Plack, 2005: Chapter 4 & Chapter 5 (only section 5.2.5)


Lecture notes


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 / Outline


Cross-section of the ear

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.
Stretched-out graph of the cochlea
 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)
Cross-section of the cochlea
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. 

    From "Introduction to Psychology," Walden University

ii) The motion is transmitted to the cochlea, within the inner ear, through three delicate bones or ossicles, within the middle ear  [malleus (hammer), incus (anvil), stapes (stirrup)].  These bones set in motion the oval window, an oval membrane attached to the stirrup via the footplate and with area ~20 times smaller than that of the eardrum. The area difference results in a pressure increase, amplifying the incoming signal and reducing the impedance mismatch between the middle and inner portions of the ear. This mismatch is further reduced through the ossicles' lever action and the "buckling" motion of the eardrum (see your textbook for details).
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

Inner-Ear Innervation

The inner ear is populated by two types of hair cells:
a) Inner hair cells, whose disturbance sends auditory messages to the brain down a complex auditory pathway, in the form of electrical impulses, and
b) Outer hair cells that send information to the base of the cells, influencing the cells' length and general response in a feedback mechanism that allows the hearing apparatus to adjust its sensitivity based on the incoming signal and on messages from the brain.
Consequently, the hair cells in the inner ear are innervated by two types of nerve fibers:
a) Afferent fibers that carry information from the ear into the brain and are connected mostly to inner hair cells and
b) Efferent fibers that carry information out of the brain into the ear and are connected mostly to outer 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.


Clip from
Gateway to the Mind
© 1958 Jack Warner / Bell Science Productions
The information in this clip is rather outdated, especially in terms of the inner ear, as is the information implied in a more recent animation posted on the Rockefeller University's website


Clip from
The Body Atlas - Vol. 5: Now Hear This
Directed by: Peter Macpherson, Vivienne King, Thelma Rumsey, Alec Nisbett
© 1994 The Learning Channel / Pioneer Productions / Ambrose Video Publishing
This clip better represents our current knowledge on the function of the inner ear.


Basilar membrane (BM): 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 intensities.

      250Hz:  Boystown Research Hospital            1kHz:  Boystown Research Hospital            4kHz:  Boystown Research Hospital

The resonance range of the BM and, therefore, the frequency range of hearing (i.e. absolute thresholds for frequency) extends from ~20Hz to ~20.000Hz (20KHz). On average, frequencies below 20Hz sound as individual pulses with no definite pitch, while frequencies above 20KHz are inaudible by humans. For high frequencies, the basilar membrane vibrates close to the entrance of the cochlea, while for low frequencies it vibrates towards the helicotrema. Tiny hair cells (nerve endings) on the organ of Corti are pushed against the tectorial membrane by the motion of the basilar membrane, translating this motion into electric impulses.

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).
Click here for more on hair-cell damage and regeneration from the Virginia Merrill Bloedel Hearing Research Center website (University of Washington).

Critical band: 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).
Harmonic components & Critical Bandwidth

Figure 6: The first 12 components of C3, shown as black circles on a stretched music-notation 'stave'.
The vertical bars indicate the approximate critical bandwidth around each component
(after Campbell and Greated, 1987).

Basilar Membrane Signals

Figure 7: A schematic diagram illustrating the signals sent to the brain when the basilar membrane is vibrating in response to a complex wave with many sine components.  Each low-frequency component sends individual signals since, as indicated in Figure 6, the frequency separation between the low-frequency components is larger than the critical bandwidth.  The upper components/harmonics are 'unresolved' because, as the component number increases, more components fall within the same critical band (after Campbell and Greated, 1987).




Overviews of Ear Anatomy and Function


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
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.
Chapter on The Ear from Gray's Anatomy (the book, that is...), available through Yahoo! Education.
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 Ear


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 department.


Finally, scan through the External and Middle Ear portions of The Cochlea Homepage (University of Padova, Italy)




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.         

_ Cochlear Mechanics From the Communication Engineering Laboratory, Boys Town National Research

_ Inner ear action animations from the Ear Works site by the Department of Neurophysiology, University
   of Wisconsin, Madison.

_ Finally, a discussion, with images, on How Information from the Cochlea Reaches Targets in the
   Brainstem, courtesy of the National Library of Medicine.
   Make sure you explore this graph discussed in class!


Columbia College, Chicago - Audio Arts & Acoustics