“Voice is a flowing breath of air, perceptible to the hearing by contact. It moves in an endless number of circular rounds, like the innumerably increasing circular waves which appear when a stone is thrown into smooth water, and which keep on spreading indefinitely from the centre unless interrupted by narrow limits, or by some obstruction which prevents such waves from reaching their end in due formation. When they are interrupted by obstructions, the first waves, flowing back, break up the formation of those which follow.”

Alternate translation: The voice is a flowing breath, made sensible to the organ of hearing by the movements it produces in the air. It is propagated in infinite numbers of circular zones, exactly as when a stone is thrown into a pool of standing water countless circular undulations are generated therein, which, increasing as they recede from the center, spread out over a great distance, unless the narrowness of the locality or some obstacle prevent their reaching their termination; for the first line or waves, when impeded by obstructions, throw by their backward swell the succeeding circular lines of waves into confusion. Quoted by Ernst Mach, The Science of Mechanics: A Critical and Historical Account of its Development (1893, 1960) Tr. Thomas J. McCormack
Source: De architectura (The Ten Books On Architecture) (~ 15BC), Book V, Chapter IV, Sec. 6