Determining internal pipe acoustic pressure using external accelerometers

The inherent coupling of a pipe's internal acoustic pressure and its external wall vibrations has given impetus to the proposed method of using externally mounted accelerometers to indirectly measure a pipe's internal acoustic pressure. A pipe wall's radial, in-phase motion (the breathing mode) is strongly coupled to the internal acoustic pressure of the pipe. Analytical modeling and experimental analysis were performed to assess the feasibility of using a ring con- figuration of accelerometers, paired with modal analysis, to extract the breathing mode component of the vibration and indirectly calculate the internal acoustic pressure. Simulation modeling affirmed the method's sensitivity to unevenly spaced accelerometers and the transverse sensitivity of the accelerometers, especially for frequencies below 200 Hz. Initial experiments performed on an aluminum pipe with a 156 mm diameter and 3.2 mm thickness revealed the accelerometers' ability to indirectly predict the internal acoustic pressure levels within 10 dB, over the range from200 Hz to 1 kHz, relative to the measured levels using hydrophones. More promising is the method's ability to accurately monitor changes in pressure levels and pressure fluctuations. The proposed method, utilizing a ring of accelerometers, proves successful in accurately predicting the relative increase or decrease of the internal acoustic pressure level, with the exception of a few deviations at higher frequencies.