Towards acoustic particle velocity sensors in air using entrained balloons: Measurements and modeling

In this article, the feasibility of using balloons for the measurement of acoustic particle velocity in air is investigated by exploring the behavior of an elastic balloon in air as it vibrates in response to an incident acoustic wave. This is motivated by the frequent use of neutrally buoyant spheres as underwater inertial particle velocity sensors. The results of experiments performed in an anechoic chamber are presented, in which a pair of laser Doppler vibrometers simultaneously captured the velocities of the front and back surfaces of a Mylar balloon in an acoustic field. From phase measurements, the motion is described in terms of contributions from odd-order vibration modes (including bulk translation) and even-order vibration modes. The measured entrainment factors for the balloon are seen to be in good agreement with a physical model based on the scattering from an entrained rigid sphere. This demonstrates the feasibility of using entrained balloons for direct measurement of acoustic particle velocity in air.