For AQA A Level Physics, understanding the microscopic layout of the human ear supports both data analysis and practical interpretation. These revision notes focus on how distinct chambers, membranes, and hair cell arrays convert sound into neural signals.
The table below outlines the main divisions and functions within the ear, linking anatomical labels to their roles in hearing and balance.
| Structure | Location | Primary Function | Key Revision Points |
|---|---|---|---|
| Pinna | Outer ear | Capture and funnel sound | Helps localise sound by time and intensity differences |
| Auditory Canal | Outer ear | Guide vibrations to the eardrum | Resonates near 3–4 kHz, enhancing sensitivity |
| Tympanic Membrane | Boundary of middle ear | Transfer sound pressure to ossicles | Vibrates in phase with incoming compressions and rarefactions |
| Malleus, Incus, Stapes | Middle ear | Amplify and transmit vibrations to oval window | Lever ratio and area difference create moderate pressure gain |
| Oval Window | Inner ear boundary | Transmit mechanical energy to cochlear fluid | Movement generates pressure waves in perilymph |
| Cochlea | Inner ear, bony labyrinth | Frequency analysis via basilar membrane | Place theory: high frequencies near base, low near apex |
| Organ of Corti | Sits on basilar membrane | Contains hair cells that transduce motion | Inner hair cells send most afferent signals |
| Auditory Nerve | Carries impulses to brainstem | Codes pitch and intensity via rate and place | Phase locking occurs up to about 4–5 kHz |
Structure of the Outer Ear
The outer ear begins with the pinna, a funnel shaped structure that captures airborne sound and directs it into the auditory canal. Its ridged shape influences how different frequencies are scattered or focused before they reach the eardrum.
Within the auditory canal, standing waves can form, particularly around 3–4 kHz, which adds a characteristic resonance to the overall transfer function. This resonance is a standard feature in AQA graphs of ear sensitivity against frequency.
Middle Ear Mechanics
Impedance Matching
The middle ear improves transmission of sound energy from air to fluid by using leverage and area differences. The ratio of the tympanic membrane area to the oval window area provides a pressure advantage, while the ossicles act as a system of levers to reduce amplitude but increase force.
Protection and Reflexes
Loud sounds trigger the acoustic reflex, where muscles attached to the ossicles contract to stiffen the chain. This reduces vibration amplitude and protects the inner ear, an important concept when discussing thresholds of hearing and discomfort.
Inner Ear and Transduction
Basilar Membrane and Tonotopy
The cochlea unrolls into a tonotopic map where specific locations respond to specific frequencies. Exam questions often require you to relate this place theory to graphs of sensitivity or to explain what happens when waves peak at different positions along the membrane.
Hair Cell Transduction
The Organ of Corti sits on the basilar membrane, and hair cells bend as the membrane moves. Stereocilia deflection opens ion channels, leading to receptor potentials and, ultimately, neurotransmitter release. Inner hair cells dominate signalling to the auditory nerve, while outer hair cells fine tune frequency selectivity and amplification.
Auditory Pathway to the Brain
After transduction, the auditory nerve transmits action potentials to cochlear nuclei in the brainstem and then onward through higher centres. Processing includes extraction of pitch, location, and timing cues, which support both speech recognition and music perception in AQA discussion questions.
FAQ
Reader questions
How does the shape of the pinna affect hearing? Its curved shape helps capture sound and provides basic direction cues by altering how waves enter the auditory canal, subtly changing frequency patterns before they reach the eardrum. Why does the middle ear use ossicles instead of direct fluid transmission?
Ossicles amplify force through area and lever ratios, overcoming the impedance mismatch between air and cochlear fluid so that more energy reaches the inner ear without excessive loss.
What is the role of the basilar membrane in frequency analysis?
Its stiffness gradient causes different sections to resonate at different frequencies, enabling the cochlea to act as a frequency analyser and supporting the place code for pitch perception.
How does the acoustic reflex protect hearing during loud noises?
Muscle contraction stiffens the ossicular chain, reducing vibration transmission and lowering the effective sound level reaching the sensitive structures of the inner ear.