 According to Albers [17], he (Westervelt) there first observed an accidental generation of low frequency sound "in air" by Captain H.J.Round (British pioneer of the superheterodyne receiver) via the parametric array mechanism.The phenomenon of the parametric array, seen first experimentally by Westervelt in the 1950s, was later explained theoretically in 1960, at a meeting of the Acoustical Society of America.