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Question 3 The external wall of a bedroom in a house is 4m by 2.5m in area. It contains one double glazing window. The wall faces a busy road and is 30m from the line of traffic on the road. Analysis of the sound pressure level of the sound from the traffic at 7.5m was undertaken and the octave band analysis given in the table below. The 280mm cavity wall and sealed double glazing window have sound reduction indices of 55dB and 44dB respectively at 1000Hz. Assume the insulation of the window and wall improves by 5dB for each doubling of frequency. The recommended maximum sound level for sleeping areas in a home is given by the noise criteria curve (NC) 35 on a noise rating (NR) curve. The table below provides the noise criteria curve 35 and the sound pressure level measured 7.5m from the road. The speed of sound in air is 343 ms!, speed of sound in glass 2300 ms-1 and speed of sound in brick 4000 ms-1. The density of air inside the room is 1.21 kgm? Frequency range (Hz) Sound pressure level (dB) Noise rating (NR) 7.5m from busy road curve for noise criteria (NC) 35 20-75 99 63 75-150 95 55 150-300 93 47 300-600 92 42 600-1200 93 37 1200-2400 90 35 2400-4800 88 33 4800-10000 85 32
(a) Calculate the sound pressure level at the window for each frequency range. (b) What values of sound insulation are needed for each frequency range in order to comply with NC35 (c) Calculate the required average transmission coefficient for the window and wall combined (d) Calculate the transmission coefficients for both the window and the wall. (e) Using the information provided and your answers above, calculate, for 1000Hz, the maximum area (to 0.1m2) of window allowed in the wall. (f) Calculate the density of the brick wall at 1000Hz. State any assumptions you make.