After reading this article you will learn about Noise Measurement and Control in Factories:- 1. Necessity for Noise Measurement and Control 2. Methods of Noise Measurement 3. Threshold Sound Levels 4. Risk Areas 5. Measuring Instruments 6. Control 7. Control of Sources 8. Prevention of Propagation 9. Remote Control and Isolation 10. Hearing Protection.
Contents:
- Necessity for Noise Measurement and Control
- Methods of Noise Measurement
- Threshold Sound Levels
- Risk Areas
- Noise Measuring Instruments
- Noise Control
- Control of Noise Sources
- Prevention of Noise Propagation
- Remote Control and Isolation of Noise
- Hearing Protection
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1. Necessity for Noise Measurement and Control:
The movement of the particles of an elastic medium to either side of their position of equilibrium generates acoustic vibration. Sound may be defined as an acoustic vibration capable of producing an auditory sensation. The frequency of audible sound ranges from about 16 Hz to approximately 20 kHz. Noise may be considered as any unpleasant or disturbing sound.
Steady-state noise is the one with negligible fluctuations in the sound pressure level during the period of observation. Non-steady-state noise, on the other hand, is a noise the level of which varies notably during the period of observation.
Non-steady state noise covers fluctuating noise, intermittent noise and impulsive noise. The last one (i.e., the impulsive noise) consists of one or several pulses of acoustic energy, each pulse lasting less than one second.
It is necessary to mention its sound pressure level as well as the frequency in order to describe a noise correctly. The sound pressure level depends on the amplitude of the acoustic vibration. The sound pressure level is measured in decibels (dB), and it defines the intensity of noise. On the other hand, the frequency (i.e., the number of vibrations per unit of time) is measured in cycles per second (c/s) or Hertz (Hz).
The A-weighted sound pressure level (LA) is defined by
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LA = 20 Iog10 (Pn/P0)dB(A), ….(1)
where Pn = effective value of the sound pressure produced by the noise to be measured, P0 = reference sound pressure = 2 x 10-5 Pa, and dB (A) = “slow” response A-scale decibel.
The equivalent continuous sound level is the sound level in dB (A) which, if present during 40 hours per week, would yield the same compound noise exposure index as the various sound levels measured in the course of one week. This implies that if the sound intensity increases by 3 dB (A), the length of exposure should be halved in order that the two degrees of noise exposure be equivalent.
2. Methods of Noise Measurement:
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The methods to be adopted for measuring and assessing the degree of noise depend on the objective to be attained by such measurements.
The objective may be any of the following:
(a) The assessment of hazards of hearing loss;
(b) The assessment of the degree of interference with verbal communication essential for safety; and
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(c) The assessment of the hazard involved in a work task.
Noise should be measured using standardised methods which are adapted to the objective to be attained. Moreover, the methods should conform to international standards or to the equivalent national standards.
When it is required to assess the hazards of noise to hearing capacity, the noise in question should be measured in such a manner that as accurate a picture as possible is obtained of the degree of exposure, and that the results obtained may be compared with the threshold limits.
When the noise levels are evaluated, one should take into account the normal conditions of work and also the circumstances under which the highest noise levels occur. If the noise is of the steady-state type, the sound level at the workplace and the equivalent continuous sound level should be determined in dB (A). The frequencies should be analysed according to standard methods.
In order to assess the actual exposure to a non-steady-state noise of the impulsive type, it is advisable to choose, between the following two methods, the one which yields the higher figures:
(A) Measurement using the sound level meter in the “impulse” position, the mean value for an exposure of 8 hours per day being calculated on the basis of the equal-energy principle; and
(B) Application of a certain (positive) correction facto (generally 3-10 dB(A) added to the “slow” response values as determined in accordance with international or national standards. (The value assigned to this correction factor should depend on the importance of the “impulse” characteristics of the noise to be measured.
When it is required to assess the degree of interference with speech communication, the noise should be measured in the following places:
(i) Noisy work areas where it is important (for reasons of safety) that the workers are able to hear a message or signal; and
(ii) Noisy work areas where the workers are liable to be subjected to a supplementary stress (or even handicapped in their work) on account of difficult speech communication.
In this connection, the maximum distance over which speech at a normal voice level is intelligible should be determined.
Finally, when it is required to make an assessment of the hazards of excessive noise involved in a work task, and to assess its potential for causing fatigue in industrial workers, noise should be measured in the following two types of places:
(i) Noisy work areas where it is important (for safety reasons) not to expose the workers to excessive noise-induced stress and fatigue; and
(ii) Noisy work areas where the nature of the task performed by the workers is such that noise is liable to handicap them, or to make the task more difficult or more demanding.
3. Threshold Sound Levels:
Threshold sound levels (or hearing damage risk criteria) should be defined in accordance with the aim to be attained.
This aim can be any of the following:
(a) Preventing, hearing damage risk;
(b) Preventing interference with communication essential for safety; and
(c) Preventing nervous fatigue (taking into account the nature of the task performed).
In order to prevent hearing damage risk, the following maximum values should be fixed according to the desired degree of protection:
(a) An alert threshold (establishing the sound level below which there is very little risk of hearing damage as a result of an exposure of 8 hours per day); and
(b) A hazard threshold (determining the sound level above which there may be a risk of hearing damage and deafness for an unprotected ear which is exposed for 8 hours per day).
For these thresholds, the following values may be recommended in view of the present state of art:
(a) An alert threshold limit of 85 dB (A); and
(b) A hazard threshold limit of 90 dB (A).
We note here that these values correspond to equivalent continuous sound levels (Leq), and should be compared with the results of noise measurements. No worker should be allowed to enter into an area (for any period of time whatsoever) where the sound level is equal to or exceeds 115 dB (A) unless he is wearing appropriate hearing protection.
Personal hearing protection should also be worn in places where isolated noise peaks are beyond 130 dB (A) “impulse” response or 120 dB (A) “fast” response. Similarly, no worker should ever be allowed to enter into an area where the sound level exceeds 140 dB (A).
4. Risk Areas:
The noise level should be measured at all of the following type of workplaces:
(a) The place where the task performed or the working environment are liable to present a noise hazard;
(b) The place where workplace supervision, medical surveillance or inspection visits reveal that there may be a noise hazard;
(c) The place where workers consider that they are exposed to a noise level which inconveniences them, or disturbs their work; and
(d) The place where speech communication at a normal voice level is interfered with at a distance of 50 cm.
Noise surveys should be conducted at workplaces to evaluate the environmental noise in the various shops of an industrial plant. The noise levels should be measured at a height of about 1.5 metres above the floor or work area, and at a distance of at least 1.0 metre from the walls. One should preferably establish the mean value of the sound levels recorded in different directions.
It is advisable to locate the sources of noise, and to determine the levels of noise generated by them, by appropriate measurements. If the results obtained by a preliminary survey show noise levels exceeding 85 dB (A), the noise levels should be mapped out at workplaces. In this way, a noise chart should be established of the areas where sound levels are equal to, or exceed, 80,85,90; 100 and 115 dB (A).
In order to assess the degree of noise exposure, measurements should be taken at places usually occupied by the workers in the area under consideration. In such cases, the noise level should be measured at the point where the worker’s head is, while he keeps his normal work posture, or at a distance of about 1 metre from either side of the worker’s head in the normal work posture.
An additional assessment of a worker’s exposure to noise may be made with an integrating noise dosimeter of an approved type. The worker wearing such a dosimeter should, however, be trained to use it correctly. He should, moreover, be supervised by a competent person.
5. Noise Measuring Instruments:
The initial attempts to assess sound levels were made by listening to sound of a given frequency, and by comparing them subjectively’. However, the measurements of this kind are time-consuming.
In addition to this, the results of such measurements differ between subjects, and also between assessments made by the same subject. This led to the idea of devising special instruments (called “sound level meters”) which incorporated certain characteristics of the human auditory system.
Sound level meters are electronic instruments consisting of the following components:
(a) A precision microphone;
(b) A high-quality amplifier;
(c) A detection system; and
(d) A galvanometer with a dial (or digital display) indicating the sound level in db.
The sound level meter may be an autonomous miniature unit, easy to carry and use. These instruments must confirm to standards laid down on an international level by the International Standardisation Organisation (ISO) and the International Electrochemical Commission (IEC). The usual measuring range of sound level meters is 35 to 130 dB (A), which is suitable for a wide variety of applications.
Whether or not a noise is disturbing or hazardous depends mainly on the following of its characteristics:
(i) The level of sound;
(ii) Its duration; and
(iii) Its frequency content.
These three factors are combined in the concept of equivalent continuous sound level (Leq). The integrating precision sound level meter is an Leq indicator, with digital or analogue display. The intensity of noise often varies near a machinery or at places where workers may be at times in the course of a work-day, for example, on the way from one department to another, or at the canteen.
A simple method for the assessment of the hearing impairment under these conditions consists in using a personal noise dosimeter worn by the worker everywhere he goes. After having been worn during an 8-hours work-day, the dosimeter indicates the percentage of the noise dose.
An indication of 100% on the noise dosimeter corresponds to an exposure to 90 dB(A) for 8 hours, or to an equivalent combination of exposures to higher or lower levels. If it is possible to obtain representative measurements results in less than 8 hours, a table is supplied for their conversion into 8-hours exposures.
6. Noise Control:
Control of noise (i.e., its prevention and reduction) is a system-related problem.
This system is composed of the following components:
(a) The source of noise;
(b) The path of sound propagation; and
(c) The receiver of noise.
Among these, the source is part of the system that produces the acoustic energy. It may, for example, be an engine, air flowing in a duct, or gear teeth. Generally speaking, the source should be considered as a group of noise generators which present different physical characteristics randomly distributed in space and time.
The acoustic energy produced by the source is transmitted to the environment in which it is propagated, and which may be a solid structure or air. The third component of the system (i.e., the receiver of sound) may be a worker operating his machine.
Noise prevention and reduction measures should be aimed at the following goals:
(i) Controlling sources of noise;
(ii) Precluding the propagation, amplification and reverberation of noise; and
(iii) Isolating the workers.
Noise control measures should also be directed at the attenuation of the noise by removing or isolating the workers from the sources of noise, or by providing personal hearing protectors. It is possible to combine different noise control measures in order to achieve an appropriate reduction of noise levels.
7. Control of Noise Sources:
It is useful to distinguish between three essential categories of noise sources, which are:
(a) Those sources where the emission of noise results from the vibration of a solid or liquid surface (due to mechanical forces)
(b) Those sources where the emission of noise results from turbulences in a gaseous environment (due to aerodynamic forces); and
(c) Those sources where the emission of noise results from electro- dynamic or magneto-dynamic forces, from an electric arc, or from an electric corona discharge (due to electric forces).
Evidently reduction of noise at the source is the most rational method of noise control. If all machines and all prime movers were sufficiently silent, there would not be many problems of excessive noise left for solution.
Of course, noise reduction at the source necessitates investigations at the design stage; and modifications of machines, prime movers and processes. All this may be costly, and this explains the reluctance of employers and machinery manufacturers for controlling the noise at the sources.
Some of the measures for controlling the noise at the source are as follows:
(i) Reduction of vibration intensity by maintaining the dynamic balance, diminishing the force acting on the vibrating part, reducing the number of revolutions per minute, and increasing the duration of the work cycle;
(ii) Reduction of response of vibrating elements by increasing their damping power, and improving their fastening;
(iii) Reduction of turbulence and of the speed at which fluids contained in pipes and ducts pass through the inlet and outlet openings;
(iv) Conversion of impact into progressive pressure;
(v) Conversion of reciprocating movements into rotational movements;
(vi) Replacement of sudden stoppage by progressive breaking;
(vii) Replacement of straight teeth spur gears by helical-teeth spur gears, and substitution (if possible) of plastics and other materials for metals;
(viii) Design of the shape and cutting speed of tools in conformity with the characteristics of the material to be machined;
(ix) Prevention of impact when objects or bulk materials are mechanically conveyed, and of their dropping freely from the conveyor end;
(x) Appropriate design of burners, combustion chambers, and explosion chambers;
(xi) Taking into account electro-dynamic, magneto-dynamic and aerodynamic noise sources when designing electrical equipment;
(xii) Installation of damping elements of points of contact between machine and plant elements;
(xiii) Appropriate design of fan blades; and finally
(xiv) Appropriate design of compressed-air lines, ventilation ducts, and pipework for liquids to prevent propagation of noise.
If there is more than one source of noise in a given area, the noisiest source should be controlled first to attain an efficient reduction of the overall noise level.
8. Prevention of Noise Propagation:
Since noise can spread from a single source taking various paths, it is essential to study its transmission with a view to preventing it in the most efficient manner.
In addition to this, one should also take measures to reduce amplification and reverberation of noise.
The control of noise propagation may be achieved by the following measures:
(a) Installation of machines on vibration-damping bases, which are isolated from the floor and walls;
(b) Insertion of damping materials between machine bases and foundations, and use of anti-vibration mountings; and
(c) Separate installation of noisy machines to avoid noise propagation by other elements of the plant and premises.
On the other hand, the following measures are useful to control the propagation and reverberation of airborne noise:
(i) Complete or partial enclosure of noisy equipment and machines;
(ii) Installation of sound barriers, sound-absorbent linings and sound- isolating partitions;
(iii) Taking into account the acoustic factor when the premises are designed and laid out; and
(iv) Soundproofing of premises (lining of walls, partitions, floors and ceilings with damping and absorbing materials).
Silencers may also be used, if necessary, to prevent the propagation of noise.
9. Remote Control and Isolation of Noise:
Sometimes the level of noise emitted by a machine or an equipment is so high that it is difficult to control it by conventional measures.
In such cases, the following approach may be helpful:
(a) Such a machine should be operated by remote control, and the process should be watched with the aid of remote-indicating displays; and
(b) The machine or equipment should be installed, if possible, in a separate room to reduce the number of workers liable to be exposed to high levels of noise.
If it is not possible to apply noise control methods in particularly noisy premises, or if conventional noise control measures do not yield satisfactory results, sound-isolated shelters or cabins (also known as “noise refuge”) should be set up, so that the operators can control the entire process (or at least most of it) from such cabins.
10. Hearing Protection:
If it is not possible to reduce the noise below the hazard level by appropriate design or installation of the machinery, the following measures should be taken to protect the workers from excessive noise:
(i) There should be noise shelters close to the workplace which may be hermetically closed (with air conditioning, if necessary) or open on one side;
(ii) The workers should wear appropriate hearing protectors; and
(iii) The length of noise exposure should be limited.