The celebrated Italian Violinist and composer Guiseppe Tartini (1692-1770) has been credited in discovering the phenomenon of an audible ‘third voice’ when playing double stops (two simultaneous notes) on stringed instruments. These have been called ‘combination tones’, ‘sum tones’, ‘difference tones’ as well as ‘Tartini tones’.
As an example, if one were to play an open ‘A’ string on the violin simultaneously with a C# on an ‘E’ string (an interval of a major 10th) in a loud and projected manner, a third ‘voice’ would be clearly discernible (that note being an ‘A’ an octave below the ‘A’ string). As the violinist adjusts the tuning of this major 10th, so the ‘third voice’ also changes pitch. There are numerous other double stops that produce a similar effect. The reason behind this acoustic phenomenon is tied up with the harmonic series and some pretty complicated physics which means that the mix of harmonics from the lower and upper note have many matching components and therefore a missing fundamental is heard. It is like the effect of a shadow and is sometimes called a ‘ghost note’.
This all ties in with a second phenomenon known as ‘binaural beats’ but referred to by string players as either ‘the beat’ or ‘sympathetic vibrations’. Where a string player tunes up their instrument, it can be observed that they play two strings simultaneously and then adjust one of them until they are completely happy that the two strings are resonating in tune. What we are actually doing is using the pure interval of a 5th (which all violins, violas and cello’s are tuned to) to pick up any subtle differences in tuning. This is done by listening out for the ‘beat’ which is an audible vibration (again taking the form of a third voice) which is rapid when the perfect fifths are impure, slowing down as the interval becomes gradually more in tune before finally stopping altogether when the notes become perfectly in tune. Although string players tune in perfect 5ths, this is the interval (when considering the whole history of different temperaments or tuning systems) which doesn’t tally with a perfect octave and therefore can cause tuning issues within the string section or orchestra.