James P. Carneal
Gary P. Gibbs
Vib. and Acoust. Labs., Dept. of Mech. Eng., Virginia Polytech. Inst. and State Univ., Blacksburg, VA 24061
A structural-based acoustic intensity (SBAI) sensor has been developed for low-frequency applications. The sensor is comprised of a structural mounted accelerometer and pressure sensor. Local acoustic intensity is calculated by the time average of the product of the velocity and pressure measurements. The ability to use a structural-mounted pressure sensor was confirmed when the phase between a near field and structural-based pressure sensor was determined to be less than 5 deg up to approximately 1500 Hz. Verification of the SBAI sensor proceeded as follows. The structural-based acoustic intensity sensor output was shown to be proportional to the output of a calibrated two microphone intensity sensor from a piston source. Active control of the piston source using the SBAI as an error sensor showed global reduction of radiated acoustic power of approximately 15 dB for several harmonic excitation frequencies. Active control of a complex structure (plate) at various frequencies displayed mixed results. For the (3,2) resonance and a position of (x/Lx=0.85, y/Ly=0.78), global reduction of radiated acoustic power of approximately 9 dB was achieved. However, other tests did not show this type of reduction primarily due to the SBAI sensor acting as a local intensity estimator. It is evident that when applied to a complex structure, the point (or points) at which the intensity measurement is taken must be chosen carefully to obtain a global estimate of the far field radiation.