Designing Effective Speech Interfaces

How Humans Hear Sound

Since the definition of sound that we are working with is any “vibration” that “is capable of being heard by the human ear” we should have some familiarity with how sound is actually heard. As discussed earlier in this chapter, any sound wave with a frequency between approximately 20 and 22,000 Hz that strikes our ears is perceived as sound. We previously said that a sound wave reaches human ears and physically strikes the eardrums. Then, our ears send an electronic signal to the brain and the message is perceived as sound. We can delve more deeply into this process by discussing the physiology of hearing.

Anatomically, the human ear has three parts: the outer ear, the middle ear, and the inner ear. The process of hearing begins when a sound wave is physically captured by the outer part of the human ear.

The outer, or external, portion of the ear is shaped in such a way that it funnels sound waves further into the ear. The hole in our outer ear is the beginning of the auditory canal. When a sound wave travels through the auditory canal, it resonates and the sound pressure level is increased by many decibels. At the end of the auditory canal is the eardrum, which marks the beginning of the middle ear.

The eardrum, also called the tympanic membrane, is a flexible membrane that vibrates when struck by a sound wave. A series of three bones, called the ossicles, touches the inside of the tympanic membrane. The ossicles vibrate in sympathetic response to the vibrations ...