The Ear: Structure and Sound Transmission Revision Notes | A Level AQA Physics

1. Why the Ear is an Engineering Marvel

Sound is a pressure wave travelling through air. Your inner ear, however, is filled with fluid. Getting a wave to transfer efficiently from a light, compressible medium like air into a dense, incompressible medium like fluid is genuinely difficult — most of the energy would bounce back. The ear solves this problem with a chain of beautifully adapted structures, and understanding that chain is exactly what AQA expects you to explain.

This lesson maps the entire journey of a sound wave: from the moment it enters the ear canal to the moment a nerve impulse fires towards the brain. Each structure you meet has a specific mechanical role, and the exam will ask you to describe and explain those roles precisely.

2. Structure of the Ear: A Three-Region Map

Picture the ear divided into three regions: the outer ear, the middle ear, and the inner ear. Each region handles a different stage of sound transmission. The outer ear collects and channels sound; the middle ear converts and amplifies it mechanically; the inner ear converts mechanical vibration into electrical signals.

The ear canal is roughly 2.5 cm long and acts as a resonant tube, giving a slight amplification to frequencies around 3–4 kHz — the range most critical for understanding speech. At the end of the canal sits the tympanic membrane, the boundary between the outer and middle ear.

The Eustachian tube equalises air pressure on both sides of the tympanic membrane. When you swallow or yawn and feel your ears 'pop', that is the Eustachian tube opening to balance the pressure — without this, the membrane could not vibrate freely.

3. The Transmission Sequence: Step by Step

Now that you have the map, here is the journey. Sound transmission through the ear is a sequence of energy conversions, and the exam frequently asks you to describe this sequence in order. Each stage must follow logically from the last.

Worked example: Tracing a Sound Wave Through the Ear

Describe, in order, how a sound wave entering the ear canal is ultimately converted into a nerve impulse.

1. Pinna → ear canal → tympanic membrane — The pinna collects and funnels the sound wave. The ear canal channels it towards the tympanic membrane. The wave causes the tympanic membrane to vibrate at the same frequency as the incoming sound.
2. Tympanic membrane → malleus → incus → stapes → oval window — The malleus is attached to the tympanic membrane and picks up its vibration. It passes this to the incus, which passes it to the stapes. The stapes presses against the oval window — a membrane-covered opening into the cochlea. This lever system of three bones amplifies the force of vibration.
3. Stapes pushes oval window → pressure wave in perilymph — The stapes acts like a piston against the oval window. This sets up a pressure wave in the fluid (perilymph) inside the cochlea. The round window at the other end of the cochlea bulges outward to accommodate the fluid movement — fluid is incompressible, so it needs somewhere to go.
4. Fluid pressure wave → basilar membrane displacement — The pressure wave travels through the cochlear fluid and causes the basilar membrane to vibrate. Different regions of the basilar membrane respond maximally to different frequencies: high frequencies near the base, low frequencies near the apex (the tip of the coil).
5. Basilar membrane movement → hair cell deflection → action potential — Sensory hair cells sit on the basilar membrane. When the membrane moves, the stereocilia (tiny hair-like projections) on the hair cells are deflected. This opens ion channels, depolarising the cell and triggering action potentials in the auditory nerve, which travel to the brain.

Answer: Sound wave → tympanic membrane vibration → ossicle transmission → oval window → cochlear fluid pressure wave → basilar membrane vibration → hair cell depolarisation → auditory nerve impulse → brain.

Notice that the sequence involves two distinct types of energy conversion. The first is mechanical throughout — from air pressure waves to membrane vibration to fluid waves. The second, at the hair cells, is a transduction: mechanical energy becomes electrical energy in the form of an action potential.

4. Impedance Matching: Why the Middle Ear Amplifies

Here is where the physics becomes genuinely interesting. When a sound wave in air hits a fluid boundary directly, roughly 99.9% of the energy is reflected — only about 0.1% is transmitted. This is because air and fluid have very different acoustic impedances (resistance to the propagation of sound). Without a solution to this problem, you would be almost completely deaf.

The middle ear solves this through two mechanisms that together constitute impedance matching. Both mechanisms increase the pressure delivered to the oval window, compensating for the energy that would otherwise be lost at the air-fluid boundary.

• Area ratio: The tympanic membrane has a much larger surface area than the oval window (roughly 17–20 times larger in humans). Because the same force is concentrated onto a smaller area, the pressure at the oval window is greatly increased. Recall that pressure = force / area, so reducing the area increases the pressure for the same applied force.
• Lever action of the ossicles: The malleus and incus act as a lever system with a mechanical advantage slightly greater than 1. The longer arm of the lever (attached to the tympanic membrane) moves with greater displacement but less force; the shorter arm (connected to the stapes) moves with less displacement but greater force. This provides an additional amplification factor of approximately 1.3.
• Combined effect: Together, the area ratio and lever action increase the pressure at the oval window by a factor of approximately 22, largely overcoming the impedance mismatch between air and cochlear fluid.

The lever action of the ossicles is a classic application of the principle of moments. The malleus and incus together form a lever pivoting about a fulcrum, with the longer arm connected to the tympanic membrane and the shorter arm driving the stapes. Because the input arm is longer than the output arm (d_in > d_out), it follows from the principle of moments (F_in × d_in = F_out × d_out) that F_out > F_in, giving a mechanical advantage greater than 1 — exactly the amplification the ear needs.

5. The Cochlea and Frequency Analysis

The cochlea does more than detect sound — it performs a frequency analysis. Think of it as a biological spectrum analyser. Different positions along the basilar membrane respond maximally to different frequencies, a property known as tonotopy.

The basilar membrane varies in both width and stiffness along its length. Near the base (closest to the oval window), it is narrow and stiff, so it resonates at high frequencies. Near the apex (the far end of the coil), it is wide and flexible, so it resonates at low frequencies. When a complex sound enters the cochlea, different regions of the membrane vibrate maximally, and the pattern of hair cell activation encodes the frequency content of the sound.

The hair cells themselves are arranged in rows along the organ of Corti, which sits on the basilar membrane. When the basilar membrane moves, the stereocilia of the hair cells are deflected relative to the overlying tectorial membrane. The outer hair cell stereocilia are directly embedded in the tectorial membrane and are sheared as the membranes move; the inner hair cell stereocilia, which are the primary afferent transducers, are not embedded in the tectorial membrane and are instead deflected by fluid movement in the subtectorial space. In both cases, this deflection opens mechanically gated ion channels, allowing potassium ions to flow in and depolarise the cell.

6. Exam Technique and Common Mistakes

AQA questions on the ear tend to use the command words 'describe', 'explain', and 'compare'. Each demands a different response structure. A 'describe' answer lists what happens in sequence without needing to give reasons. An 'explain' answer must include the mechanism — the 'because' — behind each step. Mixing these up is one of the most common ways students lose marks on this topic.

When answering questions about impedance matching, always state both mechanisms: the area ratio and the lever action. Mentioning only one will typically earn partial credit at best. Quantitative detail (the area ratio of approximately 17–20, the combined pressure amplification of roughly 22) can strengthen an explanation answer and is worth including if the question asks you to 'explain fully'.

For extended-response questions, structure your answer around the three-region framework introduced at the start of this lesson: outer ear (collection), middle ear (mechanical transmission and impedance matching), inner ear (transduction). This framework ensures you cover all the required stages and gives your answer a clear, logical progression that examiners reward.