I have been measuring transmission loss in impedance tube using 4 microphones. LMS Test lab is used for measurement. In the STL IN TUBE tab there are options to include phase calibration and ignore the same. I did measurement for TL in a foam acoustic package using both the methods. I could not observe significant difference in the TL graph between the measurements. So whats the significance of Phase calibration and when should u use it ?
Thank you all !
Phase calibration is used to correct for the phase mismatch between your measurement microphones. The transfer function that you measure using Test.Lab is a complex ratio of the acoustic pressure responses, and any variation in the amplitude or phase responses of the transfer function pairs will affect the accuracy of your results. Now this being said, the importance of this correction may depend on your microphonesand the STL result itself.
My advice would be to always try to perform a phase calibration of your tube, at least in the beginning of your measurement day. This is also a kind of a quality check on your measurement setup - is you see any changes to the Hc curve, it may mean that you should check your mics, sealing on the tube, etc. If you want to compare STL results from different samples, you should also make sure that all results use or did not use the phase correction.