Helmholtz Resonators Acting as Sound Source in Automotive Aeroacoustics

Helmholtz-resonators are discussed in technical acoustics normally in conjunction with attenuation of sound, not with amplification or even production of sound. On the other hand everybody knows the sound produced by a bottle, when someone blows over the orifice. During the investigation of the sound produced in body gaps it was found that the underlying flow physics are closely related to the Helmholtz-resonator. But different from the typical Helmholtz-resonator generated noise – as for example the blown bottle or, from the automotive world, the sun roof buffeting – there is no fluid resonance involved in the process. For body gaps the random pressure fluctuation of the turbulent boundary layer is sufficient to excite the acoustic resonance in the cavity. The sound generation is characterized by a continuous rise in sound pressure level with increasing velocity, the rise is proportional to U with varying exponents. It will be shown that the fluid resonance is shifted to much higher velocities than one would expect from Rossiters feedback model. This is the reason that it can not be found in body gaps. Nevertheless even without fluid resonance the body gaps represent the most important noise source for modern automobiles. A vehicle is much more silent in the interior if all gaps are closed as if, for example, the wing mirrors are removed and the A-Pillar-radius is doubled.

The shift of the fluid resonance to higher velocities mentioned above is associated with a reduced transport velocity for vortices over small orifices. Experimental evidence will be shown that this transport velocity is dramatically reduced for orifices buried under a thick turbulent boundary layer.