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.
Again, any help would be great and thanks for your time replying.
Chris
Did you get any further forward with this Chris? I've got the same issues with the same question.
Hi there mate, sorry late reply. I havent really got any further. And my tutors don't really understand it. Have you?
To calculate the worst case LAmax,F and LAeq, 1h in the operating theatre, LAeq, 0700-2300, LAeq 2300-0700, LAmax,S at the housing development, and LA01,30 min in the school for the helicopter noise alone, we will follow these steps:
1. Calculate the worst case LAmax,F in the operating theatre:
- Determine the maximum noise level (LAmax,S) of the helicopter at 150m.
- Add 3 dB to LAmax,S to estimate LAmax,F.
- Calculate the additional noise due to the helicopter running on the helipad before take-off and after arrival.
- Sum the noise levels to obtain LAmax,F.
2. Calculate the LAeq, 1h in the operating theatre:
- Determine the equivalent noise level (LAE) at 300m for the helicopter.
- Calculate the additional noise due to the helicopter running on the helipad before take-off and after arrival.
- Calculate the total equivalent noise by summing the noise levels at different distances.
- Convert the total equivalent noise level to LAeq, 1h using the formula LAeq, 1h = LAE + 10 * log10(1/24).
3. Calculate the LAeq, 0700-2300:
- Determine the equivalent noise level (LAE) at 300m for the helicopter.
- Calculate the additional noise due to the helicopter running on the helipad before take-off and after arrival.
- Calculate the total equivalent noise by summing the noise levels at different distances.
- Convert the total equivalent noise level to LAeq, 0700-2300 using the formula LAeq, 0700-2300 = LAE + 10 * log10(16).
4. Calculate the LAeq, 2300-0700:
- Determine the equivalent noise level (LAE) at 300m for the helicopter.
- Calculate the additional noise due to the helicopter running on the helipad before take-off and after arrival.
- Calculate the total equivalent noise by summing the noise levels at different distances.
- Convert the total equivalent noise level to LAeq, 2300-0700 using the formula LAeq, 2300-0700 = LAE + 10 * log10(8).
5. Calculate the worst case LAmax,S at the housing development:
- Determine the maximum noise level (LAmax,S) of the helicopter at 150m.
6. Calculate the LA01,30 min in the school:
- Determine the equivalent noise level (LAE) at 300m for the helicopter.
- Calculate the additional noise due to the helicopter running on the helipad before take-off and after arrival.
- Calculate the total equivalent noise level at 150 m for 30 minutes.
- Convert the total equivalent noise level to LA01,30 min using the formula LA01,30 min = LAE + 10 * log10(0.5).
Assumptions:
- The helicopter's noise levels remain constant during flight, and the rate of decay with distance is constant.
- LAmax,f is 3 dB greater than LAmax,S.
- The helicopter's speed is 28 m/s both during approach and take-off.
- The approach and departure slopes originate in the center of the helipad.
- There are no other significant noise sources.
- Noise transmission through the windows is the only contributor to internal noise in the hospital.
- The noise spectrum of the helicopter peaks at 500 Hz, and other octaves can be ignored.
- The noise in the 500 Hz band is omnidirectional.
- The hospital rooms have sealed unit glazing with a sound reduction index of 30 dB at 500 Hz.
- The school windows are open with an area of 50,000 mm2 and contribute significantly to internal noise.
- The hospital room has a volume of 30 m3, and the school rooms have a volume of 50 m3.
- The reverberation time in the hospital room at 500 Hz is 1.2 seconds, and in the school rooms, it is 0.5 seconds.
By following these steps and considering the given assumptions, you should be able to calculate the requested noise levels for the helicopter alone.