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Thread: Help on a Question.

  1. #1 Help on a Question. 
    New Member
    Join Date
    Apr 2010
    Hi there everyone. I'm a first timer on this site and trying to get some help on question that has been causing me trouble. Obviously I'm not expecting an answer, but any help towards the right answer would be great. I'm pretty new to Acoustics so I'm still getting acquainted to the concepts. I have this question:

    Helicopter Noise Transmission
    It is planned to introduce a regular helicopter service from a helipad at ground level in a hospital premises. There will be four arrivals and four departures per day but never more than one arrival and one departure in any hour. There is a school and land zoned for housing development 150 m away.

    Only one type of helicopter will be used. During flight its noise levels are 71 dB LAmax,S and 78 dB LAE at 300m and 64 dB LAmax,S and 74 dB LAE at 600 m. Once the helicopter is in flight, it may be assumed that the source noise level is unchanged and that the rate of decay with distance is constant also. On the ground its noise level is 70 dB LAmax,S at 150m and 60 dB LAmax,S at 300m. It can be assumed that LAmax,f is 3 dB greater than LAmax,S.

    The operating theatre and other rooms are 120m from the helipad. The approach path is 50m to the side of the operating theatre. The take-off path is directly over the housing area and the school. Each helicopter approaches and departs in the same direction. The engines run on the helipad for 5 minutes before take-off and two minutes after arrival. Helicopters will approach at an angle of 10 and will have an angle of climb after takeoff of 14. Its speed in both cases is 28 metres/second.

    For the purposes of calculations in this assignment it may be assumed that (a) the
    helicopters have a constant speed of 28 m/s (b) the approach and departure slopes
    originate in the centre of the helipad and (c) there are no vertical flight segments. The helicopter noise spectrum peaks strongly at 500 Hz to the extent that other octaves may be ignored. The noise in this band may be assumed to be omnidirectional.

    Each room of the hospital has one 1m1m window with sealed unit glazing. The glass has a sound reduction index of 30dB at 500Hz and noise transmission through the window is the only significant contributor to the internal noise level. The reverberation time at 500Hz is 1.2 seconds and the room volume is 30m3. The school has open windows, with openings having an area of 50,000 mm2 for each room, each of which has a reverberation time at 500Hz of 0.5 seconds and a volume of 50m3. Noise transmission through the open window is the predominant path for internal noise.

    Assuming there are no other significant noise sources, calculate the worst case LAmax,F and LAeq, 1h in the operating theatre, LAeq, 0700-2300, LAeq 2300-0700 and LAmax,S at the housing development and LA01,30 min in the school, for the helicopter noise alone. Give clear and complete explanations of the steps in your calculations and state any assumptions you make in addition to those given.

    Again, any help would be great and thanks for your time replying.


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