Acoustic properties of porous materialsused in silencers

Sammanfattning: The aim of this master thesis is an experimental investigation of the acoustic characteristics of absorbing materials used in mufflers for trucks and cars. The difference in the composition between different wool type materials consists of difference in material, fibre diameter, length, density and fibre orientation. It is also possible to construct mufflers using micro perforated plates (MPP), either solely or in combination with wool type materials. When a specific material is selected the characteristics of the performance can be altered by compressing the material to different bulk densities. It was investigated how some of these properties change the airflow resistivity of the material. When the airflow resistivity of the material is known this parameter can be used in FEM software to describe how a specific material will react, behave and perform as an absorbent. Two different methods were used to extract the airflow resistivity. The fastest method is from the ISO standard ISO 9053 were the airflow resistivity is measured over a sample with a flow speed down to 0.5 mm/s. The second method is the Transfer Matrix Method (TMM) with which the airflow resistivity is extracted from the acoustic transfer matrix of the sample. Both methods are fully described in the report. The TMM was used within a frequency range of 0‐1600 Hz at no flow conditions. Measurements using both methods were performed at room temperature . Ten different wool type materials and two different kinds of MPP were studied. For the wool type materials, the airflow resistivity was measured with the fibres parallel and perpendicular to the direction of sound and airflow. The material samples had bulk densities of 80‐210 g/l. For the MPP the specific airflow resistance was measured with the static flow perpendicular to the plates. The results from the two methods were compared and the transmission loss, absorption coefficient, reflection coefficient and the complex speed of sound were calculated using the transfer matrix from the TMM. Regarding the TMM these data were also compared to the results that can be calculated when using the measured airflow resistivity together with the empirical expressions from Delany‐Bazley & Miki. Repacking of some materials were done in order to study the differences introduced by the packing process. IV The conclusions after the measurements were: • The agreement between the two methods was very good. • The value of the resistivity was doubled for measurements with the fibres perpendicular to the direction of sound. • When the materials with a high degree of micro strands were oriented with the fibres perpendicular to the direction of sound there was a resonant behaviour in the sample. The onset frequency of this resonance increased with increasing bulk density. This resonance leads to difficulties in predicting the behaviour of real life exhaust systems. • In order to get reliable results, further studies on the micro perforated plates must be made, with even lower flow velocities and sound pressure levels and maybe with other methods.