(3) INTENSITY AND PITCH OF SOUNDS [Illustration: FIG. 321.--Graphic representations of (_a_) a noise, (_b_) a musical sound.] =328. Musical Sounds and Noises Distinguished.=--The question is sometimes raised, what is the difference between a _noise_ and a _musical sound_? The latter has been found to be produced by an even and regular vibration such as that of a tuning fork or of a piano string. A noise on the other hand is characterized by sudden or irregular vibrations such as those produced by a wagon bumping over a stony street. These differences may be represented graphically as in Fig. 321, (a) represents a noise, (b) a musical tone. [Illustration: 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