=325. Graphic Representation of Sound waves.=--It is frequently desirable to represent sound waves graphically. The usual method is to use a curve like that in (Fig. 318, 3). This curve is considered as representing a train of waves moving in the same direction as those in Fig. 318 1 and 2, and also having the same length. The part of the wave _A-B_ represents a condensation of the sound wave and the part _B-C_ represents a rarefaction. A complete wave consisting of a condensation and a rarefaction is represented by that portion of the curve _A-C_. The portion of the curve _B-D_ also represents a _full wave length_ as the latter is defined as _the distance between two corresponding parts of the adjacent waves_. The curve, Fig. (318, 3) represents not only the wave length, but also the height of the wave or the amount of movement of the particles along the wave. This is called the _amplitude_ and is indicated by the distance _A-b_. Since the _loudness_ or intensity of a sound is found to depend upon the amount of movement of the particles along the wave, the _amplitude_ of the curve is used to indicate the loudness of the sound represented. All of the characteristics of a sound wave may be graphically represented by curves. Such curves will be used frequently as an aid in explaining the phenomena of wave motion both in sound and in light. =326. Reflections of Sound.=--It is found that a wave moving along a wire spring is reflected when it reaches the end and returns along the spring. Similarly a sound wave in air is reflected upon striking the surface of a body. If the wave strikes perpendicularly it returns along the line from which it comes, if, however, it strikes at some other angle it does not return along the same line, but as in other cases of reflected motion, the _direction_ of the _reflected_ wave is described by the _Law of Reflected Motion_ as follows: _The angle of reflection is always equal to the angle of incidence_. This law is illustrated in Fig. 319. Suppose that a series of waves coming from a source of sound move from _H_ to _O_. After striking the surface _IJ_ the waves are reflected and move toward _L_ along the line _OL_. Let _PO_ be perpendicular to the surface _IJ_ at _O_. Then _HOP_ is _the angle_ of incidence and _LOP_ is the _angle of reflection_. By the law of reflected motion these angles are equal. In an ordinary room when a person speaks the sound waves reflected from the smooth walls reinforce the sound waves moving directly to the hearers. It is for this reason that it is usually easier to speak in at room than in the open air. Other illustrations of the reinforcement of sound by reflection are often seen. Thus an _ear trumpet_ (Fig. 320), uses the principle of reflection and concentration of sound. So-called _sounding boards_ are sometimes placed back of speakers in large halls to reflect sound waves to the audience. [Illustration: FIG. 319.--Law of reflection.] [Illustration: FIG. 320.--An ear trumpet.]