I never thought I’d worry myself with traffic noise predictions. Performing arts centers are usually well enough insulated for this to be designed out. We typically focus more on ground vibrations from trams, underground and aircrafts.
But this situation is different. It is not a performing arts center. It is any tower, with an undefined geometry and height, to be built within a race track “loop”.
Traffic noise propagation is a well understood process. But the methods have been developed to protect buildings from the relentless and generally increasing traffic noise from public roads. Not much of the above fits particularly well to a loop with 20-25 high speed performance vehicles.
The basic assumption that a continuous flow of vehicles can be represented by a line source simply cannot be applied to a twisty car race track. Nor can a single number sound power level that ignores spectral character and is derived from vehicle counts (per 18hrs), average speed, proportion of trucks/buses and road surface.
Racing cars typically start together before spreading over the closed loop of the track. They have very distinctive sound spectral signatures (V8, V6, Supercars, motorbikes all sound different). And they travel at much higher speeds on a very special surface.
With these conditions, CoRTN just cannot be applied and one has to look at the PROs and CONs of the European or Nordic methods. These allow at least more detailed descriptions of the cars as noise sources. More precisely, as point sources moving along small segments of a road.
This is precisely what we have implemented in Grasshopper. The definition can take any geometry and location for the tower. The process is:
- Divide the track into segments travelled for each car, given its own speed, within a 1 second interval
- Calculate the attenuation of noise through the distance between each car and each facade of the tower.
- Includes atmospherically conditions (worst case wind)
- Includes ground attenuation
- Includes shielding for the facade
- Includes shielding from noise barriers on the side of the track
- Does all the above in octave bands
- Sums up the contribution of all cars for each facade element of the tower, for every 1 second period
- Stores and trace this incident level as a noise logger would do
- Extract Lmax, L10/L90 percentiles and Leq for the duration of the race.
The model does not include yet:
- Road/tire noise component (I am not sure it is relevant for these surfaces and these noisy car/exhausts)
- Engine breaks and drag noise
- Speed variations along the track (constant speed regardless of twists and turns)
- Variable trajectories for each car.
This has been fun! It has allowed us, not only to bet on the cars (randomly assigned speeds) but also to test famous race tracks and see which is the loudest.
Ready, steady, click!
Acoustic Research & Design Ltd 