A practical pitch scale
Below 85 Hz is exceptionally low for ordinary conversational speech. Around 85–100 Hz is generally deep, while 100–120 Hz can sound moderately deep depending on resonance.
These are descriptive bands, not biological or medical boundaries.
Is 85 or 95 Hz deep?
A stable 85 Hz speaking median is very low. A 95 Hz median is also low compared with common adult male averages. The key word is median: the lowest note you can briefly produce is not your habitual speaking pitch.
Why resonance matters
A longer effective vocal tract tends to produce lower formants and a darker sound. Two speakers can both measure 100 Hz while one sounds much deeper because their resonance patterns and vocal weight differ.