Source Location and Directivity
Millions of people suffer from tyre/road noise. Vast amounts of money are yearly spent on noise control measures such as road side noise barriers, and sound insulated windows. More effective noise control measures, to lower costs, may be obtained with a better understanding of noise generation/radiation. Therefore, the Swedish Road Administration assigned us to investigate source height and directivity of tyre/road noise.
To determine source height, we built a vertical five meter tall microphone array alongside a main road in Sweden. Source height, shown in the plot here beside, is very close to the road surface.
Directivity was measured on an abandoned airport in Sweden. We recorded sound as “seen” by a two-dimensional microphone array as a vehicle passed by at different speeds, up to 120 km/h. Directivity at 2.5 kHz is shown in the plot here beside. Start the movie by clicking on the picture below. The frequency band is 3.15 kHz, and pass-by speed 120 km/h. The angle goes from -55 degrees, to +55 degrees. The video runs at slow motion, 5 frames per second (fps); full speed corresponds to 120 fps. When the car enters from the left, all four wheels are clearly seen.
Sound videos of pass-by noise, e.g. during tests simulating real driving conditions, enables manufacturers to see and control noise sources of cars, trucks, trains, aircrafts, etc., a competitive advantage as noise regulations are continuously becoming more strict.
The picture shows noise radiation at 1000 Hz from a heavy truck pass-by at 90 km/h. Main noise sources are the tyres, especially on the driving wheels with a more rugged texture. Other important sources are noise from the transmission line and a wind director at the front.