Chapter 4 Section 3

1. Chapter 4 Section 3 Hearing Obj: Describe how the ear perceives sound

2. Sound travels through the air in waves. It is caused by changes in air pressure that result from vibration. Anything that makes a sound causes vibrations. Each of these vibrations is called a cycle or a sound wave. Every sound has its own pitch and loudness.

3. Pitch Sound waves can be very fast, occurring many times per second. The pitch of a sound-how high or low the sound is-depends on its frequency, or the number of cycles per second. The more cycles per second, the higher the pitch of a sound. Women’s voices usually have a higher pitch than those of men because women’s vocal cords tend to be shorter and therefore vibrate at a greater frequency.

4. The human ear can hear sound waves that vary from 20 to 20,000 cycles per second. Many animals, including dogs and dolphins, hear sounds well beyond 20,000 cycles per second. Although we cannot hear them, the sounds emitted by dolphins help them locate others. The sound pulses echo back from fish and other objects.

5. Loudness What is the softest sound you can hear? What is the loudest? The loudness of a sound is determined by the height, or amplitude, of sound waves. The higher the amplitude of the wave, the louder the sound. The loudness of a sound is measured in decibles, a unit that is abbreviated dB. Zero dB is considered the threshold of hearing. Zero dB is about as loud as the ticking of a watch 20 feet away in a very quiet room.

6. The Ear Just as the eye is the human instrument for seeing, the ear is the instrument for sensing all the sounds around us. In fact, the ear is shaped to capture sound waves, to vibrate with them, and to transmit sound to the brain. What we normally think of as the ear is actually the outer ear. We also have a middle ear and an inner ear.

7. The eardrum is the gateway from the outer ear to the middle ear. It is a thin membrane that vibrates when sound waves strike it. As it vibrates, it transmits the sound to three small bones in the middle ear: the hammer, the anvil, and the stirrup. (The stirrup is the smallest bone in the human body). These bones then also begin to vibrate and transmit sound to the inner ear.

8. The inner ear consists of the cochlea. The word cochlea comes from the Greek word for “snail”. The cochlea is a bony tube that contains fluids as well as neurons that move in response to the vibrations of the fluids. The movement generates neural impulses that are transmitted to the brain via the auditory nerve. Within the brain, auditory input is projected onto the hearing areas of the cerebral cortex.

9. Deafness Not everyone perceives sound. About 2 million Americans are deaf. Deafness may be inherited or caused by disease, injury, or old age.

10. Conductive Deafness – If we see an older person with a hearing aid, we can assume that he or she is probably suffering from conductive deafness. Conductive deafness occurs because of damage to the middle ear. Since this part of the ear amplifies sounds, damage to it prevents people from hearing sounds that are not loud enough. Fortunately, people with conductive deafness are often helped by hearing aids. These aids provide the amplification that the middle ear does not.

11. Sensorineural Deafness – Many people do not perceive sounds of certain frequencies. This is a sign of sensorineural deafness. Sensorineural deafness is usually caused by damage to the inner ear. Most often, the neurons in the cochlea are destroyed. Sometimes sensorineural deafness is due to damage to the auditory nerve, either through disease or through prolonged exposure to very loud sounds.

12. Have you ever attended a high-volume rock concert and left with a ringing sensation in your ears? This may have meant that neurons had been destroyed in your ears. The same thing can happen to workers who operate certain drilling equipment or drive loud vehicles. The next time you are exposed to loud sounds, remember to cover your ears. Devices called experimental cochlear implants, or “artificial ears,” contain microphones that sense sounds and electronic equipment that stimulates the auditory nerve directly. However, if the auditory nerve itself is damaged, a cochlear implant cannot help.

13. Deafness in the World Today – In recent years, people who are deaf have been able to come more into the mainstream of sensory experience as a result of their own efforts, the efforts of others, and new technology. Heather Whitestone became Miss America in 1995.

14. Interpreters are often on hand to translate speeches into languages (such as American Sign Language) used by members of the audience who are hearing impaired. More and more schools are offering courses in American Sign Language. Many television shows are now “closed captioned,” which means that special decoders make captions of dialogue visible on the screen. And, as you have just read, scientists are always trying to find new ways to counteract damage inside the ear.