What makes the keys of a piano give different sounds?
Why does the moving of your fingers up and down on a violin
string make it play different notes?
Why is the whistle of a peanut roaster so shrill, and why is
the whistle of a boat so deep?
Did you ever notice how tiresome the whistle on a peanut roaster gets?
Well, suppose that whenever you spoke you had to utter your words in
exactly that pitch; that every time a car came down the street its
noise was like the whistle of the peanut roaster, only louder; that
every step you took sounded like hitting a bell of the same pitch;
that when you went to the moving-picture theater the orchestra played
only the one note; that when any one sang, his voice did not rise and
fall; in short, that all the sounds in the world were in one pitch.
That is the way it would be if different kinds of air vibrations did
not make different kinds of notes,--if there were no differences in
pitch.
PITCH DUE TO RAPIDITY OF VIBRATION. When air vibrations are slow,--far
apart,--the sound is low; when they are faster, the sound is higher;
when they are very quick indeed, the sound is very shrill and high. In
various ways, as by people talking and walking and by the running of
street cars and automobiles, all sorts of different vibrations are
started, giving us a pleasant variety of high and low and medium
pitches in the sounds of the world around us.
An experiment will show how pitch varies and how it is regulated:
EXPERIMENT 60. Move the slide of an adjustable tuning fork
well up from the end of the prongs, tap one prong lightly on
the desk, and listen. Move the slide somewhat toward the
end of the prongs, and repeat. Is a higher or a lower sound
produced as the slide shortens the length of the prongs?
Whistle a low note, then a high one. Notice what you do with
your lips; when is the opening the smaller? Sing a low note,
then a high one. When are the cords in your throat looser?
Fill a drinking glass half full of water, and strike it. Now
pour half the water out, and strike the glass again. Do you
get the higher sound when the column of water is shorter or
when it is longer? Stretch a rubber band across your thumb and
forefinger. Pick the band as you make it tighter, not making
it longer, but pulling it tighter with your other fingers.
Does it make a higher or a lower sound as you increase the
tightness? Stretch the band from your thumb to your little
finger and pick it; now put your middle finger under the band
so as to divide it in halves, and pick it again. Does a short
strand give a higher or lower pitch than a long strand?
[Illustration: FIG. 103. When the prongs of the tuning fork are made
longer or shorter, the pitch of the sound is changed.]
A violinist tunes his violin by tightening the strings; the tighter
they are and the thinner they are, the higher the note they give.
Some of the strings are naturally higher than others; the highest is
a smaller, finer string than the lowest. When the violinist plays, he
shortens the strings by holding them down with his fingers, and the
shorter he makes them the higher the note. A bass drum is much larger
than a high-pitched kettledrum. The pipes of an organ are long and
large for the low notes, shorter and smaller for the high ones.
In general, the longer or larger the object is that vibrates, the
slower the rate of vibration and consequently the lower the pitch. But
the shorter or finer the object is that vibrates, the higher the rate
of vibration and the higher the pitch.
All musical instruments contain devices which can be made to
vibrate,--either strings or columns of air, or other things that swing
to and fro and start waves in the air. And by tightening them, or
making them smaller or shorter, the pitch can be made higher; that is,
the number of vibrations to each second can be increased.
_APPLICATION 44._ Explain why a steamboat whistle is usually
of much lower pitch than is a toy whistle; why a banjo player
moves his fingers toward the drum end of the banjo when he
plays high notes; why the sound made by a mosquito is higher
in pitch than that made by a bumblebee.
_APPLICATION 45._ A boy had a banjo given him for Christmas.
He wanted to tune it. To make a string give a higher note,
should he have tightened or loosened it? Or could he have
secured the same result by moving his finger up and down the
string to lengthen or shorten it?
_APPLICATION 46._ A man was tuning a piano for a concert. The
hall was cold, yet he knew it would be warm at the time of the
concert. Should he have tuned the piano to a higher pitch than
he wanted it to have on the concert night, to the exact pitch,
or to a lower pitch?
INFERENCE EXERCISE
Explain the following:
271. A cowboy whirls his lasso around and around his head
before he throws it.
272. Furnaces are always placed in the basements of buildings,
never on top floors.
273. A rather slight contraction of a muscle lifts your arm a
considerable distance.
274. A player on a slide trombone changes the pitch of the
notes by lengthening and shortening the tube while he blows
through it.
275. Rain runs off a tar roof in droplets, while on shingles
it soaks in somewhat and spreads.
276. There is a sighing sound as the wind blows through the
branches of trees, or through stretched wires or ropes.
277. Sometimes a very violent noise breaks the membrane in the
drum of a person's ear.
278. As a street car goes faster and faster, the hum of its
motor is higher and higher.
279. If a street is partly dry, the wet spots shine more than
the dry spots do.
280. Molten type metal, when poured into a mold, becomes hard,
solid type when it cools.
CHAPTER SEVEN
MAGNETISM AND ELECTRICITY