[diesel_john]

theoretical pressure curve
Actually the curve will have high frequency noise superimposed upon it. A low pass filter, move the transducer up and down a straight section until the minimum noise is observed. I don't want to say look for a node line... The ear is good frequency analyzer and the entire event can be heard with a mechanics stethoscope.
Notice the pickup has two piezo's, and they oriented opposite each other, side to side motion will be cancelled somewhat, since piezo's have polarity. but the expanding tube will produce plus or minus voltages in both at the same time.