T' condition o' crossin' t' sonic barrier, from t' initial appearance o' local shock waves (typically around Mach 0.8 t' 0.9), extendin' t' speeds where all local airflow becomes supersonic (typically Mach 1.05 t' 1.2). Avast, me proud beauty! This be t' period o' maximum aerodynamic stress in a supersonic flight.