The Schumann resonances, in the electromagnetic spectrum, come from the roughly 2000 lightning storms occurring at any given moment around the globe, as they excite the cavity formed by Earth’s surface and the ionosphere. The fundamental frequency has been measured to be 7.83 Hz. In theory, harmonics should be a function that is proportional to sqrt(n*(n+1)), for the nth harmonic, but given reality (e.g., the earth is not a perfect sphere), these frequencies come out differently and of course change a bit over time.
Per the Wikipedia page, I used these measured frequencies: 7.83 14.3 20.8 27.3 33.8, but stepped up 3 octaves (f*8) so the relationships are easier to comprehend. Sinusoidal signals at the precise frequencies were generated with C code. The present work alternates 4-bar sections at 100 BPM between the measured and theoretical frequencies, with a shared fundamental. Three frequencies are selected at a time, to attempt to get a sense of what triads sound like in this space.
Actual frequencies used based on observations: 62.64, 114.4, 166.6, 218.4, and 270.4. And based on theory: 62.64, 108.4956626, 153.4360375, 198.0850726, and 242.6036768.