Fig. 187.
261. Whispering Galleries.—The reflection of sound from curved surfaces gives us some interesting phenomena. The waves of sound in being reflected from a concave surface are gathered together to some point. If the surface be a perfectly spherical one, and the sound issue from the centre, the reflection will be from all points to the centre. But suppose the concave surface have the curve of an ellipse, as represented in Fig. 187. This, instead of having a centre, has two foci, c and g. Now if a sound proceed from one focus, c, the waves of sound, as represented by the lines c d, c e, c f, c h, will all be reflected to the other focus, g; so that if a person speak in a very low tone or even whisper at c, he may be heard distinctly by another at g, though persons at other points may hear nothing. We may have this result with a curved wall extending even several hundred feet; and such structures are called whispering galleries. If in one of these galleries a person standing in one focus speak loudly he will be heard by others at any point by the direct waves of sound; but the reflected sound will be added to the direct in the case of one standing at the other focus.
Fig. 188.
Fig. 189.
262. Concentration of Sound.—It is by the reflection of sound that it can be concentrated in various ways. Thus in using a speaking-trumpet the waves of sound, instead of moving in all directions as soon as they escape from the mouth, are reflected by the sides of the instrument toward a central line as represented in Fig. 188. The waves or vibrations, being thus concentrated have more intensity and are thrown to a greater distance than if they issued directly from the mouth. So a speaking-tube, confining the vibrations, carries the voice to distant parts of a building. For the same reason the voice can be heard much farther through a narrow street than in an open space. So, also, a speaker can be heard more distinctly in a hall than when addressing an audience of the same size in the open air. The "sounding-board," once so fashionable in churches, was really of considerable service in preventing the escape of the vibrations of the voice of the preacher upward, and directing them downward upon the audience. In the hearing-trumpet, Fig. 189, the vibrations are collected in the broad open end of the instrument, and by reflection are thrown together into a narrow compass before they enter the ear to strike upon the drum. We often instinctively make the palm of the hand act as an ear-trumpet when we do not hear distinctly. Many animals have the external ears movable, so that they can direct their concave surface toward the point from which they wish to hear. Such ears are movable ear-trumpets.
263. Difference Between a Musical Sound and a Noise.—The difference between a musical sound and a noise is very analogous to the difference between a crystal and the same substance destitute of the crystalline arrangement. In both there are vibrations, but in the musical sound they have perfect regularity, while in a noise the vibrations are irregular, and there is confusion. Indeed so regular are the vibrations of musical sounds that the rules and principles of music have all the rigid exactness of mathematics.
264. How Different Notes are Produced.—The quicker is the vibration the higher is the note. Thus a short and small string on a violin or in a piano gives a higher note than a long and large string, because its vibrations are quicker. The tension of the string also has an influence, the note being raised by increasing the tension. In tuning a violin the right pitch is given to each string by lessening or increasing the tension by means of the screws to which the strings are attached. In playing upon it various notes are made upon each string by shortening the vibrating portion more or less by pressure of the finger.
