Echo and the rebound of sound at equal angles
How a voice striking a wall returns to the ear like a ball bounced off it
Leonardo works out that sound behaves like a thrown ball: a voice striking a wall rebounds to the ear at an angle equal to the angle of its approach. A diagram of wall f g shows ball b returning along b-c when it strikes perpendicularly, and rebounding along d-e when it strikes obliquely between the equal angles m and n. Further sketches treat multiple echoes-one shout at n answered by the four corners a, b, c, d-and the confusion of shout and echo when a listener stands close to the reflecting corner. He also notes that the distance of a thunderclap can be judged by ear once its flash has been seen.
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Sound rebounds from a wall at an angle equal to its approach
Any blow striking an object rebounds at the same angle as the percussion. Ball b thrown to c returns along c-b, while a ball thrown along b-d rebounds along d-e, since it must leave equal angles on wall f g. A voice behaves the same way and seems to return to the ear along the reflected line.
One voice answered fourfold from four corners
The voice made at n strikes the corners a, b, c and d, and each corner sends back its own return, so a single shout is answered four times over. The voice is whole at every part of the surface it strikes.
When shout and echo blur together
If someone shouts from m while a listener stands at n close to the reflecting corner r, the echo follows so hard upon the shout that the two merge and cannot be told apart. The ear receives voices by straight, curved or broken lines, and no twisting stops its office.
Judging a thunderclap's distance by its echo
Once the flash of a thunderclap has been seen, its distance can be recognized by ear, from the likeness of its sound in the echo.