cally
the same time as those which come directly from the speaker. In the
open, there are no reflecting walls or surfaces, and the original
sound has no reenforcement from reflection.

If the reflected waves reach the ear too late to blend with the
original sound, that is, come later than one tenth of a second after
the first impression, an echo is heard. What we call the rolling of
thunder is really the reflection and re-reflection of the original
thunder from cloud and cliff.

Some halls are so large that the reflected sounds cause a confusion of
echoes, but this difficulty can be lessened by hanging draperies,
which break the reflection.

260. Motion does not always produce Sound. While we know that all
sound can be traced to motion, we know equally well that motion does
not always produce sound. The hammock swinging in the breeze does not
give forth a sound; the flag floating in the air does not give forth a
sound unless blown violently by the wind; a card moved slowly through
the air does not produce sound, but if the card is moved rapidly back
and forth, a sound becomes audible.

Motion, in order to produce sound, must be rapid; a ball attached to a
string and moved slowly through the air produces no sound, but the
same ball, whirled rapidly, produces a distinct buzz, which becomes
stronger and stronger the faster the ball is whirled.

261. Noise and Music. When the rapid motions which produce sound are
irregular, we hear noise; when the motions are regular and definite,
we have a musical tone; the rattling of carriage wheels on stones, the
roar of waves, the rustling of leaves are noise, not music. In all
these illustrations we have rapid but irregular motion; no two stones
strike the wheel in exactly the same way, no two waves produce pulses
in the air of exactly the same character, no two leaves rustle in
precisely the same way. The disturbances which reach the ear from
carriage, waves, and leaves are irregular both in time and strength,
and irritate the ear, causing the sensation which we call noise.

The tuning fork is musical. Here we have rapid, regular motion;
vibrations follow each other at perfectly definite intervals, and the
air disturbance produced by one vibration is exactly like the
disturbance produced by a later vibration. The sound waves which reach
the ear are regular in time and kind and strength, and we call the
sensation music.

To produce noise a body must vibrate in such a way as t