"A comfortable environment is one in which there is freedom from annoyance and distraction, so that working or pleasure can be carried out unhindered physically or mentally"
D.J. Croome, Noise, Buildings and People.
| Noise Level | Effect |
|---|---|
| 150 dB | causes instant loss of hearing. |
| 120 dB | is physically painful and should be avoided. |
| 100 dB | short periods of exposure causes a temporary loss of acuity (threshold shift) with prolonged exposure likely to cause irreparable damage to auditory organs. |
| 90 dB | long term exposure at this level normally causes permanent hearing loss. |
| 65 dB | long periods of exposure cause both mental and bodily fatigue. |
Significant research has been carried out in this area (Shepard(1975), Freeman (1975), Ando and Hattori(1970, 1973), Gatonni and Tarnopolsky(1973), Winder(1976) and Tarzi(1976)). It has been shown that noise is a potent factor in the production and exacerbation of stress. There are even suggestions that the general level of background noise is responsible for the comparative nervousness of city-dwellers. Excessive noise clearly affects concentration, particularly if it has some sort of information content (such as speech, a radio, etc.). In a work situation, switching attention to and from such a noise may take up to 2 seconds and may be done quite often.
Low Frequency Noise
To a degree, it is useful that we have a reduced sensitivity to the lower frequency range as it relieves us of being annoyed by low frequency sounds within and around us. However, very low frequencies, subsonic or infra-sounds, can be produced in buildings by long air-conditioning ducts and wind effects.
Subsonic sounds generally act on the inner ear, the position-sensing organ, and significant levels may cause disorientation, seasickness, digestive disorders, troubled sight and dizziness.
Frequencies of between 1 and 8Hz can correspond to resonant frequencies within the body itself, 8Hz being thought to cause circulatory resonance phenomena, overloading the heart or bursting blood vessels. 7Hz also seems particularly unpleasant as it has been shown to cause tiredness, headache and nausea. It is suggested that 7 Hz corresponds to the median frequency of the brain's 'alpha' waves.
Masking Effects
Although a subdued voice is understandable in a quiet room, it is difficult to understand even a raised voice above the roar of an aircraft engine. This drowning out, or masking, occurs as a result of the general excitation of the auditory nerve cells. For a sound to be picked out of this background level of excitation, it must produce a corresponding increase in its threshold.
This is generally known as a problem with signal-to-noise ratio.
Low frequency sounds produce a considerable masking effect over higher frequency sounds. Excessive low frequency sounds therefore constitute a serious source of interference for speech and music which rely on higher frequency sounds to impart most of their auditory information. Higher frequency sounds can mask lower frequencies to a degree, but the effect is most pronounced in sounds of similar frequency.
This phenomenon can be exploited in environmental noise control by using uninterrupted, non-information carrying sounds. These can provide an acceptable background noise to suppress other objectionable noises, making them psychologically quieter. Such noise may include air-conditioning noise, traffic flow on a highway or even a water fountain.
Presbycousis and Hearing Defects
Normally people experience some hearing loss with age, particularly at high frequencies. Whether this is simply due to the ageing process or to the effects of present levels of environmental noise seems to be a matter of speculation: Croome(1977) and Rossen(1962) describe Mabaan tribesmen in the Sudan who do not exhibit this. It is, therefore, very difficult to separate completely noise induced hearing loss from presbycousis.
Temporary Threshold Shift
When a person with normal hearing is exposed to intense noise for an extended period, they first suffer a temporary loss of sensitivity or acuity called a temporary threshold shift. After a certain period the ear gradually recovers it's sensitivity. If insufficient recovery time is given before the next exposure (as in the case of a workplace), it appears that a persistent threshold shift is eventually followed by permanent threshold shift, i.e.: partial deafness at higher frequencies.
Tinnitus
This defect is experienced by most people, usually in the form of a high pitched ringing in the ears. It can occur spontaneously and last for only a few seconds or, in some cases, persist for long periods of time.
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