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639Hz
639 Hz is one steady tone, the same in both ears, with nothing behind it. What makes it odd is where the pitch falls: almost exactly halfway between two notes of standard tuning, in a gap that has no name. On its own it sounds like a clear high hum. Put it under anything tuned to concert pitch and it will fight.
Ready to play
Stack this under rain or a fire, keep the blend, and play it with your screen off. Slo, on iOS, Android and Mac.
A pitch that belongs to no scale
It lands between D sharp 5 and E5, near enough the middle that it beats audibly against either one. Musicians call that interval a quarter-tone. Alone the effect is harmless — you hear a note, just not one that exists on a piano. Alongside a tuned instrument or a tuned pad you hear the wobble instead.
The tone itself is bare. One frequency, no harmonics, nothing that moves across the 32-second loop, and nothing else audible underneath it.
What the evidence shows
This number is usually sold as the frequency of connection and relationships. No systematic review and no clinical guideline addresses solfeggio frequencies, so there is no body of work to summarise here. The set was assembled in the 1970s from a numerological reading of the Book of Numbers, and the pitch that came out of that process happens to land between two notes, which tells you something about how the numbers were chosen.
Hear the older three on the 174 Hz tone, the 285 Hz tone and the 396 Hz tone, or open the 639 Hz and gentle rain blend in the free sound mixer to soften it.
Frequently asked questions
What note is 639 Hz?
None exactly. It sits roughly halfway between D sharp 5 and E5, which is why no name fits it.
Does 639 Hz improve relationships?
Nothing supports a claim of that kind. No systematic review or clinical guideline addresses solfeggio frequencies at all.
Why does it clash with music?
Because it falls between two semitones of standard tuning, so it beats against whichever note it is closest to.
How long is the loop?
Thirty-two seconds, repeating until you stop it or the sleep timer does.