On the variation of the acoustic intensity vector near an open end of a pipe

A new laboratory facility for studying of the acoustic intensity field in a round 6m long, 150mm diameter pipe has been developed. It enables to control carefully the boundary conditions at the both ends, positions and the orientation of the acoustics sensors. It has been used to measure the variation of the acoustic intensity near the open end of the pipe with a triaxial intensity probe supplied by Microflown. The measured data have been used to validate a new efficient model based on the normal model decomposition and finite element methods. It is shown that even in the plane wave regime the acoustic intensity distribution near the pipe end is very complex and that the measured intensity vector sensitive to the radial and longitudinal position of the probe. It is demonstrated that a very small variation in the position of the intensity probe can result in a very large change in the intensity vector that is difficult to predict. The results of this work have a strong implication on the design of acoustic intensity sensors that have been developed to characterise the boundary conditions in a pipe.