In barbershop singing, there are no instruments. This is called a cappella singing. There are only human voices creating wonderful four-part harmony!
The lead sings the melody, the tenor harmonizes above, the bass provides the foundation with its low notes, and the baritone fills out the chord by harmonizing below or above the lead.
When all four parts sync perfectly in well-tuned chords, it’s like the internal mechanism of a combination lock clicks into place, creating overtones that “blisters above”, notes that nobody is singing, that makes the sound expand.