noise = "unpleasant sound"
Not all sound is noise. It is only the sound that we do not like that we call noise. Machinery that has been made more efficient and faster has increased noise levels. In large open work areas, noise is widespread and can increase in intensity as additional machines add to the noise level. The perception of noise is personal, however it is clear that any of us can have our hearing damaged, in some cases irrepairably, it the sound level is too high.
Noise can cause damage to hearing, masking (disturbing to conversation), irritation or annoyance.
At many workplaces the level of noise can be injurious to hearing. Often the noise is created by different types of processing machines. Tools powered by compressed air, or manual work hammering metal objects also creates loud noise.
Noise which is continuous at a level of 85—90 dB(A) or more is injurious to hearing. You risk damage to your hearing if you spend more than five hours a day at this level of noise.
Hearing can also be damaged by sudden loud noises from an explosion near the ear. Unfortunately, you can’t close your ears like you can do with your eyes.
Masking noise means that the sound we want to hear is drowned by another sound. This can cause accidents, because warning shouts and other signals are not heard. At a workplace the noise from machinery can mask the sound of a truck or disturb a conversation. Similar problems exist in most industrial workplaces. The level of sound should not exceed 60—70 dB(A) if we are to conduct a conversation at a normal distance.
Irritating noise exists in various forms. The extent to which we feel that a noise is irritating is in part subjective, depending on our feeling as to whether the sound is desirable or not. Even a low volume sound can be irritating.
Restaurant patrons may enjoy music being played in the background while the waiters might find it irritating. We also frequently find this problem in the home, we can feel that our neighbour’s music is irritating while he thinks that it is wonderful.
Long periods spent in areas of high noise levels can result in a measurable deterioration in hearing. The ear is a highly sensitive organ that we must protect. The upper picture shows a cross-section of an ear. Sound travels through the outer ear, and causes vibrations of the ear drum and the three ear bones (hammer, anvil and stirrup). The vibrations pass through the fluid contained in a tightly coiled tube called the cochlea. The auditory nerve which has endings in the cochlea transmits the signal to the brain. It is the cochlea which is damaged when we are exposed to an excessively high sound level. The very fine nerve endings in the interior of the cochlea are damaged and die. This damage is irreversible. It cannot be repaired. Individuals have different degrees of sensitivity to noise. Some people’s hearing can be damaged faster than the others.
Temporary noise-induced hearing loss
If after a short time in a noisy environment we leave it for a quieter one and do not at first hear the quieter sounds, our hearing has been reduced this is called temporary noise- induced hearing loss. This kind of loss is reversible. Normal hearing will return after a period of rest.
It is therefore important that those who spend time in noisy workplace be allowed to rest their ears by taking a break in quiet surroundings.
Temporary noise-induced hearing loss is a warning sign. You must avoid working for many years in such an workplace to avoid permanent hearing loss.
Hearing is an important sense which helps us establish contact with our surroundings. If we permanently or partially lose our hearing, we will find it difficult to hear what other people are saying and difficult to listen to the radio or TV Partial or total hearing loss. You may find it very difficult to understand normal speech. You may have difficulty in understanding what supervisors or workmates say and in recognising sound signals. You may suffer from a feeling of isolation as you cannot participate in discussions during breaks or conversations with friends during time off. You may need more rest and relaxation than normal because you must often use all your energy to do your job.
Sound may consist of one pure tone but mostly it is a mixture of many tones of different volumes (loudness) and pitches (high or low frequency). The volume is measured in decibels (dB). The pitch is measured in hertz (Hz). If we find a sound disturbing it is not simply due to its loudness. The pitch can also be a contributing factor. High tones irritate more than low tones. Pure tones may be disturbing and can cause more injury than complex sound.
Noises from different sources blend together. The level of total noise in any area increases with the number of sources of noise.
It is important to bear these facts in mind when discussing the intensity of noise and changes in noise intensity. Saying that the noise level has been cutfrom 90 dB(A) to 80 dB(A) doesn’t sound very significant, but with the particular characteristics of the decibel scale in mind, it means that such a reduction is the same as the effect we get in a workplace if we remove nine out often noisy machines.
The following are some methods of noise control :
Machinery and production processes
Many sources of noise produce both air-borne and structure-borne sound. We must, therefore try to reduce the level of noise both from the machinery and on the work premises. It can be difficult to reduce the amount of noise produced by machinery and production processes without interrupting or reducing production. Therefore, try to enclose entire machines, or particularly noisy parts of machines. Service the machines regularly to keep them in good condition and thereby reduce the noise. Reduce the vibration in component parts and casings, etc. Replace metal parts by parts made of sound-absorbing material, e.g. plastic, rubber or other soundproof materials. Lengthen the braking period for reciprocating parts.
A well designed soundproof casing can reduce the noise level up to 55 dB(A). If the level of noise is a workplace is higher than 80 dB(A), steps must be taken to reduce the overall noise level. This can entail the construction of soundproof casing around the machines or the use of personal hearing protectors. Building a soundproof cover around the noise source or similar measures is preferable to relying on hearing protectors. However while waiting for such improvements, always use hearing protectors.
Noise from equipment can be reduced efficiently without taking complicated measures. We can reduce this by:
Fitting a screw-on muffler on the exhaust outlet.
Changing the type of pneumatic equipment to reduce high-frequency components.
Replacing the equipment with a quieter model e.g. quieter fans with proper blades.
Properly fastening some noise-producing parts.
Isolating or screening the workplace containing noisy equipment.
Noise generated by the handling of materials can often be reduced. Avoid banging and knocking materials during handling, particularly transportation equipment. Reduce the height that goods in bins and racks can fall. Reinforce sheet metal constructions that are hit by packets or materials or cover them with sound-proofing sandwich sheets or similar materials. Reduce sharp blows by using rubber or plastic coverings. Reduce the speed of the conveyor systems. Install several conveyor belts to divide the load and thereby reduce the speed.
Here is another method of reducing noise in transporting goods : Many companies have a chute for materials. As a rule this is a large funnel made of sheet metal, plastic or wood. When the material hits the chute a high level of noise is generated. The noise can be reduced by covering the outer surface of the chute with sound-proofing material. When purchasing a new chute, make sure it is of a sandwich type construction.
The following example shows how steps can be taken to reduce the level of sound resonance, i.e. sound which is amplified through contact with e.g. discs and wheels.
The distribution of sound on the actual work premises can be reduced by covering the ceilings and walls with sound-absorbing materials, called porous material. Using sound-absorbing screens, building sound-proof control areas and rest rooms. Installing sound-proof material in the ceiling lowers the general level of sound on the premises can also be done. If the noise sources are isolated by a wall or enclosed in sound-proofing casings the noise level can be reduced both near the source of the noise and further away. Those people who are working at quieter jobs with quieter machines are not then unnecessarily disturbed by surrounding sources of noise.
By covering the ceilings and walls with sound-absorbing materials, the distribution of the sound on the premises can be reduced and the level of noise lowered. It is often necessary to place machinery which creates noise on elastic pads or feet. This isolates the vibrations and prevents them from being distributed through the structure of the building, ducts, pipes, etc. Large, heavy machinery should be placed on individual foundations separated from the main workshop floor.