2. HEARING Do you know how you are able to hear your phone ringing? A baby crying? Leaves rustling? Sound travels through the air in waves. It is caused by changes in the air pressure that results 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. 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 objects. The sound pulses echo back from fish and other objects.

4. LOUDNESS 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 decibels, a unit that is abbreviated dB. Zero dB is considerate the threshold of hearing. Zero dB is about as loud as the ticking of a watch 20 feet away in a very quiet room.

5. 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 about the ear is actually the outer ear; we also have a middle ear and an inner ear. 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 anvil, the hammer 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.

6. The Inner Ear consists of the cochlea; the word cochlea comes from the Greek word for snail, as you can see it has the shape of a snail. The cochlea is the 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.

7. Parts of the Ear

9. LOCATING SOUNDS Did you ever sit in front of a stereo, and for some reason, all the sound seemed to come from one side instead of from straight ahead? What you probably did was to adjust the balance knob until the sound seemed equally loud in each ear. Balancing a stereo set is similar to locating sounds. If a sound seems louder from the right, you think it is coming from the right because you are used to a sound from the right side reaching the right ear first. But what if a sound comes from directly in front of you, from behind, or from above? All such sounds are equally loud and distant from each ear. So what do you do? Simple you usually turn your head just a little to determine in which ear the sound increases. If you turn to the right and the loudness increases in your left ear, the sound must be in front of you. Of course you also use information from vision and other cues in locating the source of sounds.

10. Deafness Not everyone perceives sound. About 2 million Americans are deaf. Deafness may be inherited or caused by disease, injury, or old age. Conductive Deafness: If we see an older person with a hearing aid, we can assume that he or she is suffering from conducting deafness. This 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 it is due to damage of the auditory nerve, either through disease or through prolonged exposure to very loud sounds. 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 is damaged, a cochlear implant cannot help.