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

4.3. MEASURING THE ACOUSTIC ABSORPTION 754.3.2 Measuringthe absorption in areverberation roomIf one needs to find the absorption coefficient of an object or e.g. a glasswool mattress, it is necessary to have a reverberation room at its disposal.This is a room with very hard walls (see Figure 4.8). According to theinternationalstandardsforareverberationroomV ≈ 200m 3 andV ≫ V object ,the maximum dimensions of the diagonals is 12 m, and there are no parallelsurfaces. First, the total absorption A 0 of the reverberation room itself isdeterminedduringafirstmeasurement usingtheformulaofSabine: A 0 = V6T 0in which T 0 is the reverberation time of the empty reverberation room. Next,the object is introduced and a new (shorter) value of the reverberation timeT is measured. Again follows A = V (A is the total absorption of the6Treverberation room with the object).The total absorption (room plus object) is given by A = A 0 +aS with athe absorption coefficient to be determined and S the surface of the object.Therefore :a = V ( 16S T − 1 )(4.29)T 0It suffices thus to measure two reverberation times to be able to determinea in a reverberation room. The measurement is normally carried out at anumber of normalized frequencies (i.e. band limited noise in various octavebandsisusedforthedifferent measurements ofthereverberation times). Oneshould note that a can be found in a reverberation room with the aid of apractically obtained diffuse field, i.e. for practically ’omnidirectional’ soundwaves. This omnidirectional incidence is often facilitated by hanging sounddiffusing panels (plywood or plastic) in the reverberation room.4.3.3 Measuring the absorption in the Kundt tubeMeasuring sound absorption of a material in a reverberation room is rathercomplicated because a fairly large material specimen must be available andbecause only few certified reverberation rooms are available. An alternativetest method is the so-called Kundt tube , also called impedance tube. Aschematic representation of the apparatus is given in Figure 4.9. The setupconsists of a cylindrical tube with the test specimen mounted at one end.At the other end a speaker is mounted. In the tube a rod is inserted onwhich a microphone is mounted. The the sound wave (pressure) in the tubeis measured while moving the rod through the tube. Two major parametersare recorded (these can be easily visualized on an oscilloscope) :P min the mimimum amplitude of the sound pressure