Physics Part 54

Induced currents; 3 laws, ill.u.s.trations.

Construction, action, and uses of--magneto, dynamo, induction coil, transformer, motor, telephone. Mercury arc rectifier.

Terms--primary, secondary, for coils and currents, armature, commutator, slip ring, brush, rectifier, open core, series, shunt, and compound connections for dynamos.

CHAPTER XV

SOUND

(1) SOUND AND WAVE MOTION

=317. What is a Sound?=--This question has two answers, which may be ill.u.s.trated as follows: Suppose that an alarm clock is set so that it will strike in one week and that it is placed upon a barren rock in the Pacific Ocean by sailors who immediately sail away. If when the tapper strikes the bell at the end of the week no ear is within a hundred miles, is any sound produced? The two view-points are now made evident, for some will answer "no" others "yes." Those answering "no" hold that sound is a _sensation_ which would not be produced if no ear were at hand to be affected. Those answering "yes" understand, by the term sound, _a mode of motion capable of affecting the auditory nerves_, and that sound exists wherever such motions are present. This latter point of view is called the _physical_ and is the one we are to use in this study.

[Ill.u.s.tration: FIG. 314.--The tuning fork is vibrating.]

=318. Source of Sound.=--If we trace any sound to its source, it will be found to originate in a body in rapid motion usually in what is called a state of _vibration_. To ill.u.s.trate, take a tuning fork, strike it to set it in vibration and place its stem firmly against a thin piece of wood; the sound will be strengthened materially by the vibration of the wood. If now the vibrating fork is placed with the tips of the p.r.o.ngs in water, the vibration is plainly shown by the spattering of the water (Fig. 314). When one _speaks_, the vibrating body is in the _larynx_ at the top of the windpipe. Its vibration may be plainly felt by the hand placed upon the throat while speaking.

=319. Sound Media.=--Usually sounds reach the ear through the air. The air is then said to be a _medium for sound_. Other substances may serve as a sound medium, for if the head is under water and two stones, also under water, are struck together a sharp sound is heard. Also if one end of a wooden rod is held at the ear and the other end of the rod is scratched by a pin, the sound is more plainly perceived through the wood than through the air. Think of some ill.u.s.tration from your own experience of a solid acting as medium for sound. If an electric bell is placed in a bell jar attached to an air pump, as in Fig. 315, on exhausting the air the loudness of the sound is found to diminish, indicating that in a perfect vacuum no sound would be transmitted. This effect of a vacuum upon the transmission of sound is very different from its effect upon radiation of heat and light. Both heat and light are known to pa.s.s through a vacuum since both come to the earth from the sun through s.p.a.ce that so far as we know contains no air or other matter.

Sound differs from this in that it is always transmitted by some material body and cannot exist in a vacuum.

[Ill.u.s.tration: FIG. 315.--Sound does not travel in a vacuum.]

=320. Speed of Sound.=--Everyone has noticed that it takes time for sound to travel from one place to another. If we see a gun fired at a distance, the report is heard a few seconds after the smoke or flash is seen. The time elapsing between a flash of lightning and the thunder shows that sound takes time to move from one place to another. Careful experiments to determine the speed of sound have been made. One method measures accurately the time required for the sound of a gun to pa.s.s between two stations several miles apart. A gun or cannon is placed at each station. These are fired alternately, first the one at one station and then the one at the other so as to avoid an error in computation due to the motion of wind. This mode of determining the speed of sound is not accurate. Other methods, more refined than the one just described have given accurate values for the speed of sound. The results of a number of experiments show that, at the freezing temperature, 0C., the speed of sound in air is 332 meters or 1090 ft. a second. The speed of sound in air is affected by the temperature, increasing 2 ft. or 0.6 meter per second for each degree that the temperature rises above 0C.

The speed decreases the same amount for each degree C. that the air is cooled below the freezing point. The speed of sound in various substances has been carefully determined. It is greater in most of them than in air. In water the speed is about 1400 meters a second; in wood, while its speed varies with different kinds, it averages about 4000 meters a second; in bra.s.s the speed is about 3500 meters; while in iron it is about 5100 meters a second.

=321. The Nature of Sound.=--We have observed that sound originates at a vibrating body, that it requires a medium in order to be transmitted from one place to another, and that it travels at a definite speed in a given substance. Nothing has been said, however, of the _mode_ of transmission, or of the _nature_ of _sound_. Sounds continue to come from an alarm clock even though it is placed under a bell jar. It is certain that nothing material can pa.s.s through the gla.s.s of the jar.

If, however, we consider that _sound is transmitted by waves through substances_ the whole matter can be given a simple explanation. In order to better understand the nature of sound a study of waves and wave motion will be taken up in the next section.

Important Topics

Sound: two definitions, source, medium, speed, nature.

Exercises

1. Give two ill.u.s.trations from outside the laboratory of the fact that sound is transmitted by other materials than air.

2. Name the vibrating part that is the source of the sound in three different musical instruments.

3. Is sound transmitted more strongly in solids, liquids or gases? How do you explain this?

4. How far away is a steamboat if the sound of its whistle is heard 10 seconds after the steam is seen, the temperature being 20C.? Compute in feet and in meters.

5. How many miles away is lightning if the thunder is heard 12 seconds after the flash in seen, the temperature being 25C.?

6. Four seconds after a flash of lightning is seen the thunder clap is heard. The temperature is 90F. How far away was the discharge?

7. The report of a gun is heard 3 seconds after the puff of smoke is seen. How far away is the gun if the temperature is 20C.?

8. An explosion takes place 10 miles away. How long will it take the sound to reach you, the temperature being 80F?. How long at 0F.?

9. How long after a whistle is sounded will it be heard if the distance away is 1/4 mile, the temperature being 90F.?

10. The report of an explosion of dynamite is heard 2 minutes after the puff of smoke is seen. How far away is the explosion the temperature being 77F.?

(2) WAVES[N] AND WAVE MOTION

[N] A wave is a disturbance in a substance or medium that is transmitted through it.

=322. Visible Waves.=--It is best to begin the study of wave motion by considering some waves which are familiar to most persons. Take for example the waves that move over the surface of water (Fig. 316). These have an onward motion, yet boards or chips upon the surface simply rise and fall as the waves pa.s.s them. They are not carried onward by the waves. The water surface simply rises and falls as the waves pa.s.s by.

Consider also the waves that may be seen to move across a field of tall gra.s.s or grain. Such waves are produced by the bending and rising of the stalks as the wind pa.s.ses over them. Again, waves may be produced in a rope fastened at one end, by suddenly moving the other end up and down.

These waves move to the end of the rope where they are _reflected_ and return. The three types of waves just mentioned are ill.u.s.trations of _transverse_ waves, the ideal case being that in which the particles move at _right angles_ to the path or course of the wave. Such waves are therefore called _transverse_ waves.

[Ill.u.s.tration: FIG. 316.--Water waves.]

=323. Longitudinal waves.=--Another kind of wave is found in bodies that are elastic and compressible and have inertia, such as gases and coiled wire springs. Such waves may be studied by considering a wire spring as the medium through which the waves pa.s.s. (See Fig. 318.)

[Ill.u.s.tration: FIG. 317.--The compression wave travels through the spring.]

If the end of the wire spring shown in Fig. 317 is struck the first few turns of the spring will be compressed. Since the spring possesses elasticity, the turns will move forward a little and compress those ahead, these will press the next in turn and so on. Thus a _compression_ wave will move to the end of the spring, where it will be reflected and return. Consider the turns of the spring as they move toward the end.

On account of their _inertia_ they will continue moving until they have separated from each other _more_ than at first, before returning to their usual position. This condition of a greater separation of the turns of the spring than usual is called a _rarefaction_. It moves along the spring following the wave of compression. The condensation and rarefaction are considered as together forming a complete wave. Since the turns of wire move back and forth in a direction parallel to that in which the wave is traveling, these waves are called _longitudinal_.

[Ill.u.s.tration: FIG. 318.--Longitudinal waves (1) in a spring, (2) in air, and (3) graphic representation showing wave length, condensations, and rarefactions.]

=324. The transmission of a sound by the air= may be understood by comparing it with the process by which a _wave is transmitted by a wire spring_. Consider a light spring (Fig. 318, 1) attached at the end of a vibrating tuning fork, _K_, and also to a diaphragm, _D_. Each vibration of the fork will first compress and then separate the coils of the spring. These impulses will be transmitted by the spring as described in Art. 315, and cause the diaphragm to vibrate _at the same rate_ as the tuning fork. The diaphragm will then give out a sound similar to that of the tuning fork. Suppose that the spring is replaced by air, and the diaphragm, by the ear of a person, _E_, (Fig. 318, 2.) when the p.r.o.ng of the fork moves toward the ear it starts a compression and when it moves back a rarefaction. The fork continues vibrating and these impulses move onward like those in the spring at a speed of about 1120 ft. in a second. They strike the diaphragm of the ear causing it to move back and forth or to vibrate at the same rate as the tuning fork, just as in the case of the diaphragm attached to the spring.

=325. Graphic Representation of Sound waves.=--It is frequently desirable to represent sound waves graphically. The usual method is to use a curve like that in (Fig. 318, 3). This curve is considered as representing a train of waves moving in the same direction as those in Fig. 318 1 and 2, and also having the same length. The part of the wave _A-B_ represents a condensation of the sound wave and the part _B-C_ represents a rarefaction. A complete wave consisting of a condensation and a rarefaction is represented by that portion of the curve _A-C_. The portion of the curve _B-D_ also represents a _full wave length_ as the latter is defined as _the distance between two corresponding parts of the adjacent waves_. The curve, Fig. (318, 3) represents not only the wave length, but also the height of the wave or the amount of movement of the particles along the wave. This is called the _amplitude_ and is indicated by the distance _A-b_. Since the _loudness_ or intensity of a sound is found to depend upon the amount of movement of the particles along the wave, the _amplitude_ of the curve is used to indicate the loudness of the sound represented. All of the characteristics of a sound wave may be graphically represented by curves. Such curves will be used frequently as an aid in explaining the phenomena of wave motion both in sound and in light.

=326. Reflections of Sound.=--It is found that a wave moving along a wire spring is reflected when it reaches the end and returns along the spring. Similarly a sound wave in air is reflected upon striking the surface of a body. If the wave strikes perpendicularly it returns along the line from which it comes, if, however, it strikes at some other angle it does not return along the same line, but as in other cases of reflected motion, the _direction_ of the _reflected_ wave is described by the _Law of Reflected Motion_ as follows: _The angle of reflection is always equal to the angle of incidence_. This law is ill.u.s.trated in Fig.

319. Suppose that a series of waves coming from a source of sound move from _H_ to _O_. After striking the surface _IJ_ the waves are reflected and move toward _L_ along the line _OL_. Let _PO_ be perpendicular to the surface _IJ_ at _O_. Then _HOP_ is _the angle_ of incidence and _LOP_ is the _angle of reflection_. By the law of reflected motion these angles are equal. In an ordinary room when a person speaks the sound waves reflected from the smooth walls reinforce the sound waves moving directly to the hearers. It is for this reason that it is usually easier to speak in at room than in the open air. Other ill.u.s.trations of the reinforcement of sound by reflection are often seen. Thus an _ear trumpet_ (Fig. 320), uses the principle of reflection and concentration of sound. So-called _sounding boards_ are sometimes placed back of speakers in large halls to reflect sound waves to the audience.

[Ill.u.s.tration: FIG. 319.--Law of reflection.]

[Ill.u.s.tration: FIG. 320.--An ear trumpet.]

=327. Echoes.=--_An echo is the repet.i.tion of a sound caused by its reflection from some distant surface_ such as that of a building, cliff, clouds, trees, etc. The interval of time between the production of a sound and the perception of its echo is the time that the sound takes to travel from its source to the reflecting body and back to the listener.

Experiments have shown that the sensation of a sound persists about one-tenth of a second. Since the velocity of sound at 20C. is about 1130 ft. per second, during one-tenth of a second the sound wave will travel some 113 ft. If the reflecting surface is about 56 ft. distant a _short_ sound will be followed immediately by its echo as it is heard one-tenth of a second after the original sound. The reflected sound tends to strengthen the original one if the reflecting surface is less than 56 ft. away. If the distance of the reflecting surface is much more than 56 ft. however, the reflected sound does not blend with the original one but forms a distinct echo. The echoes in large halls especially those with large smooth walls may very seriously affect the clear perception of the sound. Such rooms are said to have poor _acoustic_ properties. Furniture, drapery, and carpets help to deaden the echo because of diffused reflection. The Mormon Tabernacle at Salt Lake City, Utah, is a fine example of a building in which the reflecting surfaces of the walls and ceiling are of such shape and material that its acoustic properties are remarkable, a pin dropped at one end being plainly heard at the other end about 200 ft. away.

Important Topics

1. Waves: transverse, longitudinal; wave length, condensation, rarefaction.