The double siren was one of Koenig’s more popular instruments. It consisted of two “polyphonic” or “multivoiced” sirens with more than one series of holes, and was an invention of the German physicist and former teacher of Hermann von Helmholtz, Heinrich Wilhelm Dove (1803–1879). It produced several pure tones simultaneously, in musical chords, and under greater pressure. It was ideal for demonstrating interference effects (when sound waves combined to amplify or diminish each other) and combination tones. [Pantalony 2009, 184-185]
Armstrong Cork,. 1932. “Armstrong's Acoustical Products”. Corkoustic, Ceramacoustic For Acoustical Correction Of Churches, Auditoriums, And Theatres, And Noise-Quieting Of Banks, Schools, Hospitals, Offices, Sunday School Rooms, Factories, And Other Public Buildings. Lancaster, PA.