Ol

100,000

Figure 1 Hearing threshold and range of hearing for human listeners. Shown also are the ranges of frequency and sound pressure level of common environmental sounds, including human speech. The most intense sounds are capable of damaging the inner ear receptor organ. The hearing sensitivity of the cat, a laboratory animal commonly used in studies of the peripheral and central auditory systems, is illustrated as well (adapted with permission from Geisler, C. D, From Sound to Synapse. Oxford Univ. Press, New York, 1998).

100,000

Frequency (Hz)

Figure 1 Hearing threshold and range of hearing for human listeners. Shown also are the ranges of frequency and sound pressure level of common environmental sounds, including human speech. The most intense sounds are capable of damaging the inner ear receptor organ. The hearing sensitivity of the cat, a laboratory animal commonly used in studies of the peripheral and central auditory systems, is illustrated as well (adapted with permission from Geisler, C. D, From Sound to Synapse. Oxford Univ. Press, New York, 1998).

Figure 2 Structures of the external, middle, and inner ears [adapted with permission from Brugge, J. F., Auditory system. In Encyclopedia of Neuroscience (G. Adelman, Ed.). Birkhauser, Boston, 1987].

Amplification of sound ranges from 5 to 20 dB for frequencies within the speech range (about 1.5-7 kHz). This improvement in signal-to-noise ratio, which is due mainly to the resonant properties of the external ear canal, provides a way of enhancing speech intelligibility in the presence of many unwanted competing sounds. The pinna, on the other hand, acts as a directional amplifier for high frequencies. Spectrally transformed sound reaching the tympanic membrane provides cues for localizing the source of a sound in space, especially when the sound is on the midsagittal plane, where interaural time and intensity differences are small or nonexistent. Sounds originating from sources on either side of the midline reach the near ear before reaching the far ear, thereby creating an interaural time difference (ITD). The head also acts as an acoustic barrier at high frequencies, creating an interaural intensity difference (IID). The magnitudes of the ITD and IID depend on the location of the sound on the horizontal plane, and neural circuits in the auditory central nervous system have evolved to detect them.

The middle ear cavity is located just behind the tympanic membrane. It is normally air-filled and in equilibrium with atmospheric pressure due to the periodic opening and closing of the Eustachian tube connecting the middle ear cavity with the nasopharynx. Three auditory ossicles (malleus, incus, and stapes) connect the tympanic membrane with the oval window of the inner ear. Reflex contraction of muscles attached to the stapes and malleus stiffens the ossicular chain and thereby reduces transmission of potentially damaging low-frequency sounds. The real need for a middle ear arises because the auditory receptor organ is an "underwater receiver'' operating in the fluid environment of the inner ear. If sound waves in air were to strike this fluid boundary, 99.9% of the energy would be reflected. This interruption in the flow of sound to the inner ear would result in a conductive hearing loss, possibly as much as 30 dB. Thus, the role of the middle ear is to overcome this impedance mismatch between air and fluid and to transfer to the inner ear as efficiently as possible sound energy that impinges on the tympanic membrane.

The first, and most important, mechanism used to overcome impedance mismatch relies on the relatively large area of the tympanic membrane compared to the oval window into which the stapes footplate exerts pressure on the fluid of the inner ear. The force acting on the tympanic membrane is concentrated through the ossicles onto a small area of the oval window resulting in a pressure increase proportional to the ratio of the areas of the two membranes (approximately 20:1). Second, the lever arm of the malleus is longer than that of the incus with which it articulates, giving an additional mechanical advantage of about 1.3. Third, the conical shape of the tympanic membrane imposes additional force on the malleus. Sound energy transmitted to the inner ear is then transduced, through a cascade of mechanical and electrical events, to electrical nerve impulses in axons of the auditory nerve.

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