Impedance tubes consist of tubes with a round or square cross-section and are terminated on one side with a loudspeaker and on the other side with the test object and a reflective tube termination. The loudspeaker is used to excite plane sound waves in the impedance tube, of which the sound pressure is recorded by microphones located between the loudspeaker and the test object. The surface impedance can be calculated from the determined sound pressures and the geometric distances between the microphone positions and the test object. The sound absorption coefficient and the reflection factor are calculated from the surface impedance.
As a special application, the sound transmission, i. e. the sound insulation, can be determined by means of an extension tube. The extension tube is connected to the main tube instead of the reflecting tube termination, contains as many microphone positions as the main tube and terminates with an anechoic swamp. The anechoic termination is realized by a wedge absorber placed in the extension tube. For the determination of the sound transmission, the sound pressures at the microphone positions in the main tube and in the extension tube are determined.
The use of an impedance tube to determine sound insulation can be useful for test objects whose vibration behavior is representative for the intended application despite the small sample dimensions. This includes many porous absorbers such as fabrics, foams and fibroid materials, but also sealing profiles.