124 It becomes very complicated to quote noise levels in measurements of sound
pressure Pascals or intensity Wattsmetre
2
, as the numbers are very unwieldy. We therefore relate them to a reference level in this case, the
threshold of hearing and using a log scale for the result, a much more manageable figure can be produced. This is called the decibel which is one
tenth of a Bel. The decibel dB has no dimensions as such; it is just a unit of comparison arranged in a logarithmic scale, so that increasing the number
corresponds to a multiplication of intensity. The loudness of noise is a function of both the intensity and the frequency.
Source: Canadian Centre for Occupational Health and Safety
13.4 Health Effects of Excessive Noise
It has long been known that regular exposure to high intensity noise can result in damage to the hearing mechanism, the degree of damage being
proportional to the total integrated noise energy incident upon the ears. The damage is related to the intensity, nature continuous or intermittent and
duration of the noise exposure, and has microscopically visible effects on the
125 inner ear that are essentially irreparable and incurable. There are five possible
health effects of noise: Noise Induced Hearing Loss NIHL is a cumulative effect from
repeated exposure. It is due to damage to the hair cells of the cochlea in the inner ear. First indication of hearing loss occurs with a reduction
in the ability to hear around the 4 kHz frequency range. Over time, if the exposure continues the noise-induced hearing damage shows as
an increase in the depth of the hearing loss and a widening of the 4 kHz notch to both lower and higher frequencies.
Tinnitus - Noise heard in the ear without an external cause; it frequently accompanies deafness.
Temporary Threshold Shift TTS - Damage to the hair cells of the inner ear which can impair hearing temporarily, resulting from exposure to
high noise levels. Recovery occurs once exposure to high noise levels is reduced, typically over a period of several hours.
Physical damage to the eardrum and ossicles induced by excessively high noises e.g. explosions. This type of hearing loss is referred to as
conductive hearing loss. Annoyancestress, which is difficult to measure and quantify, but may
cause psychological effects such as poor concentration, irritability and stress.
Besides causing temporary or permanent hearing loss, noise can also be a safety hazard. Most obviously, noise interferes with verbal communication,
leading to errors and failures to respond to warning sounds and shouts. Hearing damage can be induced by continuous exposure to levels in excess
of 85 dBA but an individuals response varies within a population. Continuous exposure to levels in excess of 90 dBA will result in 20 of the
exposed population suffering from NIHL.
Regular exposure to High Intensity Noise, i.e. greater than 80 dBA, will
almost invariably produce some degree of noise-induced hearing loss in those who hearing is susceptible. There is no way to predict in advance which
particular individuals are more likely to suffer from noise-induced hearing loss.
126
Moderate Intensity Noise, i.e. 55-80 dBA, although not a potential hazard
to hearing, may adversely affect concentration and will interfere with speech communication if greater than 65 dBA.
Low Intensity Noise, i.e. less than 55 dBA, may still result in complaints
from the general public of annoyance such as sleep disturbance.
13.5 Addition of Sound Levels
