Play with this lost chord here:
This is a fairly simple sound with a few low harmonics.

Base Frequency: Seconds to cycle: Loudness:

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Actually, lots of people have discovered the Lost Chord, but didn't know it or call it that. It's actually a whole class of lost chords. Coming from me, I bet you can figure out that you can't play them in equal temperament.
If you recall Harry Partch's "One Footed Bride" and other measures of interval consonance, you'll see a big lump - the foot if you will - near the octave (2/1):

The Bride's missing left foot (1/1) is actually where the Lost Chords reside. That is to say, the mind altering, fascinating, hypnotic chords of song and story are the extremely close intervals near 1/1. Anyone with a comb filter or chorus/flange effect could have told you that, but here's the thing: as the interference beats get slower, the more this timbral fusion passes from a perceived effect to a composition itself. Also, you need a really stable and pure bunch of sound sources, which my apps Droneo and Srutibox amply provide.

Sir Arthur Sullivan - or rather Adelaide Anne Proctor - appropriately provides a clue in that the Lost Chord is played on an organ, which has the feature of rather stable tuning and lack of modulation - which means a pair of really closely tuned reeds or pipes could actually produce "Lost Chords". In fact, it's hard to imagine any meaningful practical microtonal research without the stability of reed and pipe organ "oscillators". Strings - except for Ellen Fullman's Long String Instrument - have too many unstabilizing influences!

This webpage can be saved an run offline as an always available Lost Chord webapp!
        -- Henry Lowengard