Physics of hearing

Objectives: Describe the basic structure of the human ear. State and explain how sound pressure variations in air are changed into larger pressure variations in the cochlear fluid. Describe the hearing test.

The outer ear consists of : Pinna . external auditory canal. ear-drum. The middle ear consists of: a small ( around 6 cm3), irregular, air-filled cavity. ossicles - a chain of three bones called the malleus, incus and stapes (which suspended by muscles).

The inner ear: Cochlea – hearing Vestibule – static equilibrium Semicircular canals – dynamic equilibrium

The inner ear : is a complicated bony chamber filled with fluid and embedded in the bone of the skull. It is divided into two parts: The central part together with the semi-circular canals (three liquid-filled canals) that are concerned with maintaining balance and the detection of movement and the position of the body. they do not contribute to the process of hearing. A spirally coiled, liquid-filled tube about 3 mm in total diameter with a volume of 100 mm3 called the cochlea. the cochlea is connected to the brain by the auditory nerve.

:PRESSURE VARIATIONS IN AIR AND FLUID How the sound travels through the ear: The eardrum vibrates in step with air pressure waves of sound. Then malleus vibrates also in sympathy with the eardrum. The mechanical vibrations are then transmitted by the incus and stapes to the oval membrane and then to the fluid of the inner ear.

Diagram shows the lever and piston action of the ear

Just to see(organ of Corte) The basiler membrane supports the organ of corti which contains (the ends of 30.000 auditory nerves and occupying in area of 3 cm long by 0.3 mm wide).

Hearing Test: The test is normally done in a sound proof room for each ear separately through a comfortable headset through frequencies from 250 to 8000 Hz. The hearing thresholds are plotted on a chart and can be compared to normal hearing thresholds.