Sounds produced by subsonic ballistic flow

Abstract

The sounds of passing subsonic ballistic projectiles are receiving more attention yet have limited acoustic characterization. Recent accounts include buzzing sound of a rifled bullet, and the whistling sounds of spherical projectiles produced by police firing non-lethal spherical rubber bullets, paintball, and war re-enactments using muskets. Modern recording equipment with multiple simultaneous channels and high dynamic range is now available to accurately capture subsonic ballistic flow sounds. Spherical projectiles 0.69 in. in diameter were launched from smooth bore replica muskets and high pressure paintball guns, and recorded on multiple microphones. Sounds of vortex shedding at predictable frequencies were produced using smooth spherical balls with no rotation, fired at velocities between 45 and 152 m/s (150–500 fps), and with Reynolds numbers below 3.75 × 105. The paintballs shed vortices in the 500–800 Hz range (with reported whoosh sounds); while faster musket balls shed vortices in the 2000–2500 Hz range (with reported whistling sounds). Our interest originally arose from historical reports that Erastus “Deaf” Smith, a hero of the Texas Revolution, could not hear musket balls that passed near him; and recent medical speculation that he had a 2000–3000 Hz notch in his hearing spectrum.