Outer and Middle Ears. Reading: Yost Ch. 6

Transcription

1 Outer and Middle Ears Reading: Yost Ch. 6

2 The Mammalian Ear 1 Subdivided into outer, middle, and inner ear. Outer ear Pinna External canal Middle ear tympanic membrane (ear drum) middle ear bones (ossicles) middle ear muscles. From Geisler (1998) Not to scale. Inner ear Cochlea: contains the mechanosensory epithelium ( organ of Corti ).

3 The Mammalian Ear 2

4 The Outer Ear Components: Pinna External canal Functions: Protection Amplification Sound localization Pinna External canal

5 Found only in terrestrial mammals Pinna is cartilaginous, with characteristic ridges and grooves. Structure: complex and speciesspecific Pinna Mobile in many mammals, but not so much in primates. Am. Soc. Mammol. Functions as a (frequencydependent) directional receiver ( acoustic antenna ).

6 Concha (Latin cave): or bowl, deepest depression of pinna surrounding the meatus. External auditory meatus: opening to the ear canal, on the order of 5 7 mm in diameter. The Human Pinna Tragus: promontory just anterior to the concha and meatus. Gray's Anatomy of the Human Body

7 The external ear canal is 2 3 cm long in humans, and ends at the eardrum. The outer ½ of the ear canal is supported by cartilage (continuation of framework for pinna), protected by hair and wax-secreting glands. The inner ½ is bony. Focuses sound energy on eardrum. External Ear Canal

8 Sound Amplification Pinna and ear canal increase sound pressure at the eardrum in the range of khz. Gain (amplification) peaks at about 20 db at 3 khz. The gain function of the outer ear is frequency dependent. The main structures that contribute to the frequency dependence are the concha gain (peak ~5 khz) and ear canal (~2 khz). Outer Ear Transfer Function from Shaw (1974) Ear canal ~2.5 cm-long, therefore: F 0 = c/4l F 0 = 344 m s -1 /0.1 m F 0 = ~3400 Hz But: Actual F 0 2 khz because of compliance of canal and tympanic membrane.

9 Sound Localization: Directional Hearing In addition to the effects of ear canal resonance, sounds reaching the ear drum are filtered by interactions with the pinna, head, neck, and torso. Together these form a complex acoustical antenna (Shaw 1997). Variations in amplitude and spectral filtering with source direction and frequency, along with interaural disparity cues, provide physical cues for sound localization.

10 Directionality vs Frequency Directionality of ear varies with sound frequency Iso-SPL contour plots (normalized to max SPL). Ear is more directional at higher frequencies (head shadowing)

11 Head-related Transfer Functions Head-related Transfer Function: Positiondependent change in waveform at the ear caused by a change in the impulse response of the head and pinna. Measurement of HRTFs: Broadband clicks broadcast equidistant from the head from grid of horizontal and vertical locations. Recordings taken in ear canal with calibrated probe microphone. Head related impulse response for each locus is transformed by Fourier analysis into amplitude and phase spectra. Pressure gain (amplification) in the canal can be calculated relative to a free-field measure of sound level in the absence of the head. db HRTFs can be used to create a virtual 3D sound field under headphones (by inverse Fourier transformation).

12 HRTFs in Vertical Plane Free-field HRTFs of the ear at different source elevations in median plane A: Human subject C, Shaw 1980; B: Cat, from Musicant et al : straight ahead 90 : directly above General properties: Strong gain in pressure at 4 or 5 different frequencies across the spectrum; largest gain at 3 5 khz (resonance of concha). Deep spectral notches at 6 8 khz and at ~12 khz. Lower spectral notch (around 6 8 khz) shifts systematically with source elevation.

13 Individual Differences in HRTFs Subject C (see previous slide)

14 Monaural Spectral Cues for Elevation Spectral cues (peaks and notches) vary systematically in vertical plane (roughly constant in horizontal plane). Spectral notches may be associated with specific elevations. On average, HTRF similar across individuals, but precise relationship between elevation and spectral notch is specific to the ear, and to the individual. High-frequency spectral information is important cue for vertical localization. Spectral information is only reliable for broadband sounds; narrow-band sounds (e.g., tones) are difficult to localize in the vertical plane. Binaural sound localization cues

15 The Middle Ear Components: tympanic membrane (ear drum) middle ear bones (ossicles) middle ear muscles. Functions: Impedance Matching Selective oval window force Pressure equalization

16 Tympanic Membrane Tympanic membrane: Thin, cone- or funnel-shaped, transparent membrane. 90 mm 2 total area (human). Supported by radial and concentric fibers. Tympanum: Air-filled space ~2 cc in volume between tympanic membrane and bony promontory of inner ear. Eustachian tube (~35 mm long): connects middle ear to pharynx to equalize air pressure with atmosphere.

17 Ossicles Malleus (Latin, hammer): Long process (manubrium) attached to back of tympanic membrane. Head: large rounded process attached via ligament to incus Incus (L., anvil): Short process (not visible) provides support, suspended by posterior ligament. Inferior, or long process, joins the stapes. head Stapes (L., stirrup): Footplate is attached by a ligament to oval window membrane of the inner ear. incus stapes

18 Impedance Matching 1 The outer ear contains air, whereas the inner ear contains auditory receptors, and is fluid-filled (evolved from bony fish lateral line). The difference in media between the outer and inner ear results in an impedance mismatch between the air and inner ear fluids. About 99% (~30 35 db) of the acoustic energy in aerial signals would be lost (reflected) if sounds impinged directly on the inner ear. Middle ear compensates for the impedance mismatch, acting like an acoustic transformer.

19 Impedance Matching 2 The middle ear overcomes the impedance mismatch in two main ways: force concentration (areas) and lever action. Force concentration: Energy distributed over a larger area is magnified when transferred to a smaller area. Middle ear bones transfer displacements of tympanic membrane to the stapes. Ratio of (effective) area of the tympanic membrane (~55 mm 2 ) to the stapedial footplate area (~ 3.2 mm 2 ) = (55/3.2) = 17, or ~20log(17) = 25 db gain in sound pressure. Lever action: The malleus is 1.3x longer than the incus, thus the force at the stapedial footplate is magnified 1.3x over that at tympanum.

20 Middle Ear: Transfer Function The middle ear increases the pressure from air to fluid in two main ways: the high ratio of the area of the tympanic membrane to that of the stapes footplate, and the lever action of the ossicular chain. The combined (theoretical) gain effect is roughly 20log(17*1.3) = 26 db. The real gain is slightly less than that due to the compliance of structures. The gain of the middle ear varies with frequency because the lever action of tympanic membrane and ossicular chain is frequency dependent.

21 Direct Air Conduction Oval window is a membrane under stapes footplate, round window is a membrane on the opposite side of the cochlear partition from oval window, also facing into middle ear. Pressure waves deflect the cochlear partition, but only if displacement is out of phase at the oval and round windows. Gain of outer ear (~20 db) and middle ear (~20 db) not only reduces impedance mismatch almost completely, it mechanically transfers sound energy directly to the oval window. Without middle ear, sound pressure would act simultaneously on both the oval window AND the round window, displacing them in phase. Result: hearing loss without middle ear is severe, ~40 db

22 Middle Ear Muscles 1 Tensor tympani Runs in bony canal above and parallel to the Eustachian tube. Inserts on the manubrium of malleus. Innervated by trigeminal nerve (cranial nerve V). Stapedius Smallest muscle in body (6 mm-long). Inserts on head of stapes. Innervated by facial nerve (c.n. VII).

23 Middle Ear Muscle Reflex MEMs: effectors of a reflex arc activated at high sound levels (~80 db SPL). Contraction of MEM reduces gain of middle ear, attenuating stimulation of inner ear by ~0.6 db per db of sound level above 85 db SPL. Attenuates best at low frequencies (below 2 khz), so MEM reflex shifts sensitivity to higher frequency. Reflex latency varies from ms: too slow to protect from impulsive sounds. MEMs also activated during vocalization, chewing, swallowing: helps reduce self-stimulation of ear.