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Why does a tuning fork placed on the skull produce an audible sound?

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Cross-section diagram of the human ear showing the outer ear, middle ear, and inner ear, with labeled parts including the auricle, auditory canal, tympanic membrane, ossicles, cochlea, semicircular canals, and nerves. — human ear Structure of the human ear.

When a vibrating tuning fork is placed on the skull, it produces an audible sound through a process known as bone conduction. This occurs because the vibrations of the fork are transmitted directly through the bones of the skull to the inner ear, bypassing the outer and middle ear channels.12

The mechanism depends on the frequency of the sound:

Compressional bone conduction: For higher frequencies, the skull vibrates in segments. These vibrations compress the otic capsule, creating waves in the perilymph that stimulate the organ of Corti.12
Inertial bone conduction: For lower frequencies (1,500 Hz and below), the skull moves as a rigid body. Due to their inertia, the ossicles move less freely than the cochlea, causing the oval window to move relative to the stapes footplate, which effectively stimulates the inner ear.12

This phenomenon is a vital tool for otologists to distinguish between different types of hearing impairment.1