The computer is whirring close by, a phone is jangling in the background and even further away, a plane is flying overhead. Our surroundings are becoming ever louder, and more and more noises are being added to the mix. All this means unceasing work for our ears. Our ears are capable of picking up and processing a wide range of simultaneous sounds at varying sound levels within milliseconds. This filtering function was vital to the survival of our distant ancestors. If our brain had not instantly registered the sound of a twig snapping as a potential danger, our forebears might well have been devoured by a saber-toothed tiger.
"Our hearing processes about 50 impressions per second - twice as many as our eyes.
This brings us to today, where the sound of an approaching predator has been replaced by that of an approaching car. With the increase in aural stimuli, our ears have to work harder than ever to differentiate important from unimportant noises. If we close our eyes for a minute and try to identify all the individual ambient sounds, it quickly becomes clear quite how powerful our ears’ filtering mechanism is.
"Babies can already hear in the womb at the 16th week of pregnancy."
Before a sound even reaches the brain to be filtered, however, it has already gone through several stages of the hearing process, which we will now take a closer look at.
The curved, cup-like outer ear picks up sound waves, which it then funnels deeper into the ear. Sound waves then travel through the auditory canal to the eardrum – a thin membrane that forms the threshold to the middle ear.
The pressure of sound waves causes the sensitive eardrum to vibrate. It is connected to three tiny bones (or ossicles) known as the hammer, anvil and stirrup, which transmit the vibrations of the eardrum from the middle ear to the inner ear. The middle ear is also the place where pressure is equalized. It is here that we find the eustachian tube, which links the middle ear to the nasopharynx and equalizes the pressure between the middle ear and the air outside. Usually, this pressure equalization takes place automatically and we don’t even realize it is happening. When ambient pressure changes very quickly, though – when taking a cable car up a steep mountain, for example – we may experience painful pressure in our ears. Such discomfort can be eased by consciously and repeatedly swallowing; this forces the eustachian tube to open, thereby restoring the correct balance.
Together, the cochlea and the auditory nerve make up the inner ear. This part of the body is also responsible for balance.
The cochlea is full of fluid. Waves of sound travel through the cochlea, causing microscopic hairs to vibrate. These hairs, in turn, are connected to the auditory nerve. As soon as the hairs are moved by the sound wave, they send electric signals to the auditory nerve, which is linked directly to the hearing center in the brain. The brain converts these electric signals into sounds and we can hear. The more frequently and the more seriously the hairs in the inner ear are damaged, the worse our hearing becomes.
But the inner ear performs another important function, too: it regulates our balance . The organ of equilibrium (vestibular system) consists of three ring-shaped canals filled with fluid. They are arranged approximately at right angles to each other in the horizontal, vertical and frontal planes – rather like spirit levels in all three dimensions. The vestibular system also contains the vestibular sacs (saccule and utricle), which sense changes in speed, such as upward motion in an elevator or acceleration in a car. All parts of the vestibular system are continuously sending signals to the brain to communicate whether we should shift our weight in a particular direction – to avoid falling over, for example.