Final answer:
The instrument in question is the cello, a member of the string family with a distinct timbre, and its lowest tone is one octave lower than the viola's. Timbre characterizes the sound quality of an instrument and is described with evocative adjectives rather than measurable units.
Step-by-step explanation:
The subject of the question is an instrument that belongs to the string family, is held between the knees, and has the capability to produce tones that are an octave lower than the viola's lowest tone. The instrument being described is most likely the cello.
When a violin plays middle C, its sound can be easily distinguished from a piano playing the same note due to its unique timbre. Timbre refers to our perception of the subtle differences in the sound, beyond just pitch and loudness, and is described using terms such as dull, brilliant, warm, cold, pure, and rich.
Timbre greatly affects the character and quality of the sound an instrument produces, making it a crucial aspect of its identity.