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In active method, noise generation from machines can be reduced by regulating vibration and friction. It is a well established fact that metallic gear to metallic gear arrangement produces more noise compared to polymeric gear, hence polymeric gears are more suitable and are recommended. Generally polyurethane, polyester, polyamide polymers are used for this purpose. As rotating parts of the machine for example, spindles, gears, crank shaft etc. produce more vibration, these parts could be replaced by polymeric materials.
Besides, proper and regular maintenance and overhauling of machines also play an important role in reducing and keeping low noise levels.
4.2 Passive Noise Control
A damping material (isolator) is used to reduce the vibration of power loom. The damping materials are in the form of resilient pad made of rubber, neoprene, cork or plastic. The isolation systems may be placed as shown in Fig. 4.2-
Fig: 4.2 Position of isolators in foundation
Consider a case when the power loom of mass m1 is placed on a foundation block of mass m2. The spring K1 represents the foundation soil and spring K2 is an isolating spring which is placed between masses m1 and m2, in order to minimize the transmission of vibrations from the ground to the equipment. If the ground is subjected to a periodic displacement given by Z0 sin Ï‰t. The equipment of motion may be written as (Saran, 2006),
m11 + K1Z1 - K2 (Z2 -Z1) = K1 Z0 sin Ï‰t â€¦â€¦4.1
or m22 + K2 (Z2 -Z1) = 0 â€¦â€¦4.2
The value of maximum amplitude of motion are given by,
A Z1 = K1 Z0 â€¦â€¦4.3
A Z2 = K1 Z0 â€¦â€¦4.4
The displacement transmissibility of machine (TD) is defined as the ratio of displacement amplitude of mass m2 to the displacement amplitude of rigid support. Then,
TD = = â€¦â€¦4.5
or TD = â€¦â€¦4.6
where, Âµm = = mass ratio
a1 = ; a2 =
= ; =
If is taken as permissible amplitude, and Z0 is the applied dynamic displacement, then the ratio of is known. For this value, a2 can be determined which in turn will give the stiffness of the isolator spring, i.e. K2.
4.2.2 Enclosing the noise source
Noise is transmitted by vibration. Hence the property of the enclose must be such that it should not vibrate when a sound wave hits its surface; otherwise the enclosure itself can become a noise source. Since vibration is inversely related to the mass of the material, in the use of enclosures. By the mass law, an ideal enclosure is the heavy enclosure (materials of high density).
Let us consider that the noise coming from the power loom workshop is reduced by putting the looms in an ideal enclosure of masonry block of solid dense concrete 4 inch (10 cm) thick.
The intensity transmission loss may be given as:
TL = 20 log ÏlÎ³ - 47.4 â€¦â€¦4.7
Where Ïl = area density of partition
Î³ = frequency
The density of masonry block of solid dense concrete 10 cm thick = 110 lb/ft3
= 1849.6 kg/m3
Ïl = 36.67 lb/ft2 = 36.67 (0.454)
= 179.22 kg/m2
TL500 = 20 log (179.22) (500) - 47.4 = 51.6 dB
dB transmitted = 87 - 51.65 = 35.35 dB
4.2.3 Using Double leaf wall
A double leaf wall can perform better than a single leaf wall of similar mass because the sound has to pass through two barriers. If the two leaves are not connected to each other, the insulation values of the two leaves are often connected by ties or strands, and the full insulation cannot be achieved. Even where the two leaves are isolated from each other, the full benefit can only be obtained above a certain frequency that depends on the cavity width. This is because the air in the cavity behaves like a spring connecting the leaves together, and causes a resonance at the mass-spring-mass frequency. Below this frequency, the two leaves behave more like an equivalent single leaf.
Making the cavity width wide can reduce the mass-spring-mass frequency, as in the case of
sound insulating secondary glazing. The mass-spring-mass frequency (F0) may be estimated from the following equation:
F0 = 59.6 â€¦â€¦4.8
m1 and m2 = the surface masses of two leaves in kg/m2 & d = cavity width in metres (m)
Reduction in mass-spring-mass frequency (F0), barred the air in the cavity to cause resonance.
4.2.4 Screening of vibration by the use of open trenches
22.214.171.124 Active Screening
Screening of vibration is done near the source of vibration. Fig.4.3 shows a circular trench of radius R and depth H which surrounds the machine foundation that is the source of disturbance. Barken (1962) mentioned that the reduction in vibration amplitude occurs only when the trench dimensions are sufficiently large compared with the wave length of surface waves generated by the source of disturbance. The depth of trench varied from 150 mm to 600 mm and radius R of annular trench varied from 150 mm to 300 mm.
Fig: 4.3 Vibration screening using a circular trench surrounding the source of vibration-Active screening (Woods, 1968)
Wood (1968) has introduced a term amplitude reduction factor which is defined as:
ARF = amplitude reduction factor
The dimensions of the trench are expressed in non-dimensional forms by dividing H and R by the wave length Î»R of Rayleigh waves, Î»R is obtained by determining the number of waves (n) occurring at a distance x from the source (Î»R = x/n).
The field tests of Woods (1968) thus correspond to
= 0.222 - 0.910 and = 0.222 - 1.82
For satisfactory screening of vibrations, Wood (1968) recommended that ARF should be less than or equal to 0.25. The conclusions made on the basis of this study to keep ARF â‰¤ 0.25 are:
For full circle trenches (Ï´ = 360o), a minimum value of = 0.6 is required. The zone screened in this case extended to a distance of atleast 10 wave lengths (10 Î»R) from the source of disturbance.
For partial circle trenches (90o < Ï´ < 360o), the screened zone was defined as an area outside the trench extending to at least 10 wave lengths (10 Î»R) from the source and bounded on the sides by radial lines from the centre of source through points 45o from the ends of trench. In this case also, a minimum value of = 0.6 is required.
Partial circle trenches with Ï´ < 90o, effective screening of vibration is not achived.
Trench width is not an important parameter.
126.96.36.199 Passive Screening
Wood (1968) has also performed field tests to study the effectiveness of open trenches in passive screening as shown in Fig. 4.4.
Fig: 4.4 Vibration screening using a straight trench-Passive screening (Woods, 1968)
A typical layout of these tests consists of two vibration exciters and one trench of size ranges from 100 mm x 300 mm x 300 mm deep to 2440 mm x 3050 mm x 1220 mm deep.
The values of varied from 0.444 to 3.64 and from 2.22 to 9.10. It was assumed in these tests that the zones screened by the trench would be symmetrical about the 0o line.
For satisfactory screening, Wood (1968) recommended that ARF should be less than or equal to 0.25 in a semi-circular zone of radius (1/2) L behind the trench. The conclusions made on the basis of this study to keep ARF â‰¤ 0.25 are:
should be atleast 1.33
To maintain the same degree of screening, the least area of the trench in the vertical direction (i.e. LH = AT), should be as follows:
= 2.5 at = 2.0
and = 6.0 at = 7.0
Trench width had practically no influence on the effectiveness of screening.
This shall be more effective, if there are several looms inside a power loom based textile industry.
4.2.5 Design of Active and Passive Screening
188.8.131.52 Design of Active Screening
Normal operating frequency of a power loom = 100-960 rpm
(Source: Paraniri Industries 2009)
Let us design for rpm = 960
Operating frequency of power loom in terms of Hz = = 16
Rayleigh wave velocity VR = 140 m/s (assumed)
Therefore, wavelength = Î»R = = = 8.75 m
Depth of trench for active screening H = 0.6 Î»R = 5.25 m
184.108.40.206 Design of Passive Screening
Depth of trench for passive isolator is given by H = 1.33 Î»R = 11.63 m
Let the trench is provided at a distance of 20 m
= = 2.28
= 2.28, = 2.5 + = 2.696
Length of trench = = 17.74 m
4.3 Other methods of Noise Reduction
Following measures may be taken for an effective control on noise pollution.
Proper designing of buildings and workshops: Proper designing of doors and windows of a room can help in reducing the noise to a much greater extent. The sound travels through very thin cracks between the door and wall. The space between the jamb and frame may be packed with sound absorbing material. In case of windows, the transmission loss increases as the thickness of glass increases, while in case of doors, the transmission loss increases as the weight of door increases. Glazed windows with double or triple panes of glass provide the excellent sound insulation. The air space at the edges of such panes can be filled with sound absorbing material.
The noise pollution can also be controlled by using floating floors and suspended ceilings. Suitable sound absorbing materials such as hair felt, acoustical tiles, perforated plywood and various porous materials are available to be fixed on walls, floors and ceilings in order to reduce the noise level.
Using ear protective aids: The workers working in power loom based textile industries having noisy machineries, should be provided with the ear protection aids such as soft plastic and rubber ear plugs, headphones etc. Stuffing of cotton balls in the ear, covering of ears with hands under noise conditions, being away from the source of noise are some personal protection measures.
Use of building codes: Certain codes should be enforced which require sound proofing in the construction of industries, buildings and apartments. Cities should be planned in such a way that industrial noise and highways are separated from the residential areas. In USA building codes have been enforced according to which all newly constructed buildings are required to have sound proofing mechanisms.
By vegetation cover: Trees absorbs and dissipate sound energy and act as a buffer zone. Plants and trees should be planted along highways, streets and near industrial areas. Various trees minimize noise by 5-10 dB. Ashok, Neem, and Tamarind are good for this purpose. In metropolitan cities green belt vegetation and open space may have great value in sound absorption
Following recommendations were issued by West Bengal Pollution Control Board for power loom workshops operating in the residential areas:
The outer walls of the rooms should be double walled with at least six (6) inches space in between.
Such void space to be filled with saw dust.
Inside of the wall should be covered with thermocol layer (minimum 1 inch thickness).
Room should be having false ceiling made up of straw boards or boards manufactured from jute sticks.
All the doors of the room should be swing doors.
The room should have adequate ventilation arrangement.