As sound travels to the outer ear, it passes over the torso, head, and especially the pinna. All these structures attenuate and delay the passage of the sound to the outer ear, and the attenuation and delay depend on the interaction between the size of the structures and the wavelength of the sound, such that high-frequency sounds are affected more than low-frequency sounds. These transformations alter the spectrum of the originating sound and are called head-related transfer functions (HRTFs). HRTFs are significant for sound localization.
The outer ear canal has a resonant frequency near 4000 Hz (i.e., standing waves exist within the outer ear), and as such sounds with frequency components near 4000 Hz are more intense in the ear canal than are other frequency components. The middle ear ossicles (bones) provide further enhancement in sound level in the region of 2000-5000 Hz. All these increases in sound level are necessary if air pressure is to produce an effective vibration within the inner ear. The inner
ear is a fluid-filled space containing neural structures and their support. These inner ear structures offer significant impedance to the transmission of vibration from the air-filled outer ear to the inner ear. The resonance of the outer ear and the increases provided by the ossicular chain provide an impedance match between air and the fluids and structures of the inner ear so that over a significant portion of the audible range of hearing, changes in air pressure impinging on the auditory system are efficiently transmitted to the inner ear with no loss in sound level. Damage to the ossicular chain leads to significant hearing loss because ofthe loss ofthis crucial impedance matching function.
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