Octave * - in this case it is a logarithmic unit of relations between frequencies, when one octave corresponds to a doubling of the frequency. For example, a frequency greater than one octave from 40 Hz is 80 Hz.
Schumann Resonance * is the phenomenon of the formation of standing electromagnetic waves of low and ultra-low frequencies between the Earth's surface and the ionosphere.Winfried Otto Schumann
d ~ 503 * sqrt (1 / f * σ)