|t o p i c s|
|The Ear: Introduction / Outline|
Overviews of Ear Anatomy and Function
The Outer Ear & The Middle Ear
The Inner Ear
The Ear: Introduction
Figure 1: Simplified graph of the ear (above).
The picture to the right is a microphotograph of a dissected and isolated cochlea, the organ of the inner ear that translates mechanical sound motion into neurological signals. This complex structure has many different cell types, the exact functions of which are still not fully understood.
Figure 2: Top: Simplified graph of a stretched-out cochlea (after Campbell and Greated, 1987). Bottom: The middle/inner ear action (from the Venetian Institute of Molecular Medicine)
Figure 3: Cross-section schematic diagram of a stretched-out cochlea
The Ear's Transduction Process
The ear acts as a transducer and the transduction process can be summarized as follows:
i) Sound waves generated by a vibrating system (source) reach via the air the pinna (external part of the outer ear), which funnels the wave energy through the ear canal (an ~ 2.5cm-long, almost cylindrical tube amplifying frequencies relevant to speech) and towards the tympanic membrane or eardrum (a thin, stretched, approximately circular membrane ~10mm in diameter), setting it into motion.
Similarly to the outer ear but to a lesser degree, the middle ear (e.g. length and motion of the ossicles) also helps amplify frequencies relevant to speech.
iii) The oval window imparts the vibrations to a viscous fluid (perilymph) inside the scala vestibuli portion of the cochlea, in the inner ear.
iv) Vibrations in the scala vestibuli set another membrane into motion (Reissner's membrane), which imparts the vibration to a different fluid (endolymph) inside scala media.
v) Vibrations in scala media set the basilar membrane (BM) in a special kind of motion (towards the oval window for high frequencies and the helicotrema for low frequencies) that is referred to as "travelling wave," arising due to the basilar membrane's special resonant characteristics (details below).
vi) The motion of the BM disturbs the organ of Corti (laid over the BM), causing some sensitive filaments (hair cells) extending from its top to shear against the tectorial membrane (which tops the organ of Corti), generating electric impulses.
vii) The impulses travel along the auditory nerve pathways to the brain, entering a complex electrochemical network where the sensation of sound is registered. Excess fluid vibrations that reach the scala tympani portion of the cochlea (via the helicotrema), exit the cochlea through the round window.
The inner ear is populated by two types of hair cells:
The inner ear supports transduction of auditory signal information into electrical brain signals in terms of both place and temporal coding, while interaction of hair cells with the incoming signal may result in excitation or inhibition of a hair cell's electrical response. More on auditory hair cells and nerve fibers later in the semester.
The two clips below outline the pressure-variations-to-electric-impulses transduction process occurring in the ear.
A collection of interconnected, weakly coupled flexible fibers located in the inner ear (cochlea). It
is a tuned resonator that analyzes complex waves into
sinusoidal components, vibrating at
different places in response to incoming waves of different frequencies, and
with different amplitudes in response to incoming waves of different
|The intensity range of hearing extends from I0=~10-12
W/m2 (0dB) to
I=~1W/m2 (120dB) [at 1000Hz]. As mentioned in Module 2, because intensity
(physical) magnitudes relate
logarithmically to loudness (perceptual) magnitudes, sound intensity
level (SIL) is measured on a logarithmic scale, ranging from 0dB to
120dB. As a reminder, SIL in dB (decibels) is defined
as 10log10 I/I0.
We will return to the topic of loudness in the following weeks.
SILs below 0dB are inaudible, while SILs above 120dB can be damaging to the ear. These two values outline the absolute thresholds for intensity. Exposure to high intensity sounds can result in temporary (TTS or temporary threshold shift: temporary reduction of the sensitivity of hair cells) or permanent damage of the hair cells (Figure 4).
The basilar membrane gets stiffer with age, resulting in loss of sensitivity especially for high frequencies, a condition called Presbycusis.
Figure 4: Healthy (top) and damaged
(below) hair cells (after Curtis, 1979).
Term introduced by Fletcher in the 1940s to refer to the frequency bandwidth of the, then loosely defined, auditory filter. Since von Békésy’s studies (1930s-1960s), the term also refers literally to the specific area on the basilar membrane that goes into vibration in resonance with an incoming sine wave. Its
length is determined by the elastic properties of the membrane and has an average value of ~1mm,
representing ~1/3 of an octave (at middle frequencies).
The actual length of the critical band corresponds, therefore, to a frequency-difference value called critical bandwidth. If the frequency difference between two simultaneous sine waves is within the critical bandwidth, the ear will not be able to resolve the two frequencies, while the waves will interact in a specific and musically important way:
If the frequency difference is < 10-20 Hz (approx.), the wave interaction will be perceived as a slow loudness fluctuation called beating. If the frequency difference is > 20 Hz (approx.) but smaller than the critical bandwidth, the interaction of the two simultaneous waves will be perceived as a change in the character of the combined sound referred to as roughness. Based on these observations, critical bandwidth may be defined as the frequency separation in Hz. between two simultaneous sine waves necessary for beats/roughness to disappear and for the resulting tones to sound clearly apart.
Both, the beating and roughness sensations are perceptual attributes of amplitude fluctuation resulting from interference (discussed previously). Psycho-physiologically, the beating and roughness sensations can thus be linked to the inability of the auditory frequency-analysis mechanism to resolve inputs whose frequency difference is smaller than the critical bandwidth and to the resulting instability or periodic “tickling” (Campbell and Greated 1987: 61) of the mechanical system (basilar membrane) that resonates in response to such inputs.
[We will return to beating and roughness, when discussing consonance and dissonance.]
Figure 5: As the interval between two tones decreases, their respective disturbances on the basilar membrane (critical bands) increasingly overlap, resulting in the sensations of roughness and beating (after Campbell & Greated, 1987).
Overviews of Ear Anatomy
Student activities from the
National Institute of Health website.
Although the materials are designed for younger audiences (high school students), they present a good basic overview of our topic.
The Ear Pages game from Nobelprize.org.
Skip the introduction to enter the main page of the game that includes three sections: The Ear, Georg Von Békésy, and Quiz. Spend some time exploring the section on The Ear. Start with the 'Explore' subsection and continue as desired (in the 'How we Hear' subsection select the 'Expert' level).
|Very well-designed and concise overview of the anatomy and function of the ear, including basic historical information on the three major figures in the history of hearing research: Corti, Helmholtz, and Von Békésy. Part of The Cochlea Homepage, at the University of Padova, Italy.|
The Auditory System Tour slideshow at the Department of Neurophysiology, University of Wisconsin, Madison.
This overview goes into a little more detail, especially in terms of the inner ear. See the sections on the individual parts of the hearing mechanism, below, for more.
The Ear from
Gray's Anatomy (the
book, that is...), available through
More detail than you may currently need but a standard reference resource. Will be useful if your final project involves ear physiology.
The Outer Ear & The Middle
|Outer ear outline in a flash animation (to the right) from the Honk Kong Environmental Protection Department website.||
|Click here and here for two simple animations of middle-ear motion from the Ear Works site by the Department of Neurophysiology, University of Wisconsin, Madison.|
|To the right are screen captures from eardrum and ossicle motion animations at the Wada Laboratory of genetic engineering site, Tohoku University, Japan. Note the different modes of vibration on the membrane and on the ossicles for different excitation frequencies.|
Animation clip of the data from eardrum motion measurements
on an American bullfrog ear made at UCLA's Physiological Science
The Inner Ear
|Anatomy and Theory sections of The Cochlea Homepage (University of Padova, Italy). Do not worry about equations and other math details. Approach the pages conceptually and focus on the illustrations, photographs, and animations.|
|Basilar membrane vibration animations and in-vivo (alive) vs. in-vitro (dead) vibration measurements from the Wada Laboratory of genetic engineering at Tohoku University, Japan.|
|Motion of the Organ of Corti and inner hair-cell action from the Wada Laboratory of genetic engineering, Tohoku University, Japan.|
|Outer hair-cell motility, explanation, and amplification effect from the Wada Laboratory of genetic engineering, Tohoku University, Japan.|
Finally, a discussion, with images, on
How Information from the
Cochlea Reaches Targets in the
Columbia College, Chicago - Audio Arts & Acoustics