Every line on an audiogram is a measurement of one part of the auditory system. To interpret the chart you have to know what each part does and how a tone reaches the brain by two different routes. This module follows sound from the air outside the ear to the auditory cortex.

The three compartments

The peripheral ear is conventionally divided into three parts. The outer ear — the pinna and the external auditory canal — collects sound and channels it to the eardrum. The middle ear — an air-filled cavity containing the three ossicles — converts airborne sound into vibration of the inner-ear fluids. The inner ear — the cochlea — turns that vibration into the nerve impulses the brain interprets as sound.

This division is not merely anatomical. It maps directly onto the audiogram: a problem in the outer or middle ear produces a conductive loss, while a problem in the cochlea or the auditory nerve produces a sensorineural loss. The whole logic of comparing air conduction with bone conduction rests on this three-part scheme.

The outer ear

The pinna is not acoustically passive: its folds filter and slightly amplify certain frequencies and help with localising sound. The ear canal behaves like a tube closed at one end and has a natural resonance in the region of 2–4 kHz, boosting sound there by roughly 10–15 dB. This canal resonance is one reason the audiogram so often shows its earliest changes around 3–4 kHz.

The middle ear

The middle ear solves an impedance problem. Sound travelling from air into the fluid-filled cochlea would, unaided, lose most of its energy at the boundary — fluid is much harder to move than air. The middle ear recovers that energy by two mechanisms: the eardrum is far larger in area than the oval window it ultimately drives, and the ossicular chain acts as a lever. Together these give roughly a 25–30 dB gain that offsets the impedance mismatch.

Trainee The area ratio between the tympanic membrane and the stapes footplate is the larger of the two effects, contributing the majority of the gain; the ossicular lever contributes a smaller amount. When the middle ear is damaged — fluid behind the drum, a fixed stapes, a discontinuous ossicular chain — this transformer fails, and the audiogram shows the result as an air–bone gap. The size and shape of that gap is itself diagnostic, a theme developed in the conductive-loss module.

The inner ear

The cochlea is a coiled tube partitioned along its length. The basilar membrane that forms one of those partitions is not uniform: it is stiff and narrow at the base and floppy and wide at the apex. Because of this gradient, a given frequency causes a peak of vibration at a particular place — high frequencies near the base, low frequencies near the apex. This tonotopic arrangement is why audiometry can test frequencies one at a time and read off a place-specific picture of cochlear function.

Sitting on the basilar membrane is the organ of Corti, with two kinds of sensory hair cell. The single row of inner hair cells are the true sensory receptors: they convert vibration into the nerve signal. The three rows of outer hair cells are an active amplifier — they change length in response to sound and sharpen and boost the basilar-membrane response, especially for quiet sounds.

Clinician The division of labour between the two hair-cell populations explains several audiometric phenomena. Outer hair cells are the most vulnerable element of the cochlea — noise, ageing and ototoxic drugs damage them first — so the earliest sensorineural losses reflect lost amplification. Their loss also abolishes the compressive, non-linear growth of loudness they normally provide, which is the mechanical basis of recruitment: once the amplifier is gone, loudness grows abnormally steeply above threshold. That is precisely what the SISI test probes, and why a positive SISI points to a cochlear site of lesion.⁸

From cochlea to cortex

Inner hair cells synapse on the fibres of the cochlear (auditory) nerve, which carries the signal to the brainstem. From there the central auditory pathway ascends through several relay stations to the auditory cortex. For audiometric purposes the key boundary is between the cochlea and the nerve: a lesion of the nerve or its central connections is retrocochlear, and although it too produces a sensorineural audiogram, it behaves differently on the special tests — a contrast the tone-decay and retrocochlear modules return to.

Two routes to the cochlea

Audiometry exploits the fact that sound can reach the cochlea two ways. The air-conduction route runs outer ear → middle ear → cochlea and therefore tests the whole system. The bone-conduction route uses a vibrator on the skull to drive the cochlea directly, largely bypassing the outer and middle ear. Comparing the two is the single most powerful idea in audiometry: if bone conduction is normal but air conduction is not, the fault lies in the conductive apparatus; if both are equally impaired, the fault is sensorineural. The next module turns this principle into a practical test.