The impedance tube apparatus is commonly used to measure specific impedances, sound absorption coefficients (SACs), sound transmission losses (STLs) and acoustic properties (characteristic impedances, propagation wave numbers, effective densities, bulk moduli) of acoustic materials in normal incidence conditions.
Sound-absorption ability of the sample is measured in the planewave impedance tube. The sound is generated by a loud speaker at one end of the tube and the sample is placed and sealed at the other end of the tube. The sound abortion co-efficent is the absorbed fraction of the energy of a plane sound-wave when incident on the sample material. The co-effient describes the ability of the material to absorb sound in a given frequency band.
Measurement techniques are based on ASTM standards. The experiment can be conducted by two microphones and four microphones. System consists of a solid brass tube containing a speaker at one end and the other end with the capability to hold a material sample, whose properties are to be measured. Pair of microphones, separated by finite distance is connected to this tube with the help of microphone holders. These microphones are connected to a digital signal analyzer via signal conditioners (pre-amplifiers) and a data acquisition system. A function generator is used to power the speaker in the impedance tube. Termination conditions differ based on whether to measure absorption or transmission loss. For absorption coefficient measurements, a rigid backing is used. For transmission loss measurement, a hollow tube of the same diameter as the upstream tube with a pair of microphone holders is used on the downstream of the test sample. Two different termination conditions (anechoic and rigid backing) are used during transmission loss measurements.