The basic concepts of acoustics introduced in the first part of this book provide what is necessary for the specification and description of sounds of all kinds, including those of speech. The simplest possible account of a sound which reaches our ears is given by registering the amplitude of movement of an air particle which stands in the path of the sound wave; the movement changes continuously with time, both in direction and in distance travelled, and a graph of these variations is of course the waveform of the sound. Only the dimensions of time and displacement are involved in such a graph but if we note the time interval between reversals in the direction of movement or between the repetitions of a complex pattern of movement, then we introduce the dimension of frequency. We saw in Chapter 2 that any complex wave can be formed by the addition of sine waves of the appropriate frequency and amplitude and, where necessary, phase. The most practically useful account of any type of sound wave is that which specifies the frequency and amplitude of the component sine waves and the process which arrives at this information is that of acoustic analysis or frequency analysis. This chapter will be mainly concerned with the description of the most commonly used instrumental method of performing an acoustic analysis. Before embarking on this, however, we need one extension to the basic ideas which have been introduced so far.