Measuring sound power and directivity of a submerged cylinder in a reverberant water tank using intensity based nearfield acoustic holography techniques

Radiated sound power and free-field directivity of structures submerged in water are important acoustic design parameters. Techniques for determining sound power in reverberant environments using reverberation decay times exist, but are limited to broad frequency bands and high frequencies. Constructing water-filled measurement facilities large enough to acquire narrowband power spectra at low frequencies is spatially and economically unrealistic. Traditional sound directivity measurements made in anechoic environments, such as lakes, are inconvenient for diagnostic testing due to weather, scheduling, and cost constraints. Since the mid 1980's, Nearfield Acoustic Holography (NAH) has been a reliable technique for measuring sound power and directivity. One limitation of NAH is that all sources must be contained within the measurement hologram, usually restricting measurements to anechoic environments. This work combines NAH and sound intensity techniques to test sources in reverberant water environments. Holograms of sound intensity were measured around a submerged point-driven aluminum cylinder using a custom built p-a (pressure-acceleration) intensity probe. Through wavenumber filtering and reverberant signal rejection, accurate estimates of narrowband radiated sound power, directivity, and mode shapes of the cylinder were computed. Carefully chosen test parameters, combined with novel computational methods circumvented restrictions on measurement environments.