當兩個強音同時鳴響時，會產生第三個音，即合成音，其振動數為原二音振動數之差，這個（較低的）音叫“差音”，它們也能產生第四個音，頻率高而音弱，與原二音振動之和相等。此音叫“合音”
http://members.tripod.com/gia_5/fractal/kt3.doc.
牛津簡明音樂詞典 The Concise Oxford Dictionary of Music
(Fourth Edition) (第四版）
Harvard Dictionary of Music Page 185:
Combination Tone Combigation tone, resultant tone [F. son resultant; G. Kombinationston; It. torn di combinazione; Sp. tow) resultante]. In musical acoustics, a tone of different pitch that is heard when two loud tones are sounded simultaneously. Its frequency is the difference, (differential tones) or the sum (summation tones) of the frequencies of the two primary tones or of their multiples. For example, if the two primary tones have the frequencies 1200 and 700, the following differential tones (D) and summation tones (S) can be heard: Di: 1200 - 700 = 500; Di: 2 x 1200 - 700 = 1700; Da: 2x700- 1200 = 200; Si: 1200 + 700 = 1900; S,: 2. X 1200 + 700 = 3100; Sg: 2 X 700+1200 = 2600, etc.
Although the combination tones are frequently referred to as an acoustical phenomenon, they actually are aphysiological phenomenon. If the vibrations 1200 and 700 are produced, none of the vibrations 500, 1700, etc., actually exists; it is the inner ear (cochlea) that, owing to its "non-linear" organization, produces the aural sensations corresponding to the greater or lesser frequencies. "Nonlinear" here means that the combination of two sounds with the intensities a and b is not a simple linear sum, a+ b, but a more complicated formula, involving powers of a and b. The linear formula is valid only for small intensities; as a matter of fact, combination tones are heard only if the original tones are sufficiently loud.
The differential tones, which are more easily recognized than the summation tones, were discovered by G. Tartini in 1714 and described
in his Trattato di musica of 1754 (an earlier description appeared in a book by G. A. Sorge, Vorgemach der musicalischen Composition, 1745- 47). The tone known as Tartini's tone [It. teno suono, "third tone"] is the first of the combination tones above, determined by the difference of the original frequencies. The accompanying table shows Tartini's tone for various intervals (c' arbitrarily = 300).
Tartini's tone can easily be heard on the harmonium, organ, and violin. On the violin, it was recommended by Tartini and other violinists (Leopold Mozart) as a means of controlling the correct intonation, of double stops, since a slight inaccuracy results in a more recognizable change of the low-pitched differential tone. The name "beat-tones," formerly applied to differential tones, is misleading. It is derived from the theory advanced by T. Young (1773-1829) that the differential tones are quick *bcats (more than 40 per second). This theory was refuted by Helmholtz, who discovered the summation tones by calculations based on the principle of "nonlinear superposition," thus paving the way for the modern theory. Modern research has shown that certain well-established musical sounds, e.g., that of the G-string of the violin, arc physically nonexistent, being produced only auraBy as the differential tones of their upper partials [sec Tone color].
Practical application of the first differential tone is made in the *acoustic bass of organs.
For literature, see under Acoustics; also E. Dar- mois, in CP Masson.
小提琴家史勃瓦科夫斯基(Tossy Spivakovsky)的練習方法：
A 調，
在A弦第一把位奏升C及空弦E （小三度）。若奏來平穩，其合音就是低音A——主和絃
第三指奏A弦D，而第一指奏E弦升F （大三度），其合音是D——次屬和絃
第一指奏A弦B，而第二指奏E弦升G（大六度），其合音是E——屬和絃
第二指奏A弦的升C，而第三指奏E弦的A（小六度），其合音又必為主和絃
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