Acoustic Analysis: The Impedance Tube Method

The measurement principle of an impedance tube (also Kundt’s Tube) method involves generating a sound wave in a hollow tube filled with the porous material under investigation, where sound energy is partially absorbed and reflected. By analysing sound pressure levels at different positions along the tube, the method allows for the calculation of the material’s sound absorption coefficient, providing valuable insights into its acoustic properties for various applications. The Impedance Tube allows for precise measurements of sound absorption coefficients and impedance in accordance with ISO and ASTM standards. Additionally, it facilitates sound transmission loss measurements using the Transfer Function Method, which separates incident and reflected energy from the measured transfer function to estimate the acoustic properties of the tested sample within the tube.

The Impedance tube consists of a tube with a sample holder, a speaker connected to a signal generator, two or four microphones depending on the setup, and an acoustic analyzer. For the frequency range of 63 Hz to 1800 Hz a tube with an inner diameter of 100 mm is used. For frequencies from 1000 Hz up to 6300 Hz, a smaller tube with a diameter of 30 mm can be used. A tube with a diameter of 60 mm can be used for frequencies from 125 Hz up to 3150 Hz.

Two microphone setup

  • According to DIN EN ISO 10534-2 and ASTM E1050
  • For absorption, reflection and impedance measurements

Four microphone setup

  • According to ASTM E2611
  • For transmission measurements