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Upper frequency for impedance tube absorption test?

Experimenter
Experimenter

We're doing some absorption tests with our impedance setup and were wondering if anything can be done regarding the cutoff frequency. Currently we're limited by the geometry of the tube diameter of 60 mm to which is roughly 3300 Hz. I've been snooping around the Knowledge Base looking for some answers but couldn't find anything specific to this one. Is there any software magic that Test.Lab has that can help us?

1 REPLY

Re: Upper frequency for impedance tube absorption test?

Siemens Legend Siemens Legend
Siemens Legend

Unfortunately there is no software magic for this.  The impedence tube absorption measurement relies on a plane wave of sound along the length of the tube.  The upper frequency is limited by the first out-of-plane acoustic mode in the tube, which is a function of the tube diameter (the smaller the diameter, the higher the upper frequency).

 

tube1.png

 

 

The following equation determines the upper frequency for the tube.

tube2.png

 

 

For a tube of 65 mm, the equation would produce the following upper frequency:

 

3093 Hertz = (0.586*343)/(0.065)

 

A tube of 30 mm would produce an upper frequency of:

 

6700 Hertz = (0.586*343)/(0.03)

 

The characteristic function is based on the cross section of tube.  Many impedance tubes are cicle cross-sections.