sound wave to the ear-drum (also known as the tympanic membrane).

The middle ear is an air-filled cavity of about 2 cm³ containing the mechanism, which transmits the vibratory motion of the ear-drum to the inner ear. This mechanism (called the ossicular chain) consistsof three tiny bones: ,the malleus (or hammer), which is attached to the ear-drum; the incus (or anvil), which forms an interconnecting level; and the stapes (or stirrup), which attaches to the oval window thal serves as the entrance to the inner ear.

The inner ear includes three semi-circular canals at right angles to each other and the cochlea. The semicircular canals are part of the body's balancing mechanism. The cochlea or the hearing portion of the inner ear resembles a snail's shell. The auditory nerve fibers from the brain ends at the hair cells in cochlea.

As vibrations hit the ear they set into motion the ear drum and the ossicles. The ossicles produce vibrations the fluid of the cochlea. These vibrations are transduced by sensory hair cells into nerve impulses. The brain translates these impulses into sound. The hair cells are non-regenerative; thus, if they are damaged or destroyed, hearing impairment will occur.

Hearing Impairment can be either temporary or permanent. Noise-induced temporary threshold shift (NITTS) is a temporary
loss of hearing acuity experienced after a relatively short exposure to excessive noise. Pre-exposure hearing is recovered fairly rapidly after cessation of the noise. Noise induced permanent threshold shift (NIPTS) is an irreversible loss of hearing that is caused by prolonged noise exposure. Both kinds of loss, together with presbyacusis, the permanent hearing impairment that is attributed to the natural aging process, can be experienced simultaneously.

Noise-induced hearing impairment occurs predominantly in the higher frequency range of 3000 -6000 Hz, with the largest effect at 4000 Hz. The main social consequence of hearing impairment is the inability to understand speech in daily living conditions, and this is considered to be a severe social handicap. Even small values of hearing impairment ( 10 dB averaged over 2000 and 4000 Hz and over both ears) may adversely affect speech comprehension.

The extent of hearing impairment in populations exposed to occupational noise depends on the value of L_Aeq._8h, the number of noise-exposed years, and on individual susceptibility. However, hearing impairment is not expected to occur at L_Aeq8hlevels of 75 dB(A) or below, even for prolonged occupational noise exposure.

In case of environmental and leisure time noise, L_Aeq._24h of 70 dB(A) or below will not cause impairment in the large majority of people, even after a life time exposure.

For adults exposed to impulse noise at the workplace, the noise limit is set to a peak sound pressure level of 140 dB, and the same limit is assumed to be appropriate for environmental and leisure-time noise. In the case of children, however, taking into account their habits, the peak sound pressure level should never exceed 120 dB.

2 Sleep Disturbance

Sleep disturbance is a major effect of environmental noise. Uninterrupted sleep is known to be prerequisite for good physiological and mental functioning of healthy persons. It may cause primary effects during sleep, and secondary effects that can be assessed the day after night-time exposure. The primary effects of sleep disturbance are: difficulty in falling asleep; awakenings and alterations of sleep stages or depth; increased blood pressure, heart rate etc. The secondary, or after effects, the following morning or day(s) are: reduced perceived sleep quality; increased fatigue; depressed mood or well being; and decreased performance. For a good night's sleep, the equivalent sound level should not exceed 30 dB(A) for continuous background noise, and individual noise events exceeding 45 dB(A) should be avoided.

3 Interference with Speech Communication

Speech interference is basically a masking process, in which simultaneous interfering noise renders speech incapable of being understood. The inability to understand speech results in a large number of personal handicaps and behavioural changes. Problems with concentration, fatigue, uncertainty and lack of self confidence, irritation,misunderstandings, decreased working capacity, problems in human relations, and a number of stress reactions have all been identified. Particularly vulnerable are the hearing impaired, the elderly children in the process of language and reading acquisition, and individuals who are not familiar with spoken language. Speech intelligibility in every day living conditions is influenced by speech level; speech pronunciation; talker to listener distance; sound level and characteristics of the interfering noise; hearing acuity; and by the level of attention.

Indoors, speech communication is also affected by the reverberation characteristics of the room. Reverberation time over 1 s produce loss in speech discrimination and make speech perception more difficult and straining. For full sentence intelligibility in listeners with normal hearing, signal-to-noise-ratio (i.e. the difference between the speech level and the sound level of the interfering noise) should be at least 15 dB(A). Since the sound pressure level of normal speech is about 50 dB(A), noise with sound levels of 35 dB(A) or more interferes with the intelligibility of speech in smaller rooms.

4 Performance

It has been shown, mainly in workers and children, that noise can adversely affect performance of cognitive tasks. Although noise induced arousal may produce better performance in simple tasks in the short term, cognitive performance substantially
deteriorates for more complex tasks. Reading, attention, problem solving and memorization are among the cognitive tasks most strongly affected by noise. Noise can also act as a distracting stimulus and impulsive noise events may produce disruptive effects as a result of startle responses.

5 Annoyance

Noise annoyance may be defined as a feeling of displeasure evoked by noise. The annoyance inducing capacity of noise depends upon its physical characteristics, including the sound pressure level, spectral characteristics and variations of these properties with time. However, annoyance reactions are sensitive to many non- acoustic factors of a social, psychological, or economic nature and there are considerable differences in individual reactions to the same noise.

During day time, f'ew people are highly annoyed at L_Aeq levels below 55 dB(A), and few are moderately annoyed at L_Aeq levels below 50 dB(A). Sound levels during the evening and night should be 5-10 dB, lower than during the day.

6 Physiological Functions

In workers exposed to noise, and people living near airports, industries and noisy streets, noise exposure may have a large temporary, as well as permanent, impact on physiological functions. After prolonged exposure, susceptible individuals in the general population may develop permanent effects, such as hypertension and ischaemic heart disease associated with exposure to high sound levels. The magnitude and duration of the effects are determined in part by individual characteristics, lifestyles, behaviours and environmental conditions.

Workers exposed to high levels of industrial noise for 5-30 years may show increased blood pressure and an increased risk for hypertension. Cardiovascular effects have also been demonstrated after long-term expqsure to air and road traffic with L_Aeq, 24h values of 65- 70 dB(A).