When an impulse was given the motion was carried from coin to coin, and at length the last one in the row flew out. This is the case with sound. The air molecules strike one upon another, and the wave of “sound” reaches the tympanum, and thus the impression is conveyed to the brain. We say we hear—but why we hear, in what manner the movement of certain particles affects our consciousness, we cannot determine.
That the air is absolutely necessary to enable us to hear can readily be proved. The experiment has frequently been made; place a bell under the receiver of an air-pump, and we can hear it ring. But if we exhaust the air the sound will get fainter and fainter. Similarly, as many of us have experienced upon high mountains, sounds are less marked. Sound diminishes in its intensity, just as heat and light do. Sound is reflected and refracted, as are light and radiant heat. We have already shown the effect of reflectors upon heat. Sound is caught and reflected in the same way as light from mirrors, or as the heat waves in the reflectors. We have what we term “sounding boards” in pulpits, and speaking tubes will carry sound for us without loss of power. Echoes are merely reflected sounds.
The velocity of sound is accepted as 1,100 feet in a second, which is far inferior to the velocity of light. Fogs will retard sound, while water will carry it. Those who have ever rowed upon a lake will remember how easily the sound of their voices reached from boat to boat, and Dr. Hutton says that at Chelsea, on the Thames, he heard a person reading from a distance of a hundred and forty feet. Some extraordinary instances could be deduced of the enormous distances sound is said to have travelled. Guns have been heard at eighty miles distant, and the noise of a battle between the English and Dutch, in 1672, was heard even in Wales, a distance of two hundred miles from the scene of action.
Sound always travels with uniform velocity in the air in the same temperature. But sound! What is the cause of it? How does it arise? These questions can now be fully answered with reference to the foregoing observations. Phenomena of vibration render themselves visible by light, heat, and sound, and to arrive at some definite ideas of sound vibrations we may compare them to waves, such as may be produced by throwing a stone into a pond.
There are, so to speak, “standing” waves and “progressive” waves. The former can be produced (for instance) by thrumming a fiddle-string, and when the equilibrium of the cord is disturbed, the position of the equilibrium is passed simultaneously by the string-waves. In water the waves or vibrating points pass the position of equilibrium in succession.
Waves consist of elevations and depressions alternately, and when we obtain two “systems” of waves by throwing two stones into water, we can observe some curious effects. It can be seen how one series of depressions will come in contact with the other series of depressions, and the elevations will likewise unite with the result of longer depressions and elevations respectively; or it may very well be that elevation will meet depression, and then the so-called “interference” of waves will produce points of repose. These points are termed nodes. The waves of the string proceed in the plane of its axis; water waves extend in circles which increase in circumference.
The progression or propagation of sound may be said to begin when some tiny globule of matter expands in the air. The air particles strike one against the other, and so the motion is communicated to the air waves, which in time reach the ear. But the velocity of the sound is not equal in all substances. Air will convey it around our earth at the rate of 765 miles an hour, or 1,090 feet in a second. That is, we may accept such rate as correct at a temperature of 32° Fahr., and at a pressure of thirty inches, and the velocity increases almost exactly one foot per second for each degree of temperature above 32°. Therefore on an average, and speaking in “round numbers,” the estimate of 1,100 feet in a second may be accepted as correct. In hydrogen gas the rate is much higher. Through water again it is different, and still faster through iron, glass, and wood, as will be seen in the following table:—
| TAKING AIR AS 1. | |
|---|---|
| Whalebone | 6⅔ |
| Tin | 7½ |
| Silver | 9 |
| Walnut | 10⅔ |
| Brass | 10⅔ |
| Oak | 10⅔ |
| Earthen pipes | 11 |
| Copper | 12 |
| Pear-wood | 12½ |
| Ebony | 14⅔ |
| Cherry | 15 |
| Willow | 16 |
| Glass | 16⅔ |
| Iron or Steel | 16⅔ |
| Deal | 18 |
So there is a considerable difference in the velocities of sound through the solid substances quoted, but these figures cannot be taken as exact, as different samples may give different results. In wires and bells the bodies themselves produce the sounds we hear. In wind instruments and the voice the air is the cause of the sound.
The very deepest notes are produced by the fewest vibrations. Fourteen or fifteen vibrations will give us a very low note, if not the very lowest. The pipe of sixteen feet, closed at its upper end, will produce sound waves of thirty-two feet. High notes can be formed from vibrations up to 48,000 in a second. Beyond these limits the ear cannot accept a musical sound.
Fig. 172.—The vibration of strings.
We will explain the phenomenon of