The Short Version
No credible peer-reviewed evidence supports the claim that regular humming permanently deepens the human voice. High-quality biomedical sources show humming improves vocal resonance, coordination, and quality — not baseline pitch. One controlled study found no significant effect on low-pitch frequency, and a humming-based training study actually found a slight pitch increase. Claims of deepening rely on conflating pitch range expansion with habitual pitch lowering, or misinterpreting temporary post-exercise effects as lasting change.