In the military vehicle, automotive sector, the acoustic emission of internal combustion engines has a significant impact on the environment. For example, engine fans or exhaust streams exhibit low-frequency spectra with local maxima. These correspond either to the driving speed or to the idling mode, which is related to the finite amount of pistons in the engine. In the latter case, a stationary signal of the idling mode is analyzed for its local frequency peaks. In this approach, an acoustic metamaterial based on Helmholtz-resonance is developed for the passive reduction of these narrowband frequencies. A pre-dimensioning in the geometric design of cavities and necks is carried out, while a downscaling of the size is necessary for laboratory purposes. A simulation model is developed to investigate the sound pressure distribution as well as the sound power transmission loss. The results are compared with experiments using a 3-D-printed metamaterial prototype inserted into an acoustic impedance tube. The results of this study show the potential for optimizing sound emission in noise-sensitive regions as well as in tactical, and military environments where a mission vehicle should not be detected.
Clasen et al. (Tue,) studied this question.