SOUND
249. The Senses. All the information which we possess of the world
around us comes to us through the use of the senses of sight, hearing,
taste, touch, and smell. Of the five senses, sight and hearing are
generally considered the most valuable. In preceding Chapters we
studied the important facts relative to light and the power of vision;
it remains for us to study Sound as we studied Light, and to learn
what we can of sound and the power to hear.
250. How Sound is Produced. If one investigates the source of any
sound, he will always find that it is due to motion of some kind. A
sudden noise is traced to the fall of an object, or to an explosion,
or to a collision; in fact, is due to the motion of matter. A piano
gives out sound whenever a player strikes the keys and sets in motion
the various wires within the piano; speech and song are caused by the
motion of chest, vocal cords, and lips.
[Illustration: FIG. 164.--Sprays of water show that the fork is in
motion.]
If a large dinner bell is rung, its motion or vibration may be felt on
touching it with the finger. If a tuning fork is made to give forth
sound by striking it against the knee, or hitting it with a rubber
hammer, and is then touched to the surface of water, small sprays of
water will be thrown out, showing that the prongs of the fork are in
rapid motion. (A rubber hammer is made by putting a piece of glass
tubing through a rubber cork.)
If a light cork ball on the end of a thread is brought in contact with
a sounding fork, the ball does not remain at rest, but vibrates back
and forth, being driven by the moving prongs.
[Illustration: FIG. 165.--The ball does not remain at rest]
These simple facts lead us to conclude that all sound is due to the
motion of matter, and that a sounding body of any kind is in rapid
motion.
251. Sound is carried by Matter. In most cases sound reaches the ear
through the air; but air is not the only medium through which sound is
carried. A loud noise will startle fish, and cause them to dart away,
so we conclude that the sound must have reached them through the
water. An Indian puts his ear to the ground in order to detect distant
footsteps, because sounds too faint to be heard through the air are
comparatively clear when transmitted through the earth. A gentle
tapping at one end of a long table can be distinctly heard at the
opposite end if the ear is pressed against the table; if the ear is
removed from the wood, the sound of tapping is much fainter, showing
that wood transmits sound more readily than air. We see therefore that
sound can be transmitted to the ear by solids, liquids, or gases.
Matter of any kind can transmit sound to the ear. The following
experiments will show that matter is necessary for transmission.
Attach a small toy bell to a glass rod (Fig. 166) by means of a rubber
tube and pass the rod through one of two openings in a rubber cork.
Insert the cork in a strong flask containing a small quantity of water
and shake the bell, noting the sound produced. Then heat the flask,
allowing the water to boil briskly, and after the boiling has
continued for a few minutes remove the flame and instantly close up
the second opening by inserting a glass stopper. Now shake the flask
and note that the sound is very much fainter than at first. As the
flask was warmed, air was rapidly expelled; so that when the flask was
shaken the second time, less air was present to transmit the sound. If
the glass stopper is removed and the air is allowed to reenter the
flask, the loudness of the sound immediately increases.
[Illustration: FIG. 166.--Sound is carried by the air.]
Since the sound of the bell grows fainter as air is removed, we infer
that there would be no sound if all the air were removed from the
flask; that is to say, sound cannot be transmitted through empty space
or a vacuum. If sound is to reach our ears, it must be through the
agency of matter, such as wood, water, or air, etc.
252. How Sound is transmitted through Air. We saw in Section 250
that sound can always be traced to the motion or vibration of matter.
It is impossible to conceive of an object being set into sudden and
continued motion without disturbing the air immediately surrounding
it. A sounding body always disturbs and throws into vibration the air
around it, and the air particles which receive motion from a sounding
body transmit their motion to neighboring particles, these in turn to
the next adjacent particles, and so on until the motion has traveled
to very great distances. The manner in which vibratory motion is
transmitted by the atmosphere must be unusual in character, since no
motion of the air is apparent, and since in the stillness of night
when "not a breath of air" is stirring, the shriek of a railroad
whistle miles distant may be heard with perfect clearness. Moreover,
the most delicate notes of a violin can be heard in the remotest
corners of a concert hall, when not the slightest motion of the air
can be seen or felt.
In our study of the atmosphere we saw that air can be compressed and
rarefied; in other words, we saw that air is very elastic. It can be
shown experimentally that whenever an elastic body in motion comes in
contact with a body at rest, the moving body transfers its motion to
the second body and then comes to rest itself. Let two billiard balls
be suspended in the manner indicated in Figure 167. If one of the
balls is drawn aside and is then allowed to fall against the other,
the second ball is driven outward to practically the height from which
the first ball fell and the first ball comes to rest.
[Illustration: FIG. 167.--Elastic balls.]
[Illustration: FIG. 168.--Suspended billiard balls.]
If a number of balls are arranged in line as in Figure 168 or Figure
169, and the end ball is raised and then allowed to fall, or if _A_ is
pushed against _C_, the last ball _B_ will move outward alone, with a
force nearly equal to that originally possessed by _A_ and to a
distance nearly equal to that through which _A_ moved. But there will
be no _visible_ motion of the intervening balls. The force of the
moving ball _A_ is given to the second ball, and the second ball in
turn gives the motion to the third, and so on throughout the entire
number, until _B_ is reached. But _B_ has no ball to give its motion
to, hence _B_ itself moves outward, and moves with a force nearly
equal to that originally imparted by _A_ and to a distance nearly
equal to that through which _A_ fell. Motion at _A_ is transmitted to
_B_ without any perceptible motion of the balls lying between these
points. Similarly the particles of air set into motion by a sounding
body impart their motion to each other, the motion being transmitted
onward without any perceptible motion of the air itself. When this
motion reaches the ear, it sets the drum of the ear into vibration,
and these vibrations are in turn transmitted to the auditory nerves,
which interpret the motion as sound.
[Illustration: FIG. 169.--Elastic balls transmit motion.]
[Illustration: FIG. 170.--When a ball meets more than one ball, it
divides its motion.]
253. Why Sound dies away with Distance. Since the last ball _B_ is
driven outward with a force nearly equal to that possessed by _A_, it
would seem that the effect on the ear drum should be independent of
distance and that a sound should be heard as distinctly when remote as
when near. But we know from experience that this is not true, because
the more distant the source of sound, the fainter the impression; and
finally, if the distance between the source of sound and the hearer
becomes too great, the sound disappears entirely and nothing is heard.
The explanation of this well-known fact is found in a further study of
the elastic balls (Fig. 170). If _A_ hits two balls instead of one,
the energy possessed by _A_ is given in part to one ball, and in part
to the other, so that neither obtains the full amount. These balls,
having each received less than the original energy, have less to
transmit; each of these balls in turn meets with others, and hence the
motion becomes more and more distributed, and distant balls receive
less and less impetus. The energy finally given becomes too slight to
affect neighboring balls, and the system comes to rest. This is what
occurs in the atmosphere; a moving air particle meets not one but many
adjacent air particles, and each of these receives a portion of the
original energy and transmits a portion. When the original disturbance
becomes scattered over a large number of air particles, the energy
given to any one air particle becomes correspondingly small, and
finally the energy becomes so small that further particles are not
affected; beyond this limit the sound cannot be heard.
If an air particle transmitted motion only to those air particles
directly in line with it, we should not be able to detect sound unless
the ear were in direct line with the source. The fact that an air
particle divides its motion among all particles which it touches, that
is, among those on the sides as well as those in front, makes it
possible to hear sound in all directions. A good speaker is heard not
only by those directly in front of him, but by those on the side, and
even behind him.
254. Velocity of Sound. The transmission of motion from particle to
particle does not occur instantaneously, but requires time. If the
distance is short, so that few air particles are involved, the time
required for transmission is very brief, and the sound is heard at
practically the instant it is made. Ordinarily we are not conscious
that it requires time for sound to travel from its source to our ears,
because the distance involved is too short. At other times we
recognize that there is a delay; for example, thunder reaches our ears
after the lightning which caused the thunder has completely
disappeared. If the storm is near, the interval of time between the
lightning and the thunder is brief, because the sound does not have
far to travel; if the storm is distant, the interval is much longer,
corresponding to the greater distance through which the sound travels.
Sound does not move instantaneously, but requires time for its
transmission. The report of a distant cannon is heard after the flash
and smoke are seen; the report of a near cannon is heard the instant
the flash is seen.
The speed with which sounds travels through the air, or its velocity,
was first measured by noting the interval (54.6 seconds) which elapsed
between the flash of a cannon and the sound of the report. The
distance of the cannon from the observer was measured and found to be
61,045 feet, and by dividing this distance by the number of seconds,
we find that the distance traveled by sound in one second is
approximately 1118 feet.
High notes and low notes, soft notes and shrill notes, all travel at
the same rate. If bass notes traveled faster or slower than soprano
notes, or if the delicate tones of the violin traveled faster or
slower than the tones of a drum, music would be practically
impossible, because at a distance from the source of sound the various
tones which should be in unison would be out of time--some arriving
late, some early.
255. Sound Waves. Practically everyone knows that a hammock hung
with long ropes swings or vibrates more slowly than one hung with
short ropes, and that a stone suspended by a long string swings more
slowly than one suspended by a short string. No two rocking chairs
vibrate in the same way unless they are exactly alike in shape, size,
and material. An object when disturbed vibrates in a manner peculiar
to itself, the vibration being slow, as in the case of the long-roped
swing, or quick, as in the case of the short-roped swing. The time
required for a single swing or vibration is called the _period_ of the
body, and everything that can vibrate has a characteristic period.
Size and shape determine to a large degree the period of a body; for
example, a short, thick tuning fork vibrates more rapidly than a tall
slender fork.
[Illustration: FIG. 171.--The two hammocks swing differently.]
Some tuning forks when struck vibrate so rapidly that the prongs move
back and forth more than 5000 times per second, while other tuning
forks vibrate so slowly that the vibrations do not exceed 50 per
second. In either case the distance through which the prongs move is
very small and the period is very short, so that the eye can seldom
detect the movement itself. That the prongs are in motion, however, is
seen by the action of a pith ball when brought in contact with the
prongs (see Section 250).
[Illustration: FIG. 172.--The pitch given out by a fork depends upon
its shape.]
The disturbance created by a vibrating body is called a wave.
256. Waves. While the disturbance which travels out from a sounding
body is commonly called a wave, it is by no means like the type of
wave best known to us, namely, the water wave.
If a closely coiled heavy wire is suspended as in Figure 173 and the
weight is drawn down and then released, the coil will assume the
appearance shown; there is clearly an overcrowding or condensation in
some places, and a spreading out or rarefaction in other places. The
pulse of condensation and rarefaction which travels the length of the
wire is called a wave, although it bears little or no resemblance to
the familiar water wave. Sound waves are similar to the waves formed
in the stretched coil.
[Illustration: FIG. 173.--Waves in a coiled wire.]
Sound waves may be said to consist of a series of condensations and
rarefactions, and the distance between two consecutive condensations
and rarefactions may be defined as the wave length.
257. How One Sounding Body produces Sound in Another Body. In