Sound enters the ear canal as a pressure wave of varying frequency (pitch) and amplitude (volume). These pressure waves pass down the ear canal and beat on the eardrum causing it to vibrate. The external ear canal and the Pinna form the ‘outer ear’.
The cavity behind the eardrum is the ‘middle ear’, which is in turn connected to the back of the throat and facial sinus areas. The eardrum is connected to a small bone known as the Malleus (the hammer). The Malleus in turn, is connected to the Incus (known as the Anvil), which in turn is linked to the Stapes (the ‘stirrup’ the smallest, most delicate bone in the human body). This bone is attached to a small oval window at the entrance to the cochlea (the organ of hearing or ‘inner ear’).
The beating eardrum sends sympathetic vibrations through the middle ear bones, causing a lever action, into the cochlea. The cochlea is a tiny cavity (about 4mm) formed within the cranium, shaped like a snail shell, filled with a thick fluid and lined inside with thousands of microscopic hair cells / nerve endings. The travelling wave stimulates specific hair cells along the cochlea according to the exact frequency and amplitude of the continuous stream of sound signals, and triggers an electrical impulse that is sent to the brain via the auditory nerve. This energy must then pass through millions of nerve cells and various ‘junctions’ (ganglia) causing sensations in many parts of the brain. In fact we are still discovering how much hearing activates new parts of the brain although It is known that hearing activates more parts of the brain then any other sense.
Damage or obstruction to any of the components between the outer ear and the auditory pathways can therefore cause a hearing loss. Here is a brief description of the most common causes, and details of how hearing aids can help.