Comparison of Nearfield Acoustic Holography and Dual Microphone Intensity Measurements

Comparison of Nearfield Acoustic Holography and Dual Microphone Intensity Measurements,Richard W. Bono,David L. Brown,Susan M. Dumbacher

Comparison of Nearfield Acoustic Holography and Dual Microphone Intensity Measurements  
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The measurement accuracy of nearfield acoustic holography (NAH) and dual microphone intensity measurement techniques are examined in terms of source identification capabilities and sound intensity level estimates. Inherent differences in the data acquisition and post processing methodologies are investigated. The techniques are applied according to their individual limitation to best evaluate the test structure. The variance of their respective results and how those results aid in engineering solutions is thoroughly discussed. Sound and vibration laboratories are commonly required to experimentally measure a test structure's acoustic properties and locate its sound sources. Since sound pressure is a scalar quantity which greatly depends upon the acoustic boundary conditions, additional acoustic properties are required to understand the structural and acoustic behavior of a given structure. Acoustic intensity is the rate of acoustic energy flow through an area in a given direction, typically in W/m2. It is a vector quantity as it has both magnitude and direction associated with it. Additionally, acoustic intensity can be described in active and reactive components. Active intensity quantifies the net flow of energy caused by pressure gradients while reactive intensity quantifies the acoustic energy that is stored in the acoustic medium but does not cause energy to flow, such as in highly reflective environments. The dual microphone intensity probe has been utilized over the last couple of decades to measure acoustic intensity levels. The cross-spectral method of determining sound intensity is well documented in its theory and applications. Standard grid intensity tests typically involve roving a pair of phase matched microphones separated by a known distance from grid location to grid location. The intensity probe scans a given grid area long enough to sufficiently average the acoustic data. A finite difference approach approximates the required acoustic properties. The sound pressure is estimated by averaging the microphone pair's output and the particle velocity is calculated from the pressure difference between the pair. For these estimates to remain accurate, high quality, phase-matched condenser microphones must be used. The accuracy of these estimates is also limited to a certain frequency range which is directly related to the spacing between the two microphones.
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