Surely this is a velocity of sound problem - the pressure variation is determined by the acoustic propagation and hence the particulate velocity follows as a function of the signal source frequency, the humidity, and temperature. Two microphones staggered slightly apart along the tube will respond to an incoming sinusoidal acoustic wave with a time difference between them. By monitoring the time difference between the two signals, using an oscilloscope, the acoustic velocity can be measured and as such the rate of change of the particle velocity follows.

Please refer the following article.

1) Experimental determination of acoustic properties using a two-microphone random-excitation technique by Seybert and Ross ( The Journal of the Acoustical Society of America, 61, 1977).

2) Non-linear Acoustic Effects in the Rijke Tube by M A Heckl (Acustica, 72, 1990).

Hope it helps.

Regards

Balasundaram M

With two phase matched miprophones it is possible to determine the paricle speed between these mics in given frequency regions determined by the spacing between the mic diaphragms. This is a method to measure sound intensity using the two mic method. The theory and practice around this can be found in the literature.

In principle, you can estimate the particle velocity using two microphone sound pressure signals measured simultaneously. However, this method, called p-p, needs two closely located mics in order to approximate the Euler's equation well. However, errors in low-frequency are caused by the mismatched phase between the mics. Thus, you need two almost identical microphones. The theory is well known and can be found in several books and papers. I suggest reading the following:

- F. J. Fahy, Sound Intensity, 2nd ed., E&FN Spon, London, 1995.

- M. J. Crocker, Measurement of Sound Intensity, in Handbook of Acoustical Measurements and Noise Control, C.M. Harris, Ed., Acoustical Soc. Am., New York, 1998, pp. 14.1-14.17.

- M. J. Crocker and F. Jacobsen, Sound Intensity, in Handbook of Acoustics, M.J. Crocker, Ed., John Wiley and Sons, New York, 1998, pp. 1327-1340.

Hope these will help you.

You could try placing an intensity probe in the duct at each location and deriving the acoustic particle velocity from the reported intensity in Watt per square meter and rms acoustic pressure derived from the reported SPL. The biggest impediment being flow induced phantom noise that will not necessarily be remedied by a windscreen. I think intensity probes using two microphones are a bit more robust in terms of tolerance for exposure to wind and airflows when it comes to such phantom noise and you can probably measure up to 10m/s bulk velocities without much problem over the audible frequency range.