Construction Technology 5 – 200471
Damir Kukic 17038124
Q1. List and explain three ways that noise can be reduced at the source.
Noise can be reduced at the source by placing noise in sound proof enclosure. A special enclosure does not need to be constructed for the noise source. Structure-borne noise can be eliminated by flexible connections to the services and placing machinery on resilient supports.
Q2. Explain how noise attenuation differs for a point source and a line source.
For a point source, noise intensity is reduced by the square of the distance. The noise level is attenuated 6 dB for each doubling of the distance. As noise fans out from a point source, it covers a larger area. If a sound wave is double the distance from a point source, it will cover 4 times the area. The sound intensity at 2d will be a quarter the intensity at d.
For a line source, the noise level is inversely proportional to the distance. The noise level decreases by 3 dB at twice the distance. Peak-hour traffic is regarded as a line source. The sounds of individual cars are aggregated to form a continuous ribbon of sound. As sound spreads out from a line source, the area that it covers only increases at the same rate as the distance. Hence, a sound wave at 2d will cover an area 2 times as large as the area at d.
Q3. How do wind direction and temperature affect sound transmission?
Sound transmission can either be improved or reduced by the wind. The effect of wind depends on the direction downwind sound is refracted back towards the ground and upwind sound is diffused up and away from the ground. You can hear more clearly downwind than upwind.
The time of day also determines the distribution of the sound. The velocity of sound depends on the air temperature. During the day, air, near ground level, heats up when it contacts sunlit surfaces. Sound travels faster at ground level than higher up. Day time sound is bent upwards because the air temperatures are higher near the ground. Nigh time sound is bent downwards. At night-time, ground loses heat by re-radiation to space. The coolness is passed onto the air in contact with a ground surface. It is actually easier to hear faraway sounds at night, for two reasons. Sound stays close to the ground and there is generally less background noise.
Q4. How does composite construction aid in the reduction of sound transmission through a wall.
Sound transmission can be avoided at the co-incident dips by successive layers of different materials. Since brick veneer; brickwork and plasterboard have different coincident dips, acoustic transparency can be alleviated by using several materials in combination.
Q5. Explain the two methods of determining the overall acoustic resistance of a wall. Which is used in the Building Code of Australia?
Weighted sound reduction index is a better measure of a barrier’s acoustic properties. It allows for the reduced hearing sensitivity at low frequencies. The single-number rating of sound insulation is practical for several purposes: To characterize the measuring result of a building construction, for quick comparison of the sound insulation obtained with different constructions. The weighted sound reduction index Rw is based on a standardized reference curve that is defined in one-third octaves in the frequency ranges 100 Hz – 3150 Hz. The reference curve is made from three straight lines with a slope of 9 dB per octave from 100 to 400 Hz, 3 dB per octave from 400 to 1250 Hz, and 0 dB per octave from 1250 to 3150 Hz.
Q6. Explain the concept of flanking transmission in building structure. How is this best dealt with?
Flanking transmission is a more complex form of noise transmission, where the resultant vibrations from a noise source are transmitted to other rooms of the building usually by elements of structure within the building. Sound...