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Homework Help: Acoustics: Sound Level Calculations

  1. Apr 18, 2010 #1
    1. The problem statement, all variables and given/known data
    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 1m×1m 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.

    THE ACTUAL QUESTION IS:
    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.


    2. Relevant equations

    Lp = Lw - 20log r - 11

    Lp2 = Lp1 - 10log S - 10log A

    RT = 0.16V/A


    3. The attempt at a solution

    c^2 = a^2 + b^2
    a = 109m

    tan A = opp/hyp
    tan 10 = a/109
    a = 19m

    Lp = Lw - 20log r - 11
    74 = Lw - 20log 300 - 11
    Lw = 135dB

    therefore at location of hospital:

    Lp = Lw - 20log r - 8
    Lp = 135 - 20log 53 - 8
    Lp = 89dB

    absorption coefficient of room needs to be calculated:

    RT = 0.16V/A
    1.2 = 0.16x30/A
    A = 4

    using sound transmission from outside to indoors:

    Lp2 = Lp1 - 10log S - 10log A
    Lp2 = 89 - 10log 1 - 10log 4
    Lp2 = 82.97

    LAmax,f = 83dB

    thats my thoughts on the first part of the question. any help for that one or the rest would be great. thanks.
     
  2. jcsd
  3. Apr 23, 2010 #2
    Hello badcamel, I see you are doing the IOA diploma to, I have just been working on this problem and am also having a bit of trouble getting my head round the whole thing, the one thing I did notice with your transmission loss is that you forgot to deduct the SRI from the window itself:

    Using sound transmission from outside to indoors:

    Lp2 = Lp1 - 10log S - 10log A
    Lp2 = 89 - 10log 1 - 10log 4
    Lp2 = 82.97

    LAmax,f = 83dB

    Lp2 = Lp1 - SRI (30dB from window) - 10 Log1 - 10log(4) = 53 dB

    I am struggling with calculating the rest of the problems for the school and LAeq,1hr at the moment but hopefully like the rest of this assigment it will all come good.

    I hope this helps.
     
  4. Apr 27, 2010 #3
    I just stumbled on this page and found the text very familiar....realizing its the assignment im working on. The geometry aspect was confusing at first but now trouble with the Laeq 1 hr also. The speed of the chopper is given on arrival and take off, do you recon that will have to be used in some awful calculation to determine sound levels?
     
    Last edited: Apr 27, 2010
  5. Apr 27, 2010 #4
    Personally this is how I got the following:

    Assuming that rate of decay is constant as stated in the brief based on the 7dB attenuation of LAmax,S (71dB at 300 metres and 64dB at 600 metres) the difference can be calculated as:

    23.3Log(R1/R2)

    Nearest distance the helicopter will be from the operating theatre is 50 metres, assuming that the helicopter takes off adjacent to the theatre the worst case would be the inflight LAMax,F at 50 metres from the source:

    23.3Log(300/50) = 18.13

    71 + 18.13 = 89.1

    89.1 +(3dB for Fast Rating) = LAMax,F = 92.1 dB

    SPL (inside operating theatre) = 92.1 – 30 +10Log(1/4) = 56.079

    The helicopter will run on the helipad for five minutes before takeoff and two minutes after arrival there is never more than one of each in any one hour so using the worst case scenario LAmax,F noise level inside the operating theatre the equivalent continuous noise level for one hour can be calculated as :

    10Log((5X60) x 10^(56/10) + (2 x 60) x 10^(56/10))
    (1x60x60)

    = 46.669

    LAeq, 1hr = 46.7 dB

    Thats what I have done for that question, im not sure if its right but I welcome any thoughts on it, also if anyone happens to know how to answer the question regarding the schools calcualtion of an LA01,30mins and how to use an LAE or LAmax,F or LAmax,S to get it that would be very helpful as that question has stone walled me im not sure with the amount of data given that you can even calculate an LA01,30min?????
     
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