Aerodynamic noise - often referred to as Wind noise, is a major component of passenger compartment noise, which becomes dominant especially at high speed cruising. Because of its broad band and often tonal frequency characteristics, this type of noise tends to have a high hygiene factor and as such has a major influence on the overall refinement of a vehicle.
The main features that determine the level of wind noise heard inside a vehicle are:
- Smoothness of air flow around the vehicle - this should be checked at all angles of incidence as buffetting may become a problem when the wind angle is other than straight ahead and turbulence is generated on the offside.
- Transmission loss of vehicle panels - this will determine how much the noise generated outside is attenuated. It should be noted that the panels closest to the ear (normally side glazing) are the most critical.
- Sealing - the door seals need to work in both static and dynamic conditions. The dynamic loads are due to the aerodynamic flow and normally result in the side doors being pulled out as a result of the low pressure area over the surface (this will change with angle of attack).
- External features - this covers a number of items that may generate particular wind noise problems. Aerials may produce an aerodynamic disturbance at a particular frequency that may be determined from the strouhal number. Cavities such as those around doors may generate a disturbance in the air flow that will be locked to the acoustic mode of the cavity producing a whistle.