Interferometric measurements of supersonic projectile acoustic signatures

Abstract

Supersonic projectiles cause disturbances in the air that propagate and coalesce to form acoustic signatures. In the far-field, these signatures approach ideal N-waves. However, near the projectile these signatures are closely related to the projectile's shape. To study the generation and formation of acoustic signatures from supersonic projectiles, an experiment was conducted using optical methods. A z-type schlieren imaging system and laser interferometer were used in conjunction with acoustic sensors to measure the pressure field surrounding supersonic projectiles of various sizes and shapes. The rate of change of the phase difference recorded by the interferometer is inverted to obtain the ballistic pressure field by assuming cylindrical symmetry and using an inverse Abel transform. The accuracy of these reconstructed time series is evaluated by comparing N-wave parameters, such as period and peak pressure, with analytical results derived from the Whitham F-function. Improved rise time resolution obtained from the interferometric measurement is demonstrated by comparison with recordings from standard condenser microphones. Advantages of measuring acoustic signatures near the projectile by interferometric methods are discussed.