The patient attempts to produce sound at their lowest pitch, at any volume, low or high. This defines the vocal floor of their voice. Sometimes the person has excellent vocal rapport, capable of matching their voice to notes played on a piano. Some people are not so talented and the examiner may ask them to slide down in pitch and then by ear try to determine the lowest note produced.
I often start with the note they were speaking on and have them make the tone for a few seconds. I then play a tone, perhaps 3 notes lower and see if they can match it. As they have more difficulty, I step down a single semi-tone at a time. Typically the volume also diminishs as one moves toward their vocal floor, the lowest note they can hit.
Moving lower in pitch removes any compensation from the cricothyroid muscle. For example, increased air leak at lower pitches represents a gap, which could be from bowing or paresis which is often masked by cricothyroid tension at higher pitches.
I can diagram this low range in a normal individual.
This female can drop her voice to a pitch of D3. She does get softer at her lowest notes although I have diagrammed all of her low tange as soft.