E2 Auricular Pavilion and External Auditory Canal

Topic

Before airborne sound pressure enters the external auditory canal, it first passes through the folds of the pinna—the cartilaginous ridges that give the visible part of the ear its characteristic shape. Each fold of the pinna reflects a portion of the incoming sound pressure; this reflected pressure combines within the pinna itself with the sound pressure arriving directly (unreflected). This interaction reinforces certain frequencies and partially cancels others, depending on the distance the reflected sound pressure traveled along each fold before merging with the unreflected sound.

The distance the reflected sound pressure travels along the pinna's folds before combining with the direct sound depends on the direction of the original sound source: a sound arriving from above follows a different path through the pinna's folds than one arriving from below or behind. Consequently, each angle of arrival produces a unique pattern of frequency reinforcement and cancellation. This filtering pattern—distinct for every direction of sound arrival—is known as the pinna's spatial filtering. It enables the brain to determine whether a sound originated from above, below, the front, or the rear based solely on the frequency content entering the external auditory canal—information that differences in timing or intensity between the two ears alone cannot provide.

The sound pressure, having undergone spatial filtering by the pinna, then enters the external auditory canal. Due to its elongated, narrow shape, the canal acts as a cylindrical waveguide resonator: a tube closed at one end by the tympanic membrane and open at the other toward the pinna. A tube closed at one end and open at the other does not transmit all frequencies equally; it strongly reinforces frequencies with wavelengths that fit specifically within the length of the external auditory canal itself, while transmitting other frequencies with less reinforcement.

Due to its nature as a cylindrical waveguide resonator, the external auditory canal strongly reinforces a range of mid-to-high frequencies—approximately between 2,000 and 5,000 Hertz—thereby boosting the sound pressure reaching the tympanic membrane within this range by 10 to 15 decibels relative to the pressure at the canal's entrance. This boost is known as passive amplification—passive because the external auditory canal adds no energy of its own to the sound pressure; instead, it simply concentrates it within the mid-to-high frequency range thanks to its cylindrical waveguide resonator shape, without expending additional energy to do so.

Between the spatial filtering provided by the folds of the pinna and the passive amplification resulting from the external auditory canal's cylindrical waveguide resonator shape, the sound pressure that ultimately reaches the tympanic membrane differs in frequency content from the original environmental sound pressure. The resulting pattern of reinforced and attenuated frequencies encodes the sound's direction of origin—thanks to spatial filtering—while mid-to-high frequencies are boosted by passive amplification. Consequently, all subsequent spectral processing within the middle and inner ear begins with this sound pressure—already modified by the pinna and external auditory canal—rather than the original sound pressure present in the environment before reaching the ear.

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