Passive Noise Reduction in an Automotive Compartment

Fuel economy, international competition and socioeconomic factors have forced manufacturers to develop lighter automotive vehicles. However, such vehicles are typically more susceptible to noise and vibration problems. The internal sound field in the passenger compartment is affected significantly by the acoustic modal characteristics of the cavity, by the dynamic behaviour of the surrounding structure, and by the nature of the coupling of these two dynamic systems. The purpose of this research is to develop and analyse a new vibroacoustic model containing the main compartment cavity and the luggage compartment cavity. Special attention is placed on the effect of a double walled partition between the main compartment cavity and the luggage compartment. The system is studied using ANSYS finite element (FE) code. The modelling involved shell finite elements for the structure and three-dimensional (3D) acoustic elements for the cavities. The 3D FE modal analysis produced results visualizing the complex picture of acoustic-structure coupling in the lower frequency range (30–200 Hz). It was found that strong coupling between the thin walled structure and the acoustic enclosures exists in the vicinity of any acoustic resonance. The key noise reduction principle examined is the passive application of a characteristic impedance mismatch. Using an FE model, numerical simulations are conducted to study the effect of various design parameters on acoustic transmission. The results show that at least 8 dB reductions in sound pressure level may be achieved with a modest level of vacuum in the double partition between the main cabin and the luggage compartment cavity.