NOISE
According to ILO, noise defined as “all sound which can result in hearing impairment or be harmful to health or otherwise dangerous”.
or
Noise is unwanted sound judged to be unpleasant, loud or disruptive to hearing.
A source of noise (e.g. a chainsaw or hi-fi system) emits longitudinal pressure waves into the air. These pressure waves then move away from the noise source at the speed of sound, displacing the air.
The characteristics of a sound pressure wave can be visualised by plotting a graph of pressure (measured in Pascals) against time, as in the following figure.
CONTINOUS AND IMPULSE NOISE
Noise can be continuous, variable, intermittent or impulsive depending on how it changes over time.
Continuous noise is noise which remains constant and stable over a given time period.
Example: The noise of boilers
Impulse noise is a noise which includes unwanted, almost instantaneous sharp sounds.
Example: Noise from scratches recording disks
EFFECTS OF NOISE
A) Hearing Loss
Hearing loss (the process of losing auditory sensitivity) occurs to a degree naturally with age, but it may occur to a greater degree as the result of external causes. It can be classified under two broad headings:
• Conductive hearing loss occurs due to a physical breakdown of the conducting mechanism of the ear resulting from an acute acoustic trauma, e.g. an explosion or gunfire. The eardrum, ossicles or the cochlea can be damaged, often beyond repair. There is no cure, although surgery may reduce the damage to the eardrum.
This form of hearing loss is rarely caused by occupational noise exposures.
• Sensorineural hearing loss occurs when the hair cells in the cochlea are damaged. Harm may result from natural causes, such as infection, or by physical injury.
In an occupational setting, sensorineural hearing loss occurs from long-term exposure to excessive noise, and so this is the type of hearing loss most commonly found by the safety practitioner.
B) Tinnitus
Tinnitus is a condition where the sufferer hears “ringing in the ear” or other types of noise in their head without there being any external noise source. There are no observable external symptoms.
Tinnitus can occur after exposure to excessive noise levels as an acute condition which recedes with time (e.g. after attending a rock concert). The recovery period could be 12 or more hours where very high exposure levels occur. People who have chronic noise-induced hearing impairment can suffer from chronic tinnitus.
The symptoms of tinnitus suggest that damage to the nerve structure of the cochlea or the auditory nerve has occurred, or possibly both.
C) Threshold Shift
Threshold shift is a reduction in a person’s ability to hear, i.e. they need more sound intensity to stimulate their hearing. The condition can be permanent or temporary.
• Temporary Threshold Shift (TTS)
Temporary threshold shift occurs after exposure to high noise levels where hearing acuity returns with time. The condition has been described as a fatigue of the hair cells in the cochlea. The level of threshold shift is expressed in terms of the raising of sound intensity required to hear a given sound level, e.g. a 20dB shift means the sound level has been increased by a value of 20dB before the individual concerned could detect the sound.
• Permanent Threshold Shift (PTS)
Permanent threshold shift is the condition where there is a permanent reduction in hearing acuity. This reduction is irreversible, with no recovery of hearing acuity with time away from exposure. The condition follows from repeated TTS exposures, as might occur in a workplace where noise levels are high day after day and hearing protection is not worn.
D) Noise-Induced Hearing Loss (NIHL)
Noise-induced hearing loss is a condition which results from failure of the hair cells in the cochlea to respond fully to sound intensities having frequencies within the human speech range. The person does not necessarily lose the ability to hear sound, but is not able to distinguish the spoken word clearly, even if it is presented with a raised voice.
E) Other Effects of Noise on Health
In addition to the short- and long-term effects of exposure to noise on hearing, there can also be other health effects:
• Neuro-psychological disturbances such as headache, fatigue, insomnia (sleeplessness) and irritability.
• Cardiovascular system disturbances such as hypertension and cardiac disease.
MEASUREMENT & EVALUATION OF NOISE
MEASUREMENT OF NOISE:
Sound Pressure Level: a measure of the intensity of the pressure wave moving through the air. It is normally expressed using the decibel (dB) scale
Decibel (dB) – the unit of sound pressure level
Frequency: a measure of the number of pressure waves that pass a fixed point in one second, the unit is the hertz (Hz). The human ear is sensitive to noise across a wide range of frequencies: from 20 Hz (very low frequencies bass) to 20000 Hz (very high frequencies – high pitch).
Noise Measurement Instruments:
• Simple sound level meter – Measures instantaneous noise levels and can be used for spot checks or for very simple surveys
EVALUATION OF NOISE:
- Quantify the level and duration of exposure of workers and compare to exposure limits established in national or international standards.
- Identify and characterise the sources of noise and the exposed worker.
- Create a “noise map” for the determination of risk areas
- Assess the need for control measures.
- Evaluate the effectiveness of existing noise prevention and control measures.
NOISE EXPOSURE CONTROL
Wherever noise is a problem, there are three orders of priority for dealing with it. In simple terms noise exposure can be controlled in three ways:
1. Reduce the noise at source (through equipment selection or engineering controls
2. Interrupt the pathways from source to receiver (through engineering controls)
3. Protect the receiver (through engineering controls or PPE)
• Reduce noise at source
Eliminate hazardous noise at source – remove the noise source entirely. Where this is not reasonably practicable then:
– Change the source to one that generates less noise.
– Relocate the noise source.
– Redesign the equipment or task to reduce the noise at source.
– Carry out maintenance.
– Implement a purchasing policy so that only low noise equipment is purchased.
• Control (attenuate) noise transmission
Reduce the transmission of noise before it reaches the worker:
– Isolate the source to prevent transmission, e.g. using anti-vibration mounts.
– Acoustic barriers that interrupt the movement of sound waves through the air.
– Acoustic enclosure of the noise source.
• Control noise exposure at the receiver
– Acoustic havens.
– Hearing protection zones and the use of passive and active hearing protection.
– Limit exposure time.
– Health surveillance (audiometry).
Hierarchy of Noise Control
AT SOURCE:
• Eliminate the source – Completely remove the noise source, not practical in many instances
• Substitute the source – change the noise source for something else that does the same job but generates less noise. Example: change a petrol driven machine for an electric version)
• Modify the process – by changing from glass to plastic bottles, noise from a packing line can be reduced, or by replacing a compressed air rivet gun with a screw fixing manufacturing noise can be reduced
• Maintenance – machinery often produces noise because it is in need of maintenance
• Damping – machine parts (especially metal surfaces) can sometimes resonate in harmony with noise being produced by the machine (like a cymbal that rings when hit). This exaggerates the noise generated. Damping changes the resonance characteristics of the metal part to prevent it ringing in this way. This can be achieved by changing the part, stiffening it or even adding material to one side of it.
• Silencing – Any machine that produces exhaust gases (e.g., a diesel generator) should be fitted with a silencer on the exhaust to suppress noise
AT PATHWAY:
• Insulation – in some instances it is possible to build an acoustic enclosure around the noise source. Noise is generated inside the enclosure but cannot penetrate through the walls to the outside work environment. For example, a static diesel generator might be placed in a separate building with sound insulating walls, or a noisy pump may be enclosed in an acoustic hood
• Isolation – noise is often transmitted in the form of mechanical vibration from machinery into supporting structures (e.g., from a compressor into the floor it is mounted on). Isolation involves separating the machine from any supporting structure using vibration absorbent mats or springs. This breaks the transmission pathway
• Absorption – once noise has escaped from its source it may travel directly to the receiver through air or may be reflected off hard surfaces (such as walls and ceilings). Absorption involves putting sound absorbing material in the workplace to absorb these sound waves before they can reach the receiver,
e.g., a sound absorbent material might be used to line a wall, preventing reflection of sound waves
RECEIVER:
• Acoustic haven – if the workplace is inherently very noisy and it is not possible to apply the above controls, then an acoustic haven might be built that workers can retreat into to escape the noisy environment.
• Hearing protection – if none of the above is effective or possible then some form of hearing protection should be used to reduce the amount of noise that penetrates to the workers ear.
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