Does the length of REM probe tube affect the measure ? We need a little help from the acoustic theory of resonance frequencies of tubes to respond to that question. In the REM probe tube, we have the following variables:
• The softness of the material forming the tube
• The diameter of the tube
• The length of the tube
Assuming that the softness and the inner diameter of the tube are constant, or it has a uniform cross-sectional area along its whole length, then we can determine its lowest frequency resonance. This will enable us to pinpoint the frequency range of sound waves that this tube can effectively transmit without modifying its amplitude by resonating.
We can obtain that frequency with the following equation:
F=C/2L
The “typical” length of a REM probe tube is 76.2mm or 3 inches, but some models are up to 86mm.
That gives us a lowest frequency resonance of 1.7 GHz for the longest REM probe tube. There is not influence whatsoever at human sound spectrum frequencies of the tube length on the amplitude of the sound, given that the initial conditions remain the same, the softness of the material, the diameter of the tube are uniform along the tube length.
The only criteria for choosing the length of a probe will then be the depth in the ear canal allowing the tip of the probe to capture the sound as close as possible to the eardrum.
What about diameter?
The standard that was used for REM probe tubes is inner diameter of the REM probe tube of 0.5mm and external diameter of 1.0mm. Only the inner diameter will affect the amplitude of the signal. As long as it is uniform along the REM probe tube no change in amplitude will occur.
What about calibration?
Calibration will have to be redone every time the length of the tube changes, since it might change the overall amplitude of the soundwave if the length is extended or reduced. But if you calibrate the reference microphone of the analyser will be compensated for the proper length of the tube.