This chapter shows how sound is propagated through the intervening medium and discusses the origin of the sound waves which proceed from the source. It distinguishes three different qualities of sound: the pitch, intensity, and quality. The pitch is the most important of these because it is that property which enables us most frequently and efficiently to recognize a particular sound. Many difficulties confront any attempt to measure the intensity of sound. Finally there is another characteristic which one might attribute to sound sensations, and this is called timbre or quality. In general, one can easily distinguish between the note of a violin, flute, trumpet, and so on, even when they all three are playing the same note, that is a note of the same frequency. For musical purposes it is convenient to introduce more notes into the diatonic scale.