Combination tone

A combination tone, also called a sum tone or a difference tone (also occasionally resultant tone), can be any of at least three similar psychoacoustic phenomena. When two tones are played simultaneously, a listener can sometimes perceive an additional tone whose frequency is a sum or difference of the two frequencies. The discovery of some of these phenomena is credited to the violinist Giuseppe Tartini, and so the tones are also called Tartini tones.

Explanation

One way a difference tone can be heard is when two tones with fairly complete sets of harmonics make a just fifth. This can be explained as an example of the missing fundamental phenomenon.^[1] If $f$ is the missing fundamental frequency, then $2f$ would be the frequency of the lower tone, and its harmonics would be $4f,6f,8f,$ etc. Since a fifth corresponds to a frequency ratio of 2:3, the higher tone and its harmonics would then be $3f,6f,9f,$ etc. When both tones are sounded, there are components with frequencies of $2f,3f,4f,6f,8f,9f,$ etc. The missing fundamental is heard because so many of these components refer to it.

The specific phenomenon that Tartini discovered was physical. Sum and difference tones are thought to be caused sometimes by the non-linearity of the inner ear. This causes intermodulation distortion of the various frequencies which enter the ear. They are combined linearly, generating relatively faint components with frequencies equal to the sums and differences of whole multiples of the original frequencies. Any components which are heard are usually lower, with the most commonly heard frequency being just the difference tone, $f_{2}-f_{1}$ , though this may be a consequence of the other phenomena. Although much less common, the following frequencies may also be heard:

2f_{1}-f_{2},3f_{1}-2f_{2},\ldots ,f_{1}-k(f_{2}-f_{1})

For a time it was thought that the inner ear was solely responsible whenever a sum or difference tone was heard. However, experiments show evidence that even when using headphones providing a single pure tone to each ear separately, listeners may still hear a difference tone^{[citation needed]}. Since the peculiar, non-linear physics of the ear doesn't come into play in this case, it is thought that this must be a separate, neural phenomenon. Compare binaural beats.

Heinz Bohlen proposed what is now known as the Bohlen–Pierce scale on the basis of combination tones,^[2] as well as the 833 cents scale.

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Resultant tone

In pipe organs, a resultant tone is the sound of a combination of organ pipes that allows the listener to perceive a lower pitch.^[3]

This is done by having two pipes, one pipe of the note being played, and another harmonically related, typically at its fifth, being sounded at the same time. The result is a pitch at a common subharmonic of the pitches played (one octave below the first pitch when the second is the fifth, 3:2, two octaves below when the second is the major third, 5:4). This effect is useful especially in the lowest ranks of the pipe organ where cost or space could prohibit having a rank of such low pitch. For example, a 64' pipe would be costly and take up at least 32' of space (if capped) for each pipe. Using a resultant tone for such low pitches would eliminate the cost and space factor, but would not sound as full as a true 64' pipe.

This effect is most often used in the lowest octave of the organ only.

References

^ Beament, James (2001). How We Hear Music, ^{[page needed]}. The Boydell Press. ISBN 0-85115-813-7.
^ Max V. Mathews and John R. Pierce (1989). "The Bohlen–Pierce Scale", p.167. Current Directions in Computer Music Research, Max V. Mathews and John R. Pierce, eds. MIT Press.
^ James Ingall Wedgwood (1907). A Comprehensive Dictionary of Organ Stops: English and foreign, ancient and modern: practical, theoretical, historical, aesthetic, etymological, phonetic (2nd ed.). G. Schirmer. p. 1.

External links

Titchener Difference Tones Training
Difference tones on the harmonica
Pitch Perception Lecture Notes
Tartini computer program. Uses combination tones for pitch recognition. If certain intervals are played in double-stop, the program can display its Tartini-tone.
http://www.organstops.org/r/Resultant.html

[1] Beament, James (2001). How We Hear Music, ^{[page needed]}. The Boydell Press. ISBN 0-85115-813-7.

[2] Max V. Mathews and John R. Pierce (1989). "The Bohlen–Pierce Scale", p.167. Current Directions in Computer Music Research, Max V. Mathews and John R. Pierce, eds. MIT Press.

[3] James Ingall Wedgwood (1907). A Comprehensive Dictionary of Organ Stops: English and foreign, ancient and modern: practical, theoretical, historical, aesthetic, etymological, phonetic (2nd ed.). G. Schirmer. p. 1.

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v t e Acoustics
Acoustical engineering	Architectural acoustics Monochord Reverberation Soundproofing String vibration Sympathetic resonance	Spectrogram
Psychoacoustics	Pitch Bark scale Mel scale Equal-loudness contour Fletcher–Munson curves
Audio frequency and pitch	Beat Formant Fundamental frequency Frequency spectrum harmonic spectrum Harmonic Series Inharmonicity Missing fundamental Combination tone Mersenne's laws Overtone Resonance Standing wave Node Subharmonic
Acousticians	John Backus Jens Blauert Ernst Chladni Hermann von Helmholtz Carleen Hutchins Franz Melde Marin Mersenne Werner Meyer-Eppler Lord Rayleigh Joseph Sauveur D. Van Holliday Thomas Young
Related topics	Echo Infrasound Sound Ultrasound Musical acoustics Piano Violin
Category

Combination tone

Explanation

Resultant tone

See also

References

Further reading

External links