The Ear!

Sound

Sound is formed by the rapid vibrating of air, causing a pattern of compression and expansion known as sound waves. The rate at which the air is expanded and contracted relates to the frequency of the sound wave. A wave that compresses 1,000 times a second would have a frequency of 1,000 hertz. People can hear sound that is as low as 20 hertz, and as high as 20,000 hertz.

Characteristics of Sound

Pitch is the term used to describe the frequency of sound. Loudness is used to describe the amplitude of sound, and timbre is used to describe the complexity of the sound. For example, a higher pitch of sound would indicate a higher wave frequency, and a louder sound would have a greater amplitude.

Hearing

The figure below shows a cross section of the ear. The flap of skin and cartilage that we call the ear is technically called the pinna. When a sound wave reaches the ear, it moves down the auditory canal to the eardrum. The vibrations felt in the eardrum are transferred into the three bones directly behind it, the hammer, the anvil, and the stirrup. The vibration in the stirrup is then transferred into the cochlea, which contains fluid. The pressure changes in this fluid cause movement in the basilar membrane, which contains the hair cells. Hair cells are the receptors for hearing, and their movement triggers the hearing process. Above the hair cells is the tectorial membrane. This membrane moves with the hair cells, and alters their electrical potential. The auditory neurons surrounding the hair cells detect this change in electric potential, and send a pulse down the auditory nerve to the auditory cortex, where the sound is evaluated.

How Do We Percieve Pitch?

There are two theories that attempt to explain how we perceive different pitches. One is the place theory, the other is the frequency theory. The place theory was developed by Helmholtz, and states that different frequencies of sound will each hit the basilar membrane with maximum force at different locations. The place of impact on the basilar membrane, then, determines the perceived pitch of sound.

The frequency theory states that the reason we perceive different pitches of sound is due to the frequency of the incomming sound. If the sound has a higher frequency, then it will vibrate against the eardrum more rapidly. Once the sound waves are conducted to the auditory nerve, the nerve will fire faster if the sound is of a higher frequency. Therefore, the frequency theory states that if the auditory nerve fires more rapidly, we will perceive a higher pitch sound.

Deafness

Deafness is the total or partial loss of hearing ability. There are two ways which deafness can be caused. If there is a problem in conducting the air to the cochlea through the hammer, anvil, or stirrup, then the person will be deaf. This kind of deafness, caused by an inability to transfer the vibrations of the eardrum to the cochlea, is called conductive deafness. If there is a problem in the hair cells, the auditory nerve, or the cortical neurons, one will also lose hearing. This type of deafness is called perceptive deafness.