which seems to show that not only has one part of a vibrating string sympathy with another part of it so as to go into harmonic partials, as we have just seen, but as if the very air itself had sympathy with harmoniously vibrating strings; for Tartini observed that two harmonious sounds being produced and sustained as they can be, for example, by a strong bow on the violin, a third sound will be heard. Tartini's name for it was simply "a third sound." This is not an overtone, as Helmholtz has called the harmonic partials of one sounding string, but an undertone, because it is a "grave harmonic," away below the sounds of the two strings which awaken it. The subject of these undertones has been carefully studied since Tartini's day, and more insight has been obtained since we are now able to count and register the vibration of any musical sound. Helmholtz has called these third sounds of Tartini's "difference sounds," because when awakened by two strings, for example, the vibration-number of the third tone is the difference of the vibrations-numbers of the two tones which awaken it. The note C with vibration-number 512, and another C whose vibration-number is 256, the octave, awakened no third sound, because there is no difference between the two numbers - the one is just the doubled or halved; but if we take C256 and G381, its fifth, the difference number is 128; this being a low octave of C256, it has the effect of strengthening the upper one. Helmholtz found this to be the law of the third sound as to its producing, and the effect of it when produced. This third sound, mysteriously arising in the air through the sympathy it has with all concordant things, is another among many more suggestions that the whole Creation is measured and numbered to be in sympathy one part with another. The Creation is a universe.
The individual character of any note, and the comparative degree of contrast between any two notes in the system, depends on at least three different causes. The first is the genetic relation of the two notes. If the one note has 2 vibrations and the other 3, or the one 4 and the other 5, or the one 5 and the other 8, because of this, and because of the excess of the vibration of the one over the other, "a third sound" or "grave harmonic" being awakened between them, the different ratios have different degrees of complexity, and, in a general way, the greater the complexity the greater the [Scientific Basis and Build of Music, page 60]
electric potential difference
electrostatic potential difference
Figure 8.6 - Difference Tones
Ramsay - The Nature and Origin of Difference Sounds