VRIJE UNIVERSITEIT BRUSSEL Acoustics - the Dept. of ...

Chapter 4Sound AbsorptionAll(construction)materialshave, toagreaterorlesserdegree, thepropertyofsoundabsorption: concreteandstoneabsorblesssound, woodandfibreboardmore. It often occurs that too much sound is reflected, and so one can hearechoes in a room. One can decide to cover certain boundaries of the room,with materials that better absorb the sound, e.g. in homes or offices :Acoustic tiles on the ceilingAcoustically absorbing plates against the wallsCarpet on the floorCurtains in front of the windowsOne should pay attention to the fact that these sound absorbing materialsserve to attenuate the sound reflections. They are not used to improve soundinsulation, asisoftenthought. Thesoundinsulationofanabsorbing materialis quite small, and the misconception probably originates from the fact thatgood thermal insulating materials like glasswool and rockwool, are also goodsound insulating materials.4.1 Acoustic transmission between two mediaIn practice it never occurs that an acoustic wave, generated by an acousticsource, propagates exclusively in one medium. Usually the wave will propagateon to a second medium (e.g. gas/fluid, gas/solid, etc.). In this sectionwe will present a model which allows us to study to which extent energyis reflected, and to which extent energy will be transmitted to the second59