Hearing Method

Pathway of sound waves

The sound waves around us travel along the ear canal and hit the eardrum, making it vibrate.

In the middle ear, there are three adjoined bones (the chain of ossicles). The first bone (malleus) is attached to the eardrum. When the eardrum vibrates the malleus, the adjoining incus and stapesvibrate. The stapes is joined to the cochlea which is in the shape of a snail shell. Inside this bony part runs another tube of the same shape carrying endo lymph. Perilymph can be found between the bone bordering the cochlea and the inner tube.

The mechanical vibrations of the stapes make the perilymph vibrate. As a result, the hair cells within the cochlea are stimulated and nerve fibres transmit impulses along the auditory nerve to the brain. The temporal lobe of the brain will interpret the message (memories of all sounds we were exposed to since birth are stored here).

For an accurate transmission of sound waves from the ossicles to the inner ear, middle ear pressure must be equal to that of the prevailing atmospheric pressure. This is achieved by the Eustachian tube, which runs from the throat to the middle ear.Eustachian tube is made of bone and cartilage. It is usually closed.But when we swallow,with the palatal movement the tube will open carrying air at atmospheric pressure to middle ear. During a cold this tube can get blocked due to swelling of the lining.Middle ear pressure becomes negative causing a locked feeling.

When we fly, atmospheric pressure is low at high altitudes.Air in the middle ear leaks out through the tube in to the throat.The exact opposite happens when we land on the ground. If the tube is blocked due to a cold, air cannot enter the middle ear. Due to very low pressure in the middle ear, there can be bleeding in to the cavity causing severe pain during landing.It is therefore advisable to avoid air travel during a severe attack of cold.