B400 Acoustic Impedence Tube

The B400 consists of a transparent plastic tube fitted at one end with a glass reinforced plastic enclosure containing two loudspeakers and at the other with a heavy sample holder.

A small microphone may be moved axially along the tube and its position measured. The loudspeakers are fed with a common pure tone from a function generator and the sound waves pass along the tube to the sample. Dependent upon the type of material and the tone frequency, a portion of the sound energy is absorbed and the remainder is reflected back along the tube.  The portion absorbed being converted into heat by the viscous action of the airborne wave as it passes through the absorber.

Interference between the incident sound waves and those reflected result in a standing wave field whose amplitude varies along the length of the tube.

With the aid of an oscilloscope the amplitude of the maximum and minimum signals received by the travelling microphone may be measured and from these values the Sound Absorption Coefficient at the test frequency may be measured.


The unit enables students to conduct the following experiments.

  • Determine the Sound Absorption Coefficient for many of the normal building lining materials such as carpet, cork, fibre board and many of the better acoustic attenuating materials.
  • Determine the Sound Absorption Coefficient of these and some poor absorbers at a range of frequencies between approximately 300 and 4000Hz.
  • Determine the speed of sound in air at ambient temperature and comparison of this with the calculated value.


A:  300W 220/240 Volts, 50Hz. (With earth/ground)


B:  300W  110/120 Volts, 60Hz. (With earth/ground)