The fundamental problems of aeroacoustics is that most test scenarios are expensive and time-consuming and theoretical tests based on calculation software are difficult to simulate realistically. Due to the complexity of boundary layer pressure variations, simulations often require multiple validations and correctional phases.

Specifically, it is difficult to quantify and understand the turbulence and flow-induced noise (boundary layer noise) on different vehicles (aircrafts, cars, drones), simulation tools are slow and imprecise due to the stochastic behavior of turbulence, and experimental tests are costly due to the need for anechoic wind tunnels.

Aeroacoustics is a critical component in automotive vehicle design, certification and operation. Typical wind noise sources are shown below.

Illustration of vehicle components affecting wind noise.

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The main focus of wind noise testing in the automotive industry are measurements in boundary layers. In the automotive (as well as aerospace) industry, these types of measurements are of increasing importance. The main interest is in separating the acoustic signal from the flow-induced turbulent noise. Purposes span quantifying the amount of sound power radiated from a structure, over sound source localization using an array of microphones to quantifying the turbulent stresses to which a structure is subjected.

Boundary layer theory dictates that measurements must be made exactly on the surface of the structure as the statistics of the flow deviate quickly with increasing distance from the boundary. Moreover, care must be taken not to alter the geometry of the structure as such an action may drastically change the flow. See below illustration.

Due to the complexity of measuring boundary layer pressure variations, it is still very difficult to simulate aeroacoustics and often requires multiple validation and correctional phases. Increasing computational power combined with precision microphones from GRAS – allow wind tunnel and on-road testing that help investigate new automotive concepts, verify performance of innovative designs through experimental validations of CFD models, and validate prediction models.

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Aerodynamic Noise

Vehicle aeroacoustic performance has a strong influence on customer perception and also has importance for safety and comfort. Since flow behavior and noise generation are inextricably linked in any aeroacoustic measurement test, investigations into the flow around a vehicle are important in the field of automotive aeroacoustics.

Wind noise being an aerodynamic noise is actually related to both the body design as well as the turbulence from various components. In relation to the body design, there are three main categories, which affect the aerodynamic noise on a road vehicle: Vehicle form, open cavities and the body sealing. The vehicle form and open cavities are active noise sources, while the body sealing reduces wind noise - depending on the sealing quality.

Likewise, wind noise performance also depends on minimizing aeroacoustic noise sources generated by vehicle components since these all have a direct impact on the measurement of wind noise in the car. As such early concept studies are hugely important as careful design minimizes the problem. Wind noise is, however, often an issue, which is discovered late in the development process. It requires fast and efficient analysis since measures like extended seal systems or sound packages are expensive.

Vehicle Form

Important components related to the vehicle form and design and which have a direct influence on wind noise testing results are A-pillars, side mirrors, windscreen wipers, antenna, underbody and glazing.

Open Cavities

The second major aerodynamic noise source is open cavities. There are two main sources of cavity noise on a vehicle: noise generated by large cavities such as an open sunroof or window - also known as booming or buffeting - and cavity noise from smaller cavities such as door gaps, tailgate and gaps in body panels.

The contribution of wind noise in producing the overall noise heard inside the cabin of a car, however, varies at different speeds. At higher speeds, generally above 100 km/h (approx. 60 miles/h), aerodynamic wind noise tends to dominate the overall cabin noise. This is much more distinct today because the noise from the engine bay and powertrain has been significantly refined and reduced over the years. In today’s electrical vehicles (EV), this problem is becoming ever increasing, since wind noise is one of the most important noise sources for EVs.

Body Sealing

The body sealing as opposed to vehicle form and open cavities is intended to reduce the overall noise heard inside the cabin. Obviously, the sealing quality has a direct effect on the noise reduction. Here the idea is to prevent leaks in the seals around the door apertures and glass. Noise of the sealing systems tends to lead to higher frequency noise inside the cabin, generally above 500 kHz. Depending on make and model, different sealing systems and quality levels are applied.