Section 28. What sound is.

What makes a dictaphone or a phonograph repeat your words?

What makes the wind howl when it blows through the branches of trees?

Why can you hear an approaching train better if you put your ear to the rail?

If you were to land on the moon tonight, and had with you a tank containing a supply of air which you could breathe (for there is no air to speak of on the moon), you might try to speak. But you would find that you had lost your voice completely. You could not say a word. You would open and close your mouth and not a sound would come.

Then you might try to make a noise by clapping your hands; but that would not work. You could not make a sound. "Am I deaf and dumb?" you might wonder.

The whole trouble would lie in the fact that the moon has practically no air. And sound is usually a kind of motion of the air,—hundreds of quick, sharp puffs in a second, so close together that we cannot feel them with anything less sensitive than the tiny nerves in our ears.

If you can once realize the fact that sound is a series of quick, sharp puffs of air, or to use a more scientific term, vibrations of air, it will be easy for you to understand most of the laws of sound.

A phonograph seems almost miraculous. Yet it works on an exceedingly simple principle. When you talk, the breath passing out of your throat makes the vocal cords vibrate. These and your tongue and lips make the air in front of you vibrate.

When you talk into a dictaphone horn, the vibrating air causes the needle at the small end of the horn to vibrate so that it traces a wavy line in the soft wax of the cylinder as the cylinder turns. Then when you run the needle over the line again it follows the identical track made when you talked into the horn, and it vibrates back and forth just as at first; this makes the air in the horn vibrate exactly as when you talked into the horn, and you have the same sound.

All this goes back to the fundamental principle that sound is vibrations of air; different kinds of sounds are simply different kinds of vibrations. The next experiments will prove this.

Experiment 54. Turn the rotator rapidly, holding the corner of a piece of stiff paper against the holes in the disk. As you turn faster, does the sound become higher or lower? Keep turning at a steady rate and move your paper from the inner row of holes to the outer row and back again. Which row has the most holes in it? Which makes the highest sound? Hold your paper against the teeth at the edge of the disk. Is the pitch higher or lower than before? Blow through a blowpipe against the different rows of holes while the disk is being whirled. As the holes make the air vibrate do you get any sound?

This experiment shows that by making the air vibrate you get a sound.

The next experiment will show that when you have sound you are getting vibrations.

Experiment 55. Tap a tuning fork against the desk, then hold the prongs lightly against your lips. Can you feel them vibrate? Tap it again, and hold the fork close to your ear. Can you hear the sound?

The experiment which follows will show that we usually must have air to do the vibrating to carry the sound.

Experiment 56. Make a pad of not less than a dozen thicknesses of soft cloth so that you can stand an alarm clock on it on the plate of the air pump. The pad is to keep the vibrations of the alarm from making the plate vibrate. A still better way would be to set a tripod on the plate of the air pump and to suspend the alarm clock from the tripod by a rubber band. Set the alarm so that it will ring in 3 or 4 minutes, put it under the bell jar, and pump out the air. Before the alarm goes off, be sure that the air is almost completely pumped out of the jar. Can you hear the bell ring? Distinguish between a dull trilling sound caused by the jarring of the air pump when the alarm is on, and the actual ringing sound of the bell.

The experiment just completed shows how we know there would be no sound on the moon, since there is practically no air around it. The next experiment will show you more about the way in which phonographs work.

Experiment 58. Put a clean white sheet of paper around the recording drum, pasting the two ends together to hold it in place. Put a small piece of gum camphor on a dish just under the paper, light it, and turn the drum so that all parts will be evenly smoked. Be sure to turn it rapidly enough to keep the paper from being burned.

Melt a piece of glass over a burner and draw it out into a thread. Break off about 8 inches of this glass thread and tie it firmly with cotton thread to the edge of one prong of a tuning fork. Clamp the top of the tuning fork firmly above the smoked drum, adjusting it so that the point of the glass thread rests on the smoked paper. Turn the handle slightly to see if the glass is making a mark. If it is not, adjust it so that it will. Now let some one turn the cylinder quickly and steadily. While it is turning, tap the tuning fork on the prong which has not the glass thread fastened to it. The glass point should trace a white, wavy line through the smoke on the paper. If it does not, keep on trying, adjusting the apparatus until the point makes a wavy line.

Making a record in this way is, on a large scale, almost exactly like the making of a phonograph record. The smoked paper on which a tracing can easily be made as it turns is like the soft wax cylinder. The glass needle is like the recording needle of a phonograph. The chief difference is that you have struck the tuning fork to make it and the needle vibrate, instead of making it vibrate by air waves set in motion by your talking. It is because these vibrations of the tuning fork are more powerful and larger than are those of the recording needle of a phonograph that you can see the record on the recording drum, while you cannot see it clearly on the phonograph cylinder.

In all ordinary circumstances, sound is the vibration of air. But in swimming we can hear with our ears under water, and fishes hear without any air. So, to be accurate, we should say that sound is vibrations of any kind of matter. And the vibrations travel better in most other kinds of matter than they do in air. Vibrations move rather slowly in air, compared with the speed at which they travel in other substances. It takes sound about 5 seconds to go a mile in air; in other words, it would go 12 miles while an express train went one. But it travels faster in water and still faster in anything hard like steel. That is why you can hear the noise of an approaching train better if you put your ear to the rail.

Why we see steam rise before we hear a whistle blow. But even through steel, sound does not travel with anything like the speed of light. In the time that it takes sound to go a mile, light goes hundreds of thousands of miles, easily coming all the way from the moon to the earth. That is why we see the steam rise from the whistle of a train or a boat before we hear the sound. The sound and the light start together; but the light that shows us the steam is in our eyes almost at the instant when the steam leaves the whistle; the sound lags behind, and we hear it later.

Application 42. Explain why a bell rung in a vacuum makes no noise; why the clicking of two stones under water sounds louder if your head is under water, than the clicking of the two stones in the air sounds if your head is in the air; why you hear a buzzing sound when a bee or a fly comes near you; how a phonograph can reproduce sounds.

Inference Exercise

Explain the following:

251. The paint on woodwork blisters when hot.

252. You can screw a nut on a bolt very much tighter with a wrench than with your fingers.

253. When a pipe is being repaired in the basement of a house, you can hear a scraping noise in the faucets upstairs.

254. Sunsets are unusually red after volcanic eruptions.

255. Thunder shakes a house.

256. Shooting stars are really stones flying through space. When they come near the earth, it pulls them swiftly down through the air. Explain why they glow.

257. At night it is difficult to see out through a closed window of a room in which a lamp is lighted.

258. When there is a breeze you cannot see clear reflections in a lake.

259. Rubbing with coarse sandpaper makes rough wood smooth.

260. A bow is bent backward to make the arrow go forward.

Section 29. Echoes.

When you put a sea shell to your ear, how is it that you hear a roar in the shell?

Why can you sometimes hear an echo and sometimes not?

If it were not for the fact that sound travels rather slowly, we should have no echoes, for the sound would get back to us practically at the instant we made it. An echo is merely a sound, a series of air vibrations, bounced back to us by something at a distance. It takes time for the vibration which we start to reach the wall or cliff that bounces it back, and it takes as much more time for the returning vibration to reach our ears. So you have plenty of time to finish your shout before the sound bounces back again. The next experiment shows pretty well how the waves, or vibrations, of sound are reflected; only in the experiment we use waves of water because they go more slowly and we can watch them.

Experiment 59. Fill the long laboratory sink (or the bathtub at home) half full of water and start a wave from one end. Watch it move along the side of the sink. Notice what happens when it reaches the other end.

Air waves do the same thing; when they strike against a flat surface, they bounce back like a rubber ball. If you are far enough away from a flat wall or cliff, and shout, the sound (the air vibrations you start) is reflected back to you and you hear the echo. But if you are close to the walls, as in an empty room, the sound reverberates; it bounces back and forth from one wall to the other so rapidly that no distinct echo is heard, and there is merely a confusion of sound.

When you drop a pebble in water, the ripples spread in all directions. In the same way, when you make a sound in the open air, the air waves spread in all directions. But when you shout through a megaphone the air waves are all concentrated in one direction and consequently they are much stronger in that direction. However, while the megaphone intensifies sound, the echoing from the sides of the megaphone makes the sound lose some of its distinctness.

Why it is hard to understand a speaker in an empty hall. A speaker can be heard much more easily in a room full of people than in an empty hall. The sound does not reflect well from the soft clothes of the audience and the uneven surfaces of their bodies, just as a rubber ball does not bounce well in sand. So the sound does not reverberate as in an empty hall.

Application 43. Explain why a carpeted room is quieter than one with a bare floor; why you shout through your hands when you want to be heard at a distance.

Inference Exercise

Explain the following:

261. It is harder to walk when you shuffle your feet.

262. The air over a lamp chimney, or over a register in a furnace-heated house, is moving upward rapidly.

263. The shooting of a gun sounds much louder within a room than it does outdoors.

264. A drum makes a loud, clear sound when the tightened head is struck.

265. The pull of the moon causes the ocean tides.

266. Sand is sometimes put in the bottom of vases to keep them from falling over.

267. It is difficult to understand clearly the words of one who is speaking in an almost empty hall.

268. The ridges in a washboard help to clean the clothes that are rubbed over them.

269. One kind of mechanical toy has a heavy lead wheel inside. When you start this to whirling, the toy runs for a long time.

270. If you raise your finger slightly after touching the surface of water, the water comes up with your finger.

Section 30. Pitch.

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:

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.