Sound is also made more intense if a sonorous body is near its source. This is taken advantage of in musical instruments, where a sounding-board is used, as in the case of the piano, and in the violin, which has a thin shell as a body for holding the strings.
Another curious thing is shown in the speaking tube, where the sound waves are confined, so that they are carried along in one line, and as they are not interfered with will transmit the vibrations to great distances.
Velocity of Sound.—The temperature of the air has also an effect on the rate of transmission, but for general purposes a temperature of 62 degrees has been taken as the standard. The movement is shown to be about 50 miles in 4 minutes, or at the rate of 1,120 feet per second.
In water, however, the speed is four times greater; and in iron nearly fifteen times greater. Soft earth is a poor conductor, while rock and solid earth convey very readily. Placing the ear on a railway track will give the vibrations of a moving train miles before it can be heard through the air.
Sound Reflections.—Sound waves move outwardly from the object in the form of wave-like[p. 89] rings, but those concentric rings, as they are called, may be interrupted at various points by obstacles. When that is the case the sound is buffeted back, producing what is called echoes.
Resonance.—Materials have a quality that produces a very useful result, called resonance, and it is one of the things that gives added effect to a speaker's voice in a hall, where there is a constant succession of echoes. A wall distant from the speaker about 55 feet, produces an almost instantaneous reflection of the sound, and at double that measurement the effect is still stronger. When the distance is too short for the reflecting sound to be heard, we have resonance. It enriches the sound of the voice, and gives a finer quality to musical instruments.
Echoes.—When sounds are heard after the originals are emitted they tend to confusion, and the quality of resonance is lost. There are places where echoes are repeated many times. In the chateau of Simonetta, Italy, a sound will be repeated thirty times.
Speaking Trumpet.—This instrument is an example of the use of reflection. It is merely a bell-shaped, or flaring body, the large end of which is directed to the audience. The voice talking into the small end is directed forwardly, and is reflected from the sides, and its resonance also enables the[p. 90] vibrations to carry farther than without the use of the solid part of the instrument.
The ear trumpet is an illustration of a sound-collecting device, the waves being brought together by reflection.
The Stethoscope.—This is an instrument used by physicians, and it is so delicate that the movements of the organs of the body can be heard with great distinctness. It merely collects the vibrations, and transmits them to the ears by the small tubes which are connected with the collecting bell.