639 Hz is the fourth note of the original Solfeggio scale, corresponding to the syllable "Fa." It is called the "Connecting Frequency" because its primary association is with relationships — the bonds between people, the quality of communication, and the capacity for empathy and understanding.

Sound therapists use 639 Hz when working with relationship difficulties, communication breakdowns, and emotional disconnection. The frequency is believed to promote tolerance, encourage love and understanding, and help repair fractured bonds — whether between partners, family members, friends, or colleagues.

Linked to the heart chakra (Anahata), 639 Hz is associated with the capacity to give and receive love freely. Research into this frequency's effects is limited, but its use in sound therapy consistently centres on emotional balance and the harmonisation of interpersonal dynamics. Many practitioners consider it essential for anyone working through conflict, loneliness, or the aftermath of a difficult separation.