Experiment on propagation characteristics of leakage sound

Propagation attenuation of acoustic signals associated with leak has been measured in a hot-work 304 stainless steel U-pipe (60.5 mm in outer diameter, 5.5 mm thick and 50 m long), as functions of frequency range and diameters of leak holes. Water at 7 MPa and --2800C was discharged from holes (2, 4, 7, and 12 mm in diameter) drilled through the U-pipe. Acoustic signals generated were detected with model 0.65Z20 acoustic emission sensors (fabricated by NF Circuit Design Block Co.). They were guided from the pipe wall to sensors with stainless steel rods (5 mm in diameter and 30 cm long), which were tig-welded on the 20 m straight part of the U-pipe in intervals of 20 cm -- 2 m. Signals amplified by a preamplifier were fed to a main amplifier containing a band-pass filter and filtered to each frequency range of 50k -- 100kHz, 100k -- 200kHz, 200k -- 300kHz, and 300k -- 2000kHz. Acoustic signals decrease their amplitude proportionally to rsup(-0.2 -- 0.4) (r = distance from holes) within 1 m from leak source, then exponentially after 1 m. Thus propagation attenuation within 1 m is 6 -- 30 times as large as that per unit length after 1 m from the source. Attenuation within 1 m from the source decreases with increase of hole diameter, while it is practically independent of frequency range. Attenuation rates after 1 m from the source are independent of hole diameter, and increase with increase of frequency range. (author)