7:5 · 6 semitones
Septimal tritone. Overcharged by about 17 cents, and it beats 3 times a second.
- Pure
- 582.51¢
- Tempered
- 600¢
- Charge
- +17.49¢
- Beat
- 3.13 Hz
Lower note 220 Hz. Pure upper 308.00 Hz. Tempered upper 311.13 Hz.
The keyboard's tritone is exactly half an octave, 600 cents, the square root of 2 as a frequency ratio. It is its own inverse: go up six semitones twice and you are an octave higher, so the tritone and its complement are the same size.
The ratio 7:5 (about 582.5 cents, sometimes called Huygens' tritone) is one pure candidate for the interval, and the tempered tritone is about 17.5 cents sharp of it. Played together the pure and tempered upper notes beat about 3 times a second. 7:5 is a choice: its inverse 10:7 is the other septimal candidate, so the charge is against one chosen ratio. Septimal tritone (Wikipedia, CC BY-SA).