Sound is a good thing both to science, technology, engineering and the entire society at large. It is a medium or means of communication, but beyond communications, there are other uses of sound to man and its environment. The focus of this work is not on the uses of sound but on the effect of sound when it becomes unpleasant, how it can be controlled, prevented or converted and used for the benefit of man. This unpleasant sound is referred to as noise which is classified as a pollution, although there are different types of noise, some of which has to do with unwanted interference to signals, causing a deviation in output result. Throughout this work, the word noise, whenever it's being used refers to acoustic noise.
To understand the concept of noise, we must first understand what is sound, so we can truly understand noise (when sound becomes unpleasant, unwanted and too loud for the ear), Sound is a phenomenon which occurs when a form of energy is transmitted, resulting in pressure variation that can be detected by the human ear. Sound has been proven to travel through mediums such as air, water, steel etc and cannot be transmitted in a vacuum. When a disturbance is created from a source of sound such as vocals, musical instruments, etc, it causes a vibration of the transmitting medium and generates pressure waves that can be audible to human ear.
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Sound is characterized by two major properties, i.e frequencies of transmission and loudness. These properties are characterized by two measures, i.e amplitude and pitch. The amplitude talks about the maximum height or extent of vibration or disturbance measured from the equilibrium position, while pitch is a measure which tells us how high or low a sound tone is. The amplitude is proportional to the loudness while pitch is proportional to the frequency as shown in the figure.
The frequency of a sound is a major factor in the study of acoustics, it is a measure of the number of variation of pressure with time, it has a unit called Hertz (Hz) which signifies the number of circles completed in one second or the number of pressure fluctuations per second. The ear responds to sound based on frequency, i.e the higher the frequency the more high-pitch sound the ear perceives. The response of the human ear to sound is a product of the generated sound frequency. A low-frequency sound has a low response, while peak response falls around sound with frequencies of about 2500 to 3000(Hz).
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Another major characteristic of sound as mentioned above is loudness of the sound. The pressure variation is usually larger with a loud sound when compared to that of a weak sound. For the convenience of expression, the loudness of sound is now measured in decibel (dB) which is a logarithm scale, because measuring pressure variation in the normal pressure scale in pascal (Pa) will give very small values such as 20(Pa) and very large values up to 2,000,000,000(Pa), making it difficult to measure. The faintest sound that the human ear can pick is one decibel (1(dB)), while sound loudness up to eighty-five decibels (85(dB)) is ok to the human ears. The decibel of a sound decreases as the distance from source increases, at a very far distance the sound cannot be captured when decibel falls below one which is the minimum the human ears can capture.
Noise From Industrial Activities
There are different sources of noise, which can de be classified into three different classes namely; industrial noise, transport noise and neighborhood noise. The concept of noise to a physicist is indistinguishable from sound, to him they both involve the variation of pressure through a medium that is audible to the ear, but to a health and safety personnel, noise is an unwanted, unpleasant sound and in certain conditions harmful to human. The health effect of noise on a human is a function of the loudness, frequency and the duration of exposure to the sound. In the workplace or any other high noise producing environment, the number of hours exposed to high sound/noise producing machines or areas are very important, as continuous exposure becomes harmful. Noise is responsible for ailments such as partial or complete deafness, high blood pressure, imbalance sleep, injuries, ischemic heart disease and poor academic performance. There is a relationship also between noise and some psychological effects such as mental disorders, annoyance etc. With these health effects of noise on man, there is a need to control or reduce industrial noise pollution.
Noise Control
There are several factors to be considered before controlling noise, these are the noise source, the medium of noise propagation, room (eg. engine-room) acoustics, frequency distributions and pattern and level of noise. The first thing to be considered when controlling noise is the source. Noise control should be initiated at the source, especially during design. The amount of noise to be produced should be evaluated during design and a less noise producing design such as soundproofing if possible should be adopted.
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As established above, noise ought to be controlled or reduced from the design stage, in the absence of this or the prevalence of noise after the design stage, other mediums or ways ought to be employed to check and reduce noise in the workplace. Three terms must be considered in line with their interrelationship in producing the noise, this is to help develop a better solution. These terms are; the sources, transmission part and finally the receiver (worker). There are four basic ways to reduce noise pollution in the workplace, these are; the use of absorbers, vibration barriers, vibration isolators and vibration damping.
Absorbers
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These are sound absorbing porous materials (e.g fiberglass, open cell foam, mineral wool etc) which convert sound energy into heat energy. It is applied to absorb energy while reducing reflection of sound at a reflective surface, it is more effective when the noise frequency is very high. The property of a material which tells its ability to absorb sound is called sound absorbing coefficient.
Vibration Barrier
This is the used of high-density materials such as bricks, concrete, metals etc to block, prevent or reduce the transmission of sound/noise. The characteristic of a material which shows its ability to prevent or block sound is known as transmission loss (TL). Transmission loss (TL) is a function of frequency, i.e it increases with increase in frequency. Openings should be reduced to the barest minimum as little opening will result in a large reduction of transmission loss (TL).
Viberation Insulator
This involves the introduction of a flexible element or physical break to reduced transmitted vibrations. Noise is mostly produced by the impact of one body on another, the use of this vibration insulators will reduce this impact and as a result of this, the noise is reduced or isolated. Some materials used as vibration insulators (pads) are; rubber, cork, urethane and other similar materials.
Vibration Damping
This involves the use of materials such as sound deadened steel to extract vibration energy from a body usually a thin sheet and dissipating this vibration energy as heat. This method involves the application of damping materials on vibrating surfaces to convert mechanical motion being produced into the very small amount of heat.
Conclusion
The world today is full of many industrial activities, this activities most times involve mechanical motions of parts, therefore sound is produced as a result of these interactions. We cannot go back in time where there was no industrialization, but we can reduce the harmful effect of our industrial activities. Noise pollution has been a harmful effect of industrialization, but little or no attention is being paid to it, this is responsible for most health challenges and psychological predicaments such as total or partial deafness, imbalance sleep, annoyance, mental disorder etc. To solve or prevent this problem, some measures such as soundproofing in design stages, the introduction of absorbers, vibration barriers, vibration isolators, vibration damping etc must be put in place. If these measures are taking into consideration and things are put in their proper positions, we will have lesser industrial noise pollutions thereby resulting in a better and safer society for man.