Illustration of vehicle components affecting wind noise.

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Boundary Layer Measurements

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.

Aerodynamic Noise

Vehicle aeroacoustic performance has a strong influence on customer perception and also has importance for safety and comfort. Today, wind noise performance depends on minimizing aeroacoustic noise sources generated by various vehicle components.

These include a long range of aerodynamic sound sources, which all have a direct impact on the measurement of wind noise in the car. Since flow behavior and noise generation are inextricably linked in any aeroacoustic measurement test, investigations into the flow around areas of a vehicle are important in the field of automotive aeroacoustics. Within automotive testing in particular, there is a direct link between vehicle form and wind-induced noise.

Wind noise being an aerodynamic noise is thus related to the body design and turbulence from various components. 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.

There are three main categories within automotive aeroacoustics, 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.

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.

Solutions

Due to the stochastic behavior of turbulence and flow-induced noise – also referred to as boundary layer noise – it is very difficult to quantify and understand these phenomena. In the development and testing stages of the automotive, aerospace and renewables industries. It has, therefore, been a key focus area for years – but has been hampered by a number of facts.

First of all, experimental tests have proven imprecise due to traditional microphone profile heights. Secondly, the mounting, demounting and remounting of sensors has been a tedious and not very user-friendly process. Finally, simulations and tests in anechoic wind tunnels are costly and most often require careful planning and coordination, making it very time-consuming.

GRAS measurement microphones for boundary layer applications cover UTP microphones, surface microphones and flush-mount microphones – available for surface mounting, “blind window” mounting, destructive mounting or wire-mesh applications.

Wind noise is an important noise source besides road noise, which is the single most important noise source for EVs/HEVs at speeds above 55 km/34 mph. However, more aeroacoustic testing is required in order to limit this particular noise source.

The amount of wind noise is the same in ICEs and EVs. The difference is that in EVs wind noise makes up a much larger part (30%) of the entire noise exposure because other noises tend to mask wind noise.

As with ICEs, wind noise measurements on EVs can be performed either by using nose cones, wall-mounted microphones or surface-mounted microphones.

Because wind is one of the predominant noise sources in EVs, there is a genuine need to look into new microphone positioning possibilities, which will allow a more comprehensive understanding of noise fields and sources.

GRAS aeroacoustic sensors are an excellent choice, and we recommend GRAS 40LS 1/4" CCP Precision Surface Microphone.

See our complete application offering within electric vehicles here.