Micro-Perforated Rim as Sound Absorber
The sound level of vehicles dominated by engine and flow noise in the beginning has been continuously reduced and optimized. Consequently, other previously inconsiderable noise components are given priority. This is primarily the tire noise for exterior noise occurring already at low velocities between approx. 40 and 50 km/h. Besides the wheel arch also the tire-rim-system can be used for the integration of absorbers. Multiple approaches for this purpose can be found in literature. In this context, however, the use of fibrous materials fails primarily due to the handling of tire change, which these materials cannot endure. Moreover, the absorber must not influence the driving behavior and driving safety. The solution presented here is an approach to integrate a robust and purely metallic resonator with micro-perforation in the rim.
In this context, the focus is on the standing wave generated in the circular air cavity of the tire called torus mode. The wave length of this first torus mode is determined by the mean circumference in the air volume. A frequency of approx. 200 Hz is achieved for customary tire sizes, which generates a measurable sound level increase in the passenger cabin. This noise transmitted by structure- and air-borne sound is perceived as unpleasant. The tire absorber is placed in the cavity of the wheel. The micro-perforated surface is directly integrated in the structure of the rim. For this purpose, the necessary holes are drilled into the well. A separately manufactured chamber necessary for the air volume and the sealing of the wheel is integrated at the selected position in the rim contour and hermetically sealed.
A complete set of tires was manufactured as prototype for the measurements in the passenger compartment. The measurements were carried out on the road as well as in the acoustic roller test bench of the Fraunhofer IBP with roller shells to simulate rough asphalt with increased noise exitation. The sound levels were measured by an artificial head on the front passenger’s seat. In a comparison with the standardized wheel a sound level reduction of the torus mode of 5 dB could be achieved on the road as well as on the roller test bench. The drilling of the rim, to produce micro-perforation, however, is certainly neither practicable nor economical for a serial product. Therefore, the solution with integration in the rim is further developed and currently near to serial testing with comparable acoustic efficiency.