Institutio Oratoria
Quintilian
Quintilian. Institutio Oratoria, Volume 1-4. Butler, Harold Edgeworth, translator. Cambridge, Mass; London: Harvard University Press, William Heinemann Ltd., 1920-1922.
For the voice is like the strings of a musical instrument; the slacker it is the deeper and fuller the note produced, whereas if it be tightened, the sound becomes thinner and shriller. Consequently, the deepest notes lack force, and the higher run the risk of cracking the voice. The orator will, therefore, employ the intermediate notes, which must be raised when we speak with energy and lowered when we adopt a more subdued tone.
For the first essential of a good delivery is evenness. The voice must not run joltingly, with irregularity of rhythm and sound, mixing long and short syllables, grave accents and acute, tones loud and low, without discrimination, the result being that this universal unevenness produces the impression of