15. Hearing

15.1 The structure of the ear

The human ear is divided into the outer, middle and inner ear. The outer ear includes the pinna (earlobe) and the auditory (ear) canal. The earlobe collects sound waves into the ear canal. Unlike in many other animals, the human earlobes do not move. This has not always been the case, as vestigial muscles whose function is to move the ear still exist in the human body.



The structure of the ear.

Your earlobe funnels sound waves into your outer ear canal. The waves travel along this passage until they hit your tympanic membrane (=eardrum), and cause it to vibrate. As a result, your ossicles (the smallest bones in the human body) start moving. They, in turn, pass on vibrations to a thin layer of tissue at the entrance of your inner ear called the oval window. The movement of the oval window then sets off wave-like motions in the fluid in your cochlea.

The hearing part of the inner ear and is called the cochlea which comes from the Greek word for ‘snail’ because of its distinctive coiled shape. The cochlea, which contains many thousands of sensory cells (called ‘hair cells’), is connected to the central hearing system by the hearing or auditory nerve. The cochlea is filled with special fluids which are important to the process of hearing.

The semi-circular canals and the round window are also located in the inner ear. They help us sense movement and the position of the head.

15.2 Hearing

The sounds we hear are sound waves that travel through the air. Sound waves travel along the ear canal to the eardrum. Sound waves cause the tympanic membrane to vibrate, and the ossicles start moving. Here, the sound waves are converted into mechanical motion.

The ossicles cause the fluids in the cochlea to oscillate. The fluid stimulates the 'hair cells,’ creating a nerve impulse. High-pitched sounds will stimulate ‘hair cells’ in the lower part of the cochlea, whereas low-pitched sounds will stimulate the cells in the upper part of the cochlea. Nerve impulses travel along the auditory nerve to the auditory area of the brain, just above the ear.

Image on the right: a tympanic membrane examination will determine if there is inflammation in the middle ear.

These nerve impulses follow a complicated pathway in the brainstem before arriving at the hearing centres of the brain, the auditory cortex. This is where the streams of nerve impulses are converted into meaningful sound.

The speech and language comprehension centers of the brain are located near the auditory cortex. These are needed to understand and categorize the things that we hear. At birth, a baby does not understand the sounds they hear, but knows to be afraid of sudden loud noises. Thus, things like speech, music and the sounds of nature are distinguished through learning. This is why it is easier for a child to learn a language they have listened to since birth. Therefore, even though we do not have a dog-like ability to distinguish between different wavelengths, we know how to distinguish between the various things that we hear very efficiently in the brain.

The ear can attenuate continuous loud noise for a long time, but a sudden loud noise can damage your hearing. Hearing loss can result from a single loud sound near your ear. When hearing is impaired, the high frequencies usually disappear first and then the low frequencies. Some hearing loss is naturally impaired with age, as the structures of the ear stiffen.

Volume can be measuredin decibels (dB). In decibels, a value of zero dB means the hearing threshold of an average person. Volume of speech is about 50 to 60 dB. The intensity of more than 110 decibels, in turn, exceeds the pain threshold.

 Hearing test (Hz, use headphones)
Use headphones to test your hearing in a quiet location. You should be able to distinguish a sound coming at a frequency of 20 Hz. Many elderly people cannot distinguish the sound of the grasshopper, which is heard on a wavelength of above 4 000 Hz.