On the intensimetric analysis and monitoring of flue organ pipes

A short review of intensimetric analysis and monitoring of flue organ pipes based on the new concepts of radiating and oscillating sound intensities will be addressed in this paper. This kind of general analysis dates back to a decade ago when the concept of velocity of acoustic energy transport was rigorously introduced in 1994. In that paper it was clearly shown that the acoustic particle velocity is always decomposable into a component in-phase with the sound pressure and another one orthogonal to it in the Hilbert space. Roughly speaking this second component coincides with the phase-quadrature component when the field is a monochromatic one. From this general property it follows that acoustic energy can travel with an average velocity bounded in modulus between 0 and c (the sound speed). A further progress was made some years later in 1996 and in 1997 when the polarization property of sound intensity was discovered and measured. The story culminates in 2003 with a great synthesis when the four-dimensional treatment of the linear acoustic field was outlined in strict analogy with the electromagnetic field. The flue organ pipe was a faithful witness all along this research path and this paper will briefly summarize results obtained for sound intensity measurements performed inside and outside the pipe. There is no doubt about it: the flue organ pipe is a real paradigm of acoustics.