Impedance tubes
Different impedance tubes are used to measure the acoustic impedance of sound absorption materials. It can characterize the sound transmission loss and surface impedance as well. The working frequencies of the commercial Brüel & Kjær Type 4206 impedance tube are 50-1600 Hz (with diameter of 100 mm) and 500-6400 Hz (with diameter of 29 mm), respectively. A fabricated impedance tube has square cross-section of 50x50 mm, and the working frequency range is 500-3430 Hz.