Particle velocity field measurement using an ultra-light membrane

Abstract

The aim of this paper, is to present an innovative experimental approach to assess images of the acoustic particle velocity field using a laser vibrometer scanning an ultra-light membrane. Firstly, a theoretical part is devoted to the infinite membrane governing equations, and to its response to an acoustic incident plane wave. An impedance of the membrane is defined, and a mass correction factor is obtained for a plane wave in normal incidence, allowing to assess the particle velocity of the acoustic field without membrane from the measured velocity of the membrane. The practical realization of a finite membrane is reported in a second section, showing difficulties linked to membrane modes generated by an uncontrolled residual tension, also showing how those difficulties are overcome by weighing down the membrane and by applying the mass correction. Third and fourth parts of the article are dedicated to two-dimension applications, illustrating the implementation of the membrane approach in the case of a plate excited by a shaker, and in the case of a loudspeaker driven with a white noise. Results are encouraging, showing that a “heavy” membrane should be used in low frequency to avoid difficulties due to membrane modes, and a light one in high frequency to minimize the mass effect (that can be corrected only for plane wave in normal incidence) and to avoid potential interactions between the membrane and the air gap between the membrane and the source, when the membrane is placed in the near field.