The cochlea contains about 16,000 hair cells, each of which holds a bundle of fibers known as cilia on its tip. The movements of the hair cells trigger nerve impulses in the attached neurons, which are sent to the auditory nerve and then to the auditory cortex in the brain. The movements of the fluid in the cochlea bend the hair cells of the inner ear, much in the same way that a gust of wind bends over wheat stalks in a field. The vibrations cause the oval window, the membrane covering the opening of the cochlea, to vibrate, disturbing the fluid inside the cochlea. The resulting vibrations are relayed into the middle ear through three tiny bones, known as the ossicles-the hammer (or malleus), anvil (or incus), and stirrup (or stapes)-to the cochlea, a snail-shaped liquid-filled tube in the inner ear. At the end of the canal, the sound waves strike the tightly stretched, highly sensitive membrane known as the tympanic membrane (or eardrum), which vibrates with the waves. Prolonged exposure to sounds above 80 decibels can cause hearing loss.Īudition begins in the pinna, the external and visible part of the ear, which is shaped like a funnel to draw in sound waves and guide them into the auditory canal. The human ear can comfortably hear sounds up to 80 decibels. The sound of a typical conversation (about 60 decibels) is 1,000 times louder than the sound of a faint whisper (30 decibels), whereas the sound of a jackhammer (130 decibels) is 10 billion times louder than the whisper. Each increase in 10 decibels represents a tenfold increase in the loudness of the sound (see Figure 4.29 “Sounds in Everyday Life”). Zero decibels represent the absolute threshold for human hearing, below which we cannot hear a sound. Loudness is measured using the unit of relative loudness known as the decibel. The amplitude, or height of the sound wave, determines how much energy it contains and is perceived as loudness ( the degree of sound volume).

Longer sound waves have lower frequency and produce a lower pitch, whereas shorter waves have higher frequency and a higher pitch. The wavelength of the sound wave (known as frequency) is measured in terms of the number of waves that arrive per second and determines our perception of pitch, the perceived frequency of a sound. Unlike light waves, which can travel in a vacuum, sound waves are carried within mediums such as air, water, or metal, and it is the changes in pressure associated with these mediums that the ear detects.Īs with light waves, we detect both the wavelength and the amplitude of sound waves. Vibrating objects (such as the human vocal chords or guitar strings) cause air molecules to bump into each other and produce sound waves, which travel from their source as peaks and valleys much like the ripples that expand outward when a stone is tossed into a pond. Just as the eye detects light waves, the ear detects sound waves. In a fraction of a second, our auditory system receives the sound waves, transmits them to the auditory cortex, compares them to stored knowledge of other voices, and identifies the identity of the caller. A mother can pick out her child’s voice from a host of others, and when we pick up the phone we quickly recognize a familiar voice. But the ear is particularly sensitive to sounds in the same frequency as the human voice. The human ear is sensitive to a wide range of sounds, ranging from the faint tick of a clock in a nearby room to the roar of a rock band at a nightclub, and we have the ability to detect very small variations in sound. Sound waves that are collected by our ears are converted into neural impulses, which are sent to the brain where they are integrated with past experience and interpreted as the sounds we experience. Like vision and all the other senses, hearing begins with transduction. Describe the process of transduction in hearing.Draw a picture of the ear and label its key structures and functions, and describe the role they play in hearing.

Auditory canal function