10.1 Skull vibration and the bone-conduction route

Sound normally reaches the cochlea via air conduction: a pressure wave in the ear canal drives the tympanic membrane, the membrane drives the ossicular chain, the chain pushes the stapes footplate into the oval window, and the cochlea is set into motion (see Hearing 3 — The middle ear refresher →). But there is a second, parallel route: a mechanical vibration delivered directly to the skull is conducted through the temporal bone and into the cochlear capsule, where it sets the cochlear fluids in motion. This bone-conduction route bypasses the outer and middle ear entirely.

We use bone conduction routinely in audiology — the bone-conduction audiogram (Lesson 2.3) is acquired using a bone oscillator pressed against the mastoid prominence, and the difference between air-conduction and bone-conduction thresholds (the air-bone gap) is the diagnostic basis for distinguishing conductive from sensorineural loss. A bone-conduction device uses the same principle for therapy: turn an acoustic signal into a sustained vibration of the skull, and the cochlea responds as if to natural sound.

Three modes of skull-to-cochlea transmission

Three physical mechanisms transmit a skull vibration to the cochlea. They operate in parallel, with different frequency-dependent contributions:

1. Inertial cochlear mode

The cochlear capsule is rigidly fused into the temporal bone, but the fluids and structures inside the cochlea — perilymph in the scalae, the organ of Corti, the basilar membrane — have inertia. When the temporal bone vibrates, the surrounding bone moves first; the cochlear contents lag by inertia. The relative motion between bone and contents produces a pressure differential between scala vestibuli and scala tympani that drives the basilar membrane just as if the stapes had pushed against the oval window. This is the dominant bone-conduction mechanism above ~1 kHz.

2. Compression cochlear mode

The cochlear capsule, though stiff, is not perfectly rigid. A vibration that periodically compresses the capsule wall causes the inner cochlear volume to change very slightly, displacing fluid and exciting the basilar membrane. The asymmetry between the vestibular compliance (where the oval window provides a soft elastic boundary) and the tympanic compliance (where the round window provides another soft boundary) creates an effective volume velocity that drives the cochlea. This mode dominates between roughly 500 Hz and 1.5 kHz.

3. Osseo-tympanic mode

The vibrating skull transmits motion to the soft tissues around the external auditory canal and the cartilaginous portion of the canal itself. The canal walls vibrate, creating sound pressure within the canal that drives the tympanic membrane through the conventional air-conduction route. This mode contributes mostly below 1 kHz, and is the only bone-conduction mode that does involve the middle ear — it is blocked by middle-ear pathology.

The three modes together produce an overall bone-conduction sensitivity within about 10 dB of air-conduction sensitivity in normal-hearing listeners. The audiometric bone-conduction thresholds, when measured carefully, agree with air-conduction thresholds in normal ears.

Why the interaural attenuation is essentially zero

Air-conducted stimulation has substantial interaural attenuation — about 40 dB for supra-aural earphones, 60–70 dB for insert earphones. When we stimulate one ear, the contralateral ear hears only a much weaker version of the stimulus (because the head shadow and tissue between the two ears attenuate the signal in transit).

Bone-conducted stimulation has interaural attenuation of essentially zero. A vibration applied to the right mastoid travels through the skull and reaches both cochleae with comparable amplitude — typically within 0–10 dB depending on the exact stimulation point and frequency. This is why bone-conducted thresholds tell us about the better cochlea, not the ear that the oscillator is touching.

Two clinical consequences:

In audiometry, bone-conduction testing nearly always requires masking of the non-test ear (Lesson 2.4) — without masking, we cannot tell which cochlea is responding to the stimulus.
In BCD therapy, the device delivers its stimulus to both cochleae regardless of which mastoid it’s mounted on. For patients with one good cochlea, the BCD on either side will reach the working cochlea via the skull. This bidirectional skull-borne path is what makes the BCD especially useful for single-sided deafness — covered in Lesson 10.3.

The transducer

A modern BCD transducer is a small electromagnetic device — typically a piezo-electric or moving-coil actuator — that converts an AC electrical signal into linear mechanical motion. The transducer is rigidly coupled to the patient’s skull via one of three mechanisms (Lesson 10.2): a percutaneous abutment that protrudes through the skin (BAHA), a magnetic transcutaneous coupling that holds an external transducer against the skin (BAHA Attract), or an implanted active transducer that vibrates the skull directly (Osia, Bonebridge).

In all cases, the transducer’s mechanical impedance matches reasonably well to the impedance of the skull at the mastoid (or temporal-bone) prominence. Mismatched impedance — e.g., a transducer that is too compliant or too rigid relative to the bone — wastes energy as heat or as motion of the transducer body rather than the skull. The matching is straightforward in implanted devices (the implant is bonded to bone); it is a clinical fitting concern with skin-coupled devices, where soft-tissue absorption introduces frequency-dependent losses of 5–15 dB above 2 kHz.

What bone conduction cannot do

The bone-conduction route preserves access to both cochleae regardless of which mastoid is stimulated. But this is also bone conduction’s fundamental limitation:

No binaural cues. With a single BCD on one side, both cochleae receive the same signal — there’s no interaural time or level difference for localisation. The patient cannot tell which side a sound came from based on BCD output alone (though residual contralateral acoustic hearing can supply the comparison).
Limited maximum output. Bone vibrators cannot deliver arbitrarily high output without producing significant transducer rattle or unbecoming sensations. Typical BCDs cap at about 65–75 dB HL of equivalent acoustic output — sufficient for mild-to-moderate sensorineural reserves but inadequate for severe-to-profound cochlear loss.
Cochlear reserve is essential. The BCD bypasses the conductive apparatus but does nothing for the cochlea. If the cochlea is damaged, the BCD’s output will still be filtered through that damaged cochlea — patient outcomes are bounded by the bone-conduction audiometric thresholds (the cochlear reserve).

Next lesson: the device classes audiologists actually fit, and the surgical and non-surgical options for coupling the transducer to the skull.