The Set of 13  (Spiral of 5ths) is INNER SOUND’s original tuning-fork set, developed in 2002 and still central to the INNER SOUND tuning fork collection.
It includes 13 forks, both weighted and unweighted, arranged as 12 related pairs in accompanying cloth pouch with rubber activator and wooden hammer. The frequency range was calculated in 2000 using 7.865 Hz as the reference value for the first Schumann Earth Cavity resonance. The lowest fork is tuned two octaves above that at 31.46 Hz, the first audible frequency to humans. The original frequencies have remained unchanged since the set was developed.
Each pair is tuned in a true fifth: a precise 3:2 relationship between the frequencies of the two forks. The phrase Spiral of Fifths describes how these true fifths continue from pair to pair. In contrast with the familiar circle of fifths used in modern, equal-tempered piano tuning, a sequence of twelve pure 3:2 fifths does not close perfectly after twelve steps. It continues as a spiral. The small difference between twelve true fifths and seven octaves is known in music theory as the Pythagorean comma.
Pair 5 and the Personal Tuners are part of the Set of 13 and are also available separately.
Made in the USA from high-grade aluminum alloy, every fork is tuned twice for accuracy so that each pair forms a true fifth.
Technical specifications: View the complete Set of 13 frequency reference.