The science · Solfeggio
639 Hz, explained
639 Hz is one of the original six solfeggio values, a set first put forward in the 1970s. Popularly linked with connection and relationships, it is a description of what people say about it, not of an effect. In Sonor it is a carrier pitch: it changes the note you hear, not the band.
What the research says
The numbers were derived in the 1970s by Joseph Puleo from a pattern of verse numbers in the Bible's Book of Numbers (Science Feedback, 2022). A popular story says medieval monks sang these pitches. There is no evidence for that, and the values do not fit any musical scale in use. A fact-check found no scientific or clinical evidence that any solfeggio frequency regenerates the body or removes toxins.
A PubMed search for 639 Hz combined with music, sound, tone, solfeggio, healing or acoustic returned no records.
No peer-reviewed study was located that tested a 639 Hz tone for any effect.
How Sonor uses it
639 Hz is a named carrier pitch for the isochronic tone. Choosing it changes the note you hear. The pulse rate, and so the band, stays the same. See What are carrier frequencies? for what a carrier does.
You can hear Sonor's tones on the listen page.