No abstract

Acoustic surface impedance of sound absorbing materials can be measured by several techniques such as the impedance tube for normal impedance or the Tamura method for normal and oblique surface impedance. In situ, the acoustic impedance is mostly measured by use of impulse methods or by applying two-microphone techniques. All these techniques are based on the determination of the sound pressure at specific locations. In this paper, the authors use a method which is based on the combined measurement of the instantaneous sound pressure and sound particle velocity. A brief description of the measurement technique and a detailed analysis of the influence of the calibration, the source type, the source height, the sound incidence angle, and the sample size are included.

The low-frequency (20–250 Hz) airborne sound transmission of single partitions is investigated. Three theoretical models are used for the prediction: an infinite plate model, a baffled plate model and a room-plate-room model. The calculation models are verified by detailed comparisons with experimental results obtained in the laboratory. A parametric study is carried out to examine the influence of the dimensions of the room and the partition. The results demonstrate the strong modal behaviour of the low-frequency sound transmission. As a result, the low-frequency sound insulation depends not only on the properties of the test wall, but also on the geometry and the dimensions of the room-wall-room system.

When considering the sound transmission through a wall in between two rooms, in an important part of the audio frequency range the local response of the rooms is highly sensitive to uncertainty in spatial variations in geometry, material properties and boundary conditions, which have a wave scattering effect, while the local response of the wall is rather insensitive to such uncertainty. For this mid-frequency range, a computationally efficient modeling strategy is adopted that accounts for this uncertainty. The partitioning wall is modeled deterministically, e.g. with finite elements. The rooms are modeled in a very efficient, nonparametric stochastic way, as in statistical energy analysis. All components are coupled by means of a rigorous power balance. This hybrid strategy is extended so that the mean and variance of the sound transmission loss can be computed as well as the transition frequency that loosely marks the boundary between low-and high-frequency behavior of a vibro-acoustic component. The method is first validated in a simulation study, and then applied for predicting the airborne sound insulation of a series of partition walls of increasing complexity: a thin plastic plate, a wall consisting of gypsum blocks, a thicker masonry wall and a double glazing. It is found that the uncertainty caused by random scattering is important except at very high frequencies, where the modal overlap of the rooms is very high. The results are compared with laboratory measurements, and both are found to agree to within the prediction uncertainty in the considered frequency range.

The sound transmission loss (STL) of finite lightweight multilayered structures with thin air layers is studied in this paper. Two types of models are used to describe the vibro-acoustic behavior of these structures. Standard transfer matrix method assumes infinite layers and represents the plane wave propagation in the layers. A wave based model describes the direct sound transmission through a rectangular structure placed between two reverberant rooms. Full vibro-acoustic coupling between rooms, plates, and air cavities is taken into account. Comparison with double glazing measurements shows that this effect of vibro-acoustic coupling is important in lightweight double walls. For infinite structures, structural damping has no significant influence on STL below the coincidence frequency. In this frequency region, the non-resonant transmission or so-called mass-law behavior dominates sound transmission. Modal simulations suggest a large influence of structural damping on STL. This is confirmed by experiments with double fiberboard partitions and sandwich structures. The results show that for thin air layers, the damping induced by friction and viscous effects at the air gap surfaces can largely influence and improve the sound transmission characteristics.