These last two chapters have only scratched the surface of one of perception's most highly developed and rapidly growing areas of research, hearing. Acoustics and hearing were among the first fields of study to develop quantitative measures for describing their shared subject. These developments date back to the early Greeks, who showed that pitch was related to the length of a vibrating string. Hearing researchers were also among the first to come up with experimental techniques (such as direct scaling) for measuring perceptual reactions to sensory stimulation. These measurements were needed in order to know what acoustic events actually sound like in terms of loudness, pitch, and so forth. Contemporary research in hearing has made possible a very sophisticated understanding of the initial mechanical and neural events that eventually culminate in auditory perception. In the next chapter, we consider in more detail two aspects of hearing that are hallmarks of human auditory experience: perception of speech and perception of music.