Tone class
A class of sound (engl. Pitch class ) contains all tones, share certain characteristics with each other. From a formal point of view, a tone class is therefore an equivalence class of tones with respect to a certain feature. Accordingly, different classifications of the tones are possible, depending on which characteristic is used. In the modern 12-part European tone system , three main types of tone classes are distinguished:
- Tone classes with octave equivalence :
- All tones that are exactly one octave apart belong in the same tone class. So you get a total of 35 different tone classes:
- unchanged master tones : AHCDEFG
- Root tones raised by a semitone: A # -H # -C # -D # -E # -F # -G #
- Root tones increased by two semitones: A x -H x -C x -D x -E x -F x -G x
- Root tones lowered by a semitone: A b -BC b -D b -E b -F b -G b
- Master tones lowered by two semitones: A bb -H bb -C bb -D bb -E bb -F bb -G bb
- This version of the pitch classes is preferred in major-minor tonal music, as it enables the distinction between two differently written notes on the same pitch (e.g. D b and C # ).
- The equal tuning mostly used today enables the tone classes with so-called enharmonic equivalence . All tones that sound with the same frequency belong to one tone class. For example, the tones d ', c x ' and e bb ' belong to the same pitch class because they all designate exactly the same pitch. On the other hand, d '' does not belong to the same pitch class because the pitch is not identical to the other notes.
- If you combine these two, octave with enharmonic equivalence, you get twelve tone classes:
- A (G x , H bb )
- B (A # , C bb )
- H (A x , C b )
- C (H # , D bb )
- C # (H x , D b )
- D (C x , E bb )
- D # (E b , F bb )
- E (D x , F b )
- F (E # , G bb )
- F # (E x , G b )
- G (F x , A bb )
- G # (A b )
- This version is preferred in atonal music.
Tone classes are useful if you want to talk about certain functions that a tone can fulfill regardless of its pitch (e.g. as a leading tone within a key ).