3Outer & middle ear

FThe outer ear — collector, resonator, filter

The pinna (the visible ear) and the ear canal do more than funnel sound. The canal is a tube closed at one end by the eardrum, and like any such tube it resonates — boosting sound by as much as ~20 dB in the frequency region most important for speech (around 2–4 kHz). So before the middle ear has done anything, the outer ear has already amplified the speech band.[2009]

The folds of the pinna also filter sound in a direction-dependent way: a sound from above, in front, or behind reaches the eardrum with a slightly different spectral colouring. The brain learns these subtle spectral cues and uses them to judge elevation and to resolve front–back confusions — a monaural contribution to localisation that complements the two-eared mechanisms of Module 13.[2012]

FTThe air–fluid problem

Hearing ultimately depends on moving the fluid inside the cochlea. But fluid has a far higher acoustic impedance than air — it resists being moved much more strongly. When a wave meets a sudden jump in impedance, most of its energy is reflected rather than transmitted. A direct air-to-fluid interface would waste almost all the sound — a loss of roughly 30 dB.[2012]

This is the single problem the middle ear exists to solve. It is a mechanical impedance-matching transformer, and it works by trading the large, easily-moved eardrum against the small, hard-to-move cochlear window.

TThe impedance-matching transformer

Two mechanisms combine. The dominant one is the area ratio: the eardrum is far larger than the stapes footplate, so the force the eardrum collects over its large area is concentrated onto the small area of the footplate, raising the pressure. The second is the ossicular lever: the malleus arm is a little longer than the incus arm, so the chain trades a larger, gentler movement at the eardrum for a smaller, more forceful one at the stapes. Together they recover most of the energy that the bare air–fluid boundary would have lost. Adjust the ratios — or bypass the middle ear entirely.[2012, 2009]

FThe ossicular chain

The transformer is built from the three smallest bones in the body, the ossicles, linked in series:

• Malleus (hammer) — its handle is embedded in the eardrum, so it moves whenever the drum moves.
• Incus (anvil) — the middle link, articulating with both neighbours.
• Stapes (stirrup) — its footplate sits in the oval window, the door into the cochlea; its piston-like motion drives the cochlear fluid.

Sound therefore travels eardrum → malleus → incus → stapes → oval window → cochlear fluid. Anything that stiffens, breaks, or loads this chain degrades the transformer.

FTConductive hearing loss

When the outer or middle ear fails to deliver sound efficiently to the cochlea, the result is a conductive hearing loss — wax occlusion, a perforated eardrum, middle-ear fluid (otitis media), ossicular discontinuity, or fixation of the stapes (otosclerosis). The cochlea and nerve are intact; the problem is purely mechanical delivery, and the loss can reach up to about 60 dB.[2009]

TThe stapedius reflex

The middle ear is not passive. A small muscle, the stapedius, attaches to the stapes and contracts reflexively in response to loud sound, stiffening the ossicular chain and reducing transmission. This acoustic reflexoffers some protection against sustained loud sounds and helps reduce the masking of speech by one's own voice and by low-frequency noise.[2012]

It matters directly in the cochlear-implant clinic: the same reflex can be driven electrically through an implant, and the lowest level that triggers it — the electrical stapedius reflex threshold — is one of the most useful objective anchors for setting comfortable stimulation levels (see Objective Measures, Module 5).

FWhy a cochlear implant bypasses all this

Everything in this module — the pinna's resonance, the ossicular transformer, the oval window — exists to get airborne sound into cochlear fluid so it can move the hair cells. A cochlear implant skips the entire conductive chain: its microphone captures sound and its electrodes deliver current straight to the auditory nerve. A recipient's conductive status is therefore largely irrelevant to electric hearing — a profound conductive loss on top of a sensorineural one does not impede a cochlear implant, because the implant never uses the middle ear at all.[2009]

The one piece the implant re-uses is the reflex: the stapedius still contracts to loud electrical stimulation, giving the clinician an objective handle on loudness. With the conductive front end behind us, the next module enters the cochlea itself and the travelling wave.