Amplification of the difference tones (f2–f1)
Pierre Dutilleux · The Journal of the Acoustical Society of America · 1999
Difference tones have been known by composers and performers for a long time. Nevertheless, these tones are still difficult to use in a musical context because their appearance is fairly unpredictable. A cello player, Michael Bach, has developed a sophisticated playing technique to produce these tones. Unfortunately the public was not aware of it because it could not here them as well as the player did. Evidence of the presence of the difference tones in the acoustic signal has been found in some cases. In most cases these tones develop in the head of the listeners only at fairly high sound-pressure levels. In order to control the amount of difference tones in a musical sound, a processing method has been developed that artificially produces the difference tones. The squared and filtered instantaneous envelope of the acoustic signal delivers them. These tones can then be mixed to the original sounds as needed and the public can perceive the difference tones even at moderate sound-pressure levels. The system was implemented and performed well during concerts. With a celletto (cello without any sounding box) it is possible to hear only difference tones.