Airborne vector sensor experiments within an anechoic chamber

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A technique to measure acoustic particle velocity in air is demonstrated in a controlled set of experiments conducted within an anechoic chamber at the Army Research Lab (ARL-Adelphi). This air-borne vector sensor measures acoustic pressure with an omnidirectional microphone and the 3D acceleration of a lightweight, rigid sphere with an embedded high-sensitivity tri-axial accelerometer. Two experiments are presented to demonstrate fundamental properties of the acoustic vector field: sound directionality, and the relative phase of pressure and particle velocity in the near to far field transition. Directional studies are implemented in the far-field of a speaker producing tones, defined by kr>>1, where r is the range from the speaker and k is the acoustic wavenumber. The pressure-velocity phase relationship is examined through the transition kr << 1, kr = 1, and then kr >> 1 by changing frequency and sensor range. For comparison, a tri-axial Microflown sensor simultaneously recorded the acoustic field. As neither sensor measures particle velocity directly, the phase characteristics of sensors depend on the sensing technique and corrections must be made to account for the true phase of particle velocity. Advantages and disadvantages of each method are discussed.