In this work, reference is made to a set of methodologies, measures and procedures to be able to control one of the most aggressive agents of the environment: noise.
methodologies-evaluation-control-noise-business-productionDevelopment
1. Noise control measures
This section will deal with some of the possible measures, which must be taken into account to achieve acceptable sound pressure levels in the industry. In whichever case it is used, the following points should be taken into account:
1. Noise control is everyone's problem (Man, machine and a half).
2. Your goal is to achieve an acceptable noise level at an acceptable cost.
3. The success of the control is measured by the noise reduction achieved.
4. The control can be carried out at any point in the set.
5. Control represents a compromise between success and cost.
Authors such as (MAGGIOLO 2004) and (ABÁSOLO 2000), have taken on the task of defining the actions to be carried out, in order to obtain a good control of the noise that affects workers.
They can be included in three types that are: Actions on the source of noise, on the environment and on the worker. Figure 1 presents the schematic representation of these measurements. Each of them will be analyzed below.
Actions on the noise source
They are the most suitable; provided that they are feasible, since their objective is to eliminate noise at its source, for that it is necessary to carry out actions that are aimed at:
- Modify production processes.
- Replace pneumatic tools and equipment with power tools.
- Eliminate friction in moving machinery, surface finishing and greasing.
- Achieve machine balancing and alignment.
- Place silencers on air leaks and / or turbulence in fluid movements.
- Avoid the transmission of vibrations between components by placing elastic joints.
- Incorporate shock-absorbing materials between colliding surfaces and insert anti-vibration.
- Give a good maintenance to the work equipment.
Actions on the media
The actions on the environment in which the noise expands (enclosure), consist of stopping the passage of sound energy from the source of generation to the worker's ear. They should only be used when those mentioned above fail, for the achievement of their objective actions such as:
- Encapsulation or enclosure of noise equipment, (capsule design).
- When the capsule is not feasible, resort to isolating the focus and the worker, conditioning the latter in a cabin.
- Acoustic conditioning of the premises.
Actions on the receiver
The actions that are carried out for the prevention of risks in relation to the worker, must only be used when all the methods mentioned and analyzed above have been ineffective or unfeasible, either due to the characteristics of the work, the cost of control or for any other circumstance. The main actions frame:
1. Surveillance of the worker's health whenever there is a risk to him, through:
- Carrying out audiometry tests.
- Trials with sounds of determined frequencies.
2. It is mandatory for the company's safety managers to inform and / or train the worker about the risk to which their health is exposed if they do not comply with what is provided for their protection:
- Use of individual protection means: earplugs, ear muffs, helmets, etc.
2. Noise control: Development of methodologies
Not only is it vital to carry out a correct noise assessment, but it is also necessary to know what the methods or procedures are for its elimination, either completely, or at least to reduce it to the maximum possible value. That is why this section will explain what each of the methods consists of, in cases where it is difficult to understand, the procedure will be developed with a practical example, to achieve a better understanding. Among the main methods are:
1. Use of elastic materials
Elastic materials have the property of being sensitively affected by the slight pressures caused by a sound wave.
- Elastic panel: They are made of a wood veneer and a support. Each panel has its own frequency and this is of great importance for the attenuation of the sound, because when its frequency coincides with that of the panel, resonance appears and the sound energy will be converted into oscillations to a maximum degree. Therefore the absorption of an elastic panel is maximum for its natural frequency. (Figure 2.).
The natural frequency is calculated by:
Where:
P: Panel weight (Kg / m2)
e: Space between the wall and the panel (cm).
The absorption of the panels improves if a material with a high absorption coefficient is placed in the air chamber that remains between it and the wall that acts as an elastic medium, being its most advantageous use for low frequencies.
- Resonator: The acoustic resonator consists of a cavity that communicates to the outside through a conduit or neck in whose mouth B, sound waves fall. It has a shape similar to that of a bottle. (Figure 3).
When a wave penetrates through B and advances through the neck it reaches N, in cavity V a sound is produced from N. But in this cavity a reverberation will originate and therefore, there will be an energy that will propagate through N outwards in a pulsating way and according to the set's own frequency, which travels in the opposite direction to the incident sound.
When this secondary emission receives a new wave that affects B, its actions will be counteracted and the resonator will act as an absorber, being maximum when the frequency of the incident sound coincides with the natural frequency of the resonator, producing the resonance in opposition or cancellation.
The natural frequency of a resonator is given by:
Where:
v: speed of sound (cm / sec).
S: neck section (cm).
L: Neck length (cm).
V: Volume of the cavity (cm3).
Conductivity (c) is the coefficient of the neck section divided by its length.
So substituting values is:
The conductivity term C will take different values depending on the particular characteristics of the neck: if it is circular, square, rectangular, etc.
Resonators are practically achieved by drilling a plasterboard or aluminum and suspending at a distance from the ceiling and interposing a layer of mineral fiber. Each hole will act as an individual resonator, its absorption being maximum for high frequencies. (Figure 4).
2. Acoustic Treatment
It is one of the most used techniques to reduce high levels of sound pressure when there is a reverberant field, it is the use of absorbent materials. It consists of covering the walls and / or surfaces with these materials so that when the sound hits them, their reflection is reduced.
This method is of interest in jobs in which the problem is lack of intelligibility, for example in the service sector and the
teaching.
The steps to follow for the application of this methodology are displayed below:
Step 1. Evaluation of the existing noise in the premises
To comply with this first step, we start from defining the type of noise to which the worker is exposed. If this is constant noise, use table 1. "Evaluation of noise by criterion N". If the noise is not constant, the expressions of the continuous equivalent sound level exposed in chapter I of this thesis are used.
Step 2. Find the maximum allowable NPS in the premises according to the corresponding work activity.
To find the maximum allowable SPL, look at Table 1. “Maximum allowable noise levels”, the value that corresponds to the activity being analyzed.
The maximum permissible value for the case that the activity is not described in this table, activity I is taken, referring to all positions and premises. In this table, the values of maximum levels admitted are presented, either for constant noise or for non-constant noise.
Step 3. Calculate the level of reduction that should be achieved to eliminate the effects of noise.
Where:
NR: Reduction Level, (dB)
NPSE: Existing sound pressure level (dB), is the one that exists in the workplace for the analyzed frequency.
NPSR: Recommended sound pressure level (dB), looking at 1.1 with the frequency of interest and the NPS allowed in the room.
Step 4. Calculation of A1 (Room noise absorption before treatment).
Where:
A1: Room noise absorption before treatment, (metric sabines).
ST: Surface to be treated (m2).
α AT: Absorption coefficient before treatment (sabinos).
It must be taken into account for the calculation of A1 to all the surfaces that are described and that are present in the room.
Step 5. Calculation of A2 (Room noise absorption after treatment, metric sabinos).
Step 6. Calculation of the absorption coefficient (αT) that the material with which the surface is to be treated must have.
In this case, the roof of the premises will be treated.
A2 = ST * α T + A1 - A AT
Where:
A AT: Absorption of the surface to be treated before treatment (metric sab.).
Solving for the previous expression, we have:
With this value, you enter Table 1.2, of the absorption coefficients, and look for which of them can be used to cover the surface of the treatment.
An element to consider for the selection of the material are the costs, and the availability in the warehouse of these.
Step 7. Determine the maximum area to be coated with the selected material.
Where:
AR: Maximum area to be covered in the premises (m2).
α R: Absorption coefficient resulting from the treatment (Sabino).
Therefore, it can be said, the m2 necessary to cover, with the material selected to solve the problem of noise in the premises.
3. Use of Capsules
When noise cannot be controlled at its source, it is sometimes convenient to isolate it or confine it in closed rooms to avoid the propagation of its energy to other areas, where workers work. Within this closed enclosure, whose dimensions will depend on the characteristics of the noise, there will be a very high SPL, so it will be tried to avoid, by all means, the entry of people.
If such a situation is essential as in the forced draft fans of thermoelectric plants, individual protection measures must be taken extreme and exposure times controlled.
The calculation methodology is presented below.
Step 1. Evaluation of the existing noise in the premises
It is done in a similar way to step 1 of the acoustic treatment methodology.
Step 2: Determine the level of reduction to be achieved by the designed capsule.
Where:
∆ Lcr: Attenuation to be achieved by the capsule, (dB)
NPSE: Existing sound pressure level, (dB)
NPSE: Recommended sound pressure level, (dB).
∆ LCR = NPSE - NPSR
Step 3. Determination of the minimum distance (D) between the equipment and the inner surface of the capsule.
Where:
D: Distance from the edges or external surfaces of the
noisy object or equipment to the internal surface of the projected capsule, (m).
c: Speed of sound (343 m / s).
λ: Wavelength of sound, (m).
f: Minimum frequency that exceeds the allowed limit, (Hz).
m: mass of the material with which the capsule is to be manufactured, (Kg / m3).
To know if it is possible to use the material with which it is available, the condition shown below must be fulfilled:
Therefore, if this relationship is fulfilled, it is possible to use the material (steel plate), for the manufacture of the capsule.
Step 4. Determine the size of the capsule.
Where:
LC, AC, HC: Length, Width and Height of the capsule, (m).
LE, AE, HE: Length, Width and Height of the equipment, (m).
Step 5. Determine the capsule surfaces.
Note: If the width of the material to be used for the design of the capsule is less than 10 mm, the inner surface of the capsule does not have to be considered, therefore the inner and outer surfaces are assumed to be the same. Otherwise, it must be calculated, in the same way as the previous one but with the corresponding dimensions.
Step 6: Calculate the attenuation that the capsule will achieve. (∆ LC0).
Where:
∆ LC0: Attenuation that the capsule will achieve, (dB).
RRES: Resulting insulation coefficient, Table 2.2.
SC: External surface of the capsule, (m2).
AC: Equivalent absorption of the capsule, (m2).
Therefore, if until now the capsule attenuates more than it should (Nr), it can be said that it is efficient.
Step 7. Analysis of the influence of the orifice.
Where:
∆LT: Attenuation achieved by the capsule, taking into account the influence of the orifice, (dB).
∆L0: Amount of decibels lost due to the hole, (dB).
S0: Orifice surface, (m2).
At this time a final comparison is made to find out if the levels attenuated by the capsule are higher than necessary.
If this condition had not been met, a material with greater sound insulation could be sought, or the capsule could be lined with an absorbent material.
4. Use of Cabins
Sometimes the noise comes from various sources, scattered throughout the area, so preventing the worker from receiving it becomes complex.
A possible solution for this case is to isolate the worker from the environment, that is, confine him in a booth that prevents or limits the waves from penetrating inside.
From a practical point of view this technique, to apply it requires certain characteristics of the workplace, such as: they do not require movement (or very limited), try to benefit the thermal exchange, since the cabin increases heat, use of glass To let the vision pass
It has been used in the tandem operators of the sugar mills.
Step 1. Perform a frequency analysis of the noise by determining the lowest frequency that exceeds the maximum allowed value.
Step 2. Select the dimensions of the cabin, its characteristics and materials.
Step 3. Calculate the resulting insulation coefficient (Rres).
Where:
S1: Internal surface of the cabin, (m2).
S2: External surface of the cabin, (m2).
R1: Internal transmission losses, (Kg / m2).
R2: External transmission losses, (Kg / m2).
Step 4. Determine the equivalent absorption area as a function of frequency.
Where:
A: Equivalent absorption area, (m2).
α: Internal absorption coefficient, (sab / m2).
Yes: Internal surface of the cabin, (m2)
Step 5. Attenuation that the booth will achieve (∆ Lf), (dB).
Step 6. Calculation of the NPS inside the cabin.
Where:
Lc: SPL inside the cabin, (dB).
L: SPL outside the cabin, (dB).
∆ Lf: Attenuation achieved, (dB).
5. Use of silencers (mufflers or silencers).
These are useful for locating them at the outlet of equipment that emits gases or vapors, such as internal combustion engines, boilers, etc.
Its principle is to place a device at the outlet or exhaust that decreases its energy sharply, thus reducing the NPS. Its most widespread use is in automobiles.
There are different types of silencers, which differ by their use, they are described below, they can be found in: (Total Quality in Silencers).
Types of silencers
1. Reactive Silencer: Its simplest form is the expansion chamber that is inserted into the conduit that carries the gas flow. Each change in section produces a change in impedance that causes a reflection of the acoustic wave. The combination or interference between the incident and reflected waves results in a decrease in the sound level.
The factors that intervene in the attenuation level are:
- The vacuum of the chamber section with respect to the inlet section.
- The length.
- The speed of sound.
- The frequencies.
These types of silencers are effective in reducing discrete frequencies in the spectrum at low or medium frequencies.
2. Absorption silencers: They are basically used for the attenuation of high frequency noise, between 500 and 8000 Hz, allowing attenuations of 20 to 45 dB, they give an efficiency in a wider frequency band than that of reactive silencers.
Its construction is made from an absorbent product: rock wool, glass, foam, etc., protected by a perforated sheet. (Figure 6).
The parameters that allow to achieve the acoustic characteristics of this silencer are:
- The diameter of the holes in the perforated sheet.
- The density of voids or the proportion of the open area of the perforation.
- The nature of the absorbent product, its density and thickness.
- Length and diameter of the gas flow passage.
Application of absorption silencers
- Inlet for medium-sized blowers.
- Entrance or discharge of centrifugal blowers.
- Intermediate size dry vacuum pump discharge.
- Inlet of gas turbines.
- Input of high speed screw compressors.
- Any source of high frequency noise.
- Inlet and outlet of industrial fans.
3. Atmospheric discharge silencers: Individually designed to reduce excessive noise produced by gas and vapor discharge to the atmosphere.
Noise attenuation is achieved by means of diffusers and absorbent products. Diffusers modify the noise spectrum by transforming low frequencies into higher frequencies that will be more efficiently attenuated by absorbing products. (Figure 7).
The length and diameter of the tubes inserted into the absorbent products are selected to achieve greater noise attenuation in the bands where they are most needed.
The absorbent products are chosen according to the characteristics of the gas that passes through the silencer (chemical composition, speed, temperature).
4. Exhaust Silencers: They are used for noise exhausts in Diesel engines.
The selection of the type of silencer takes into account the attenuation curve that must be achieved and the back pressure that must not be exceeded.
The dimensions of the silencer are defined by the engine power, the exhaust gas flow, the temperature and the available space.
5. Silencers for gas turbines: The noise spectrum is similar to that of a steam pipe: the two highest level points are the suction channel and the outlet of the gas evacuation chimney. Such a spectrum is rich in high frequencies, and therefore requires an absorption silencer to be placed behind the turbine inlet filters. (Figure 8).
The precautions that must be taken into account in the design are:
- The pressure drop.
- Protection of absorbent products against the high velocity of gases.
If a noise level of low frequencies is calculated at the outlet of the turbine, which is too high, it is also necessary to place a thicker absorption silencer, with an absorbent layer and study in depth the risks of corrosion due to the dew point of the acid. sulfurous or sulfuric.
6. Silencers for fans: They can be placed at the inlet or outlet of a fan. For this, noise absorption systems (by absorbent fibers or foam) can be used.
The pressure drop of the silencer that affects the performance of the fan, that is, its flow, in addition to the noise level of the fan in its previous operating conditions, as well as the noise regenerated by the silencer due to the flow of air that passes through it, to anticipate the resulting noise at the muffler outlet.
7. In-line silencers: Depressurization valves are sources of noise that can be of three types:
- The mechanical vibrations of the different components of the valve.
- Cavitation of the fluid.
- The noise of the fluid.
When the noise level of the valve exceeds the admissible level and there is no possibility of modifying the valve, its opening or those with dynamic flow conditions, a consistent solution is to insert a silencer in the piping circuit or under the valve. (Figure 2.9).
8. Spark arrester: In particular cases the silencers must be equipped with a device called spark arrester, its role is to prevent incandescent particles from leaving the open air. (Figure 10).
The devices can be more or less sophisticated, they consist of giving the gases a rotary movement to remove heavy particles by centrifugal effect and collect them in a suitable tank.
9. Silencers for Reciprocating Compressors: Compressor noise can reach dangerously high levels.
The predominant noise comes from a compressor that comes from large variations in pressure or pulsations, when the gas enters the compressor. Thus the noise is transmitted by air, directly from the suction port of the compressor. (Figure 11).
The basic problem of compressors lies in the low frequencies (less than 250 Hz).
10. Silencers with parallel elements: They are used mainly in ventilation and air conditioning. They consist of a smooth sheet metal structure, containing inside a series of parallel absorbent panels. (Figure 12).
The acoustic attenuation, which can be 15 to 40 dB, depends on the noise frequencies, the thickness of the panels, the length of the silencer and the nature of the absorbent product.
11. Silencers for Combustion Gas Exhaust: These are large silencers made of high-density materials to definitively control the noises produced by the exhaust of combustion engine gases and other diverse applications. They withstand high temperatures outdoors.
12. Silencers Air Filters for Intake: Designed to reduce the noise levels of the intake air veins of compressors and engines in general, providing a system with a high performance filter element.
The housings are made of different thicknesses of iron plates, treated with anti-rust and acrylic lacquer paint. The intake pipe is provided threaded, according to the most suitable models for your work and pressure needs. For greater protection in the open or dusty environments, a cover covers the filter element, it is made of 100% polyester fabric, non-woven, absolutely uniform in terms of density and thickness.
13. Combined Silencers for Ventilation Ducts & Fan Discharge: Its design and construction with high-density and quality materials ensure excellent performance and long useful life even outdoors and in high salinity environments.
Steps of the muffler design methodology
Step 1. Noise evaluation.
It is carried out in a similar way to the rest of the methodologies
Step 2. Determine the speed of sound.
Where:
c: Speed of sound, (m / s).
t: Fluid temperature, (K).
Step 3. Determine the wavelength of the sound (λ).
Where:
f: Minimum frequency of interest, (Hz).
Step 4. Determine the wave number.
Where:
k: Number of compressions and depressions of the wave.
Step 5. Determine the length of the muffler (L).
Step 6. Determine the muffler section (S2), (m2).
Where:
S1: Section of the tube through which the fluid escapes, (m2).
m: Constant obtained from Figure 13, with the variables:
- ∆ Ls: Difference between the real NPS and the regulated NPS for the minimum frequency of interest.
- k * L: wave number (k) multiplied by length (L).
r: silencer radius (m).
Figure 13. Graph for the calculation of m.
Step 7. Determine the diameter of the silencer D (m).
To determine the diameter of the silencer it is necessary to know the radius, as well as the sections that form it.
Where:
r: Radius of the silencer, (m).
Download the original file
