Subharmonics are a relatively recent development in double bass technique.
This technique requires an advanced level of fingerboard knowledge, bow control and bow placement. As the placement of the bow is paramount to the effective usage of this technique, the bassist must experiment to find the correct bow position for each fingered pitch. In all cases for an octave subharmonic, the position will always be at the 6th partial of the harmonic series as it directly relates to the fingered pitch.
Place the bow at the 6th partial of the harmonic series of the fingered pitch and bow with excessive pressure and an absolutely consistent speed. The increased pressure will distort the vibration of the string, producing a phase loop which, in turn, produces the subharmonic.
I have found the technique to be most effective when less bow hair is in contact with the string and that the most effective results have been produced using a cello bow.
Dynamic Range: pp-ff
Ashley John Long Imaginary Numbers (2015)