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FYP

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by

JH Cai

on 21 April 2016

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Transcript of FYP

SOUND PROPAGATION OVER NOISE BARRIER
To investigate how the material and geometrical changes to the noise barrier affects its effectiveness

Noise
Traffic noise
Construction and industrial operations noise
Frequency: 100 to 1300 Hz
THEORETICAL MODEL
COMSOL 5.0
Mesh Convergence
Element size= /N
= 344/1300
Converge when the element size was 5.29cm
Element size: 5.29cm
MATERIAL
GEOMETRY
MATERIAL OF THE BARRIER
MATERIAL OF THE GROUND
Method
Method
Result
Analysis
Analysis
Result
1) Most and least effective barrier fluctuate between Stainless steel and Plywood
2) Insignificant deviation of insertion loss

Possible explanation:

1) Interferences at the point analyzed

2) Only acoustic impedance was taken into account
Verification

Interference was observed
Possible explanation:

1) Only the acoustic impedance was taken into account

2) Acoustic absorption not considered
Observation:
Effect of ground in negligible for all frequencies analyzed


Cap: length of 0.6m
EFFECTS OF CAP
Method
Result:
Effects on Frequency
Analysis
Result:
Determining the most effective barrier
Analysis
Verification

USEFULNESS OF ADDITIONAL CAP
Method
Result:
Effects on Frequency
Result:
Most effective barrier
Result:
GEOMETRICAL OPTICS
Method
Result:
Analysis
RELATIONSHIP
BETWEEN DISTANCE OF SOURCE AND ANGLE OF CAP
Hypothesis
Method
Result
1) Barriers are almost ineffective when the receiver is at a higher height, regardless of the frequency of the sound source.

2) Barriers tend to be more effective for higher frequencies and also at lower receiver height
Observation:
1) Decrease in insertion loss as height of receiver increase
2) Tend towards zero insertion loss as height increase
3) At lower frequencies, there was lower insertion loss with lesser fluctuation
Observation:
1) Barrier more effective when there was a cap regardless of angle
2) Most effective angle of the cap was at 90°
1) Different angles of the barrier will result in different geometric optics leading to different area of shadow zone.

2) A larger shadow zone area implies a more effective barrier.
Hypothesis was consistent with the observation that the 3m barrier with 90° cap is the most effective.
Observation and analysis:

Similar trend to the 3m barrier



Observation and analysis:

1) Similar trend to the 3m barrier

2) Most effective angle of the cap was also 90° or 120°, depending on the frequency.


Comparing 3m barrier and 3.3m barrier
Observation and analysis:
1) 3m barrier with 90° cap and 120° cap more efficient than the taller barrier of 3.3m


Observation:
1) No significant change at predicted heights
2) Changes seemed to be higher than the predicted height
Result:
Comparing 3m barrier and no barrier
Observation:
1) Pressure for the receiver point nearer to the source is higher regardless of the presence of a barrier
2) Pressure of all cases, with or without barriers, start to converge as well.

1) Geometrical optics theory predict that the nearer the receiver is to the source, the larger the area of the bright zone.
2) This is coherent with the result achieved earlier.
3) Not entirely accurate
4) Geometrical optic can be used to estimate the general trend of the various type of barrier, it may not necessarily produce an accurate solution.
Method 2
Result
Analysis
Difficult to determine what the best angle of the cap was.
Comparing two sources using line average
1) The further the noise source is from the barrier, the greater the required angle of the cap on the barrier.
2) Due to limitations of the domain size, the differences in the noise sources distance are too small to conclude.
3) Taking line average of the pressure might not be accurate as the sound pressure might vary greatly at different heights.
1) Analysis is limited to thin vertical barriers.
2) Atmospheric effects of turbulence, temperature and wind were neglected.
3) The boundary conditions were uniform.
4) Noise sources were generally unstable but were approximated to a stable point
source to simplify analysis.
5) Noise absorption of the material was neglected.
6) Assuming that there are no openings or gaps in the barrier material, some
materials, such as wood, were likely to develop openings or gaps due to
shrinkage, splitting, weathering, or warping.
1)The material of the noise barrier will not affect effectiveness of the noise
barrier.
2) The different material of the ground has negligible effects on the eventual
noise received by the receiver.
3) The geometry of the noise barrier significantly alters the effectiveness of the
noise barrier.
4) Using geometrical optics to predict the shadow zone of the noise barrier is
not accurate.
5) A greater angle of noise barrier cap is needed when the distance between the
noise source and the noise barrier increases.
LIMITATIONS
CONCLUSION
Future Work
1) Absorption factor of the material

2) Other possible geometrical changes that can be made to the barrier

3) To further check the hypothesis, further experiments should take into consideration factors in reality such as wind and temperature that may alter the effectiveness of the noise barrier.
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