Experience shows that the human voice, under favourable circumstances,
is capable of filling a larger space than was ever probably enclosed
within the walls of a single room. Lieutenant Foster, on Parry’s third
Arctic expedition, found that he could converse with a man across the
harbour of Port Bowen, a distance of 6696 feet, or about one mile and a
quarter. Dr. Young records that at Gibraltar the human voice has been
heard at a distance of ten miles. If sound be prevented from spreading
and losing itself in the air, either by a pipe or an extensive flat
surface, as a wall or still water, it may be conveyed to a great
distance. Biot heard a flute clearly through a tube of cast-iron (the
water-pipes of Paris) 3120 feet long: the lowest whisper was distinctly
heard; indeed, the only way not to be heard was not to speak at all.