Fig. 322.—Curve b represents a tone of greater intensity.
329. Characteristics of Musical Sounds.—Musical tones differ from one another in three ways or are said to have three characteristics, viz., intensity, pitch, and quality. Thus two sounds may differ only in intensity or loudness, that is, be alike in all other respects except this one, as when a string of a piano is struck at first gently, and again harder. The second sound is recognized as being louder. The difference is due to the greater amplitude of vibration caused by more energy being used. Fig. 322 shows these differences graphically. Curve b represents the tone of greater intensity or loudness, since its amplitude of vibration is represented as being greater.
330. Conditions Affecting the Intensity of Sound.—The intensity of sounds is also affected by the area of the vibrating body. This is shown by setting a tuning fork in vibration. The area of the vibrating part being small, the sound is heard but a short distance from the fork. If, however, the stem of the vibrating fork is pressed against the panel of a door or the top of a box, the sound may be heard throughout a room. The stem of the fork has communicated its vibrations to the wood. The vibrating area, being greater, the sound is thereby much increased in intensity, producing a wave of greater amplitude. The same principle is employed in the sounding boards of musical instruments as in the piano, violin, etc. It is a common observation that sounds decrease in loudness as the distance from the source increases. This is due to the increase of the surface of the spherical sound waves spreading in all directions from the source. Careful experiments have shown that in a uniform medium the intensity of a sound is inversely proportional to the square of the distance from its source. If a sound is confined so that it cannot spread, such as the sound moving through a speaking tube, it maintains its intensity for a considerable distance. An ear trumpet (see Fig. 320) also applies this principle. It is constructed so that sound from a given area is concentrated by reflection to a much smaller area with a corresponding increase in intensity. The megaphone (Fig. 323), and the speaking trumpet start the sound waves of the voice in one direction so that they are kept from spreading widely, consequently by its use the voice may be heard several times the usual distance. The intensity of a sound is also affected by the density of the transmitting medium. Thus a sound produced on a mountain top is fainter and thinner than one produced in a valley. The sound of a bell in the receiver of an air pump becomes weaker as the air is exhausted from the latter. Four factors thus influence the intensity of a sound, the area of the vibrating body, its amplitude of vibration, the distance of the source and the density of the transmitting medium. It is well to fix in mind the precise effect of each of these factors.
Fig. 323.—The megaphone.
331. Pitch.—The most characteristic difference between musical sounds is that of pitch. Some sounds have a high pitch, such as those produced by many insects and birds. Others have a low pitch as the notes of a bass drum or the sound of thunder. How notes of different pitch are produced may be shown by the siren (Fig. 324). This is a disc mounted so as to be rotated on an axis. Several rows of holes are drilled in it in concentric circles. The number of holes in successive rows increases from within outward. If when the siren is rapidly rotated air is blown through a tube against a row of holes a clear musical tone is heard. The tone is due to the succession of pulses in the air produced by the row of holes in the rotating disc alternately cutting off and permitting the air blast to pass through at very short intervals. If the blast is directed against a row of holes nearer the circumference the pitch is higher, if against a row nearer the center the pitch is lower. Or if the blast is sent against the same row of holes the pitch rises when the speed increases and lowers when the speed lessens. These facts indicate that the pitch of a tone is due to the number of pulses or vibrations that strike the ear each second; also that the greater the rate of vibration, the higher the pitch.
Fig. 324.—A siren.
332. The Major Scale.—If a siren is made with eight rows of holes, it may indicate the relation between the notes of a major scale. To accomplish this, the number of holes in the successive rows should be 24, 27, 30, 32, 36, 40, 45, 48. If a disc so constructed is rapidly rotated at a uniform rate, a blast of air sent against all of the rows in succession produces the tones of the scale. These facts indicate that the relative vibration numbers of the notes of any major scale have the same relation as the numbers 24, 27, 30, 32, 36, 40, 45, 48.