The French maker of acoustical instruments, Albert Maroloye (1785-1874), added the resonant box to the tuning fork in 1839; Koenig took over Marloye's busniess in 1858. The tops of these boxes are made of spruce, like the top of a violin. The depth of the box is slightly less than one quarter of the wavelength of the sound that it produced (allowance is made for an end correction), and so the lowest frequency standing wave is set up. The instrument is thus highly directional, with much of the acoustic radiation coming out of the open mouth of the box. 

   This 128 Hz tuning fork cost 100 francs ($20) in the 1889 Koenig catalogue