Characterization of Sound Field Diffuseness Inside Payload Fairing Using Flight-Validated Numerical Model

This paper describes the results of investigating the diffuseness of sound field inside payload fairing during launch. Accurate estimation of sound-induced vibration of spacecraft requires thorough understanding of the sound field inside payload fairing. However, in general, measurement data at a limited number of points during flight does not allow specifying details of the sound field such as spatial distribution. Ground acoustic tests are performed under the assumption of diffuse field, but the diffuseness of actual in-flight sound field has remained unclear. This paper reports the results of examining the diffuseness by analyzing the spatial distribution of the sound field during launch using vibroacoustic finite element method validated by flight data. The analyzed sound field shortly after liftoff exhibited a higher homogeneity than that in transonic condition, whereas both conditions showed lower homogeneity than ideal diffuse field. These findings indicate that the diffuseness during launch is not constant but changes over time as flight progresses. Consideration in the wavenumber domain showed that the external pressure field shortly after liftoff, which is considered to be formed from axial plane progressive waves, tends to excite breathing mode of cylindrical shell, causing more uniform internal sound field at low frequencies.