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					            Course overview
• What does the auditory system (ear + brain)
  look like and how does it work? (2½ wk)
• How is sound coded in the auditory pathway
  and how does the nature of the code affect
  hearing? (5 wk)
• How does the brain use the code for sound to
  construct the “auditory scene”? (2½ wk)
What does the auditory system look
   like and how does it work?
The Conductive Apparatus

How sound is conducted into the ear
 and how sound is affected by that
Anatomy of the Conductive Apparatus

• The layout of the ear
• Anatomy of external and middle ear
• Response of external and middle ear to
   The bottom line
Although the conductive apparatus does
not (for the most part) actively modify
sound that enters the ear, its passive
interactions with sound modify incoming
sound in a way that will turn out to be
important for hearing.
The Ear
                                  External Ear
The pinna
                                Middle Ear

 1999, Electronic Deaf Education Network
                            Middle Ear Ossicles

                     Middle Ear Mechanics

                              Eustachian Tube

How the external ear “works”
    Frequencies near the resonant
 frequency of a device or object have
A. Smaller amplitudes than other frequencies
B. No amplitude. The incident and reflected
   waves cancel out.
C. Larger amplitudes than other frequencies
   Which of the following is the best
   description of the external ear?
A. A tube open at both ends
B. A tube open at one end
C. A tube open at one end with a shell-like
The resonant frequency of a tube open at
 one end will have a wavelength equal to
 A.   ¼ the length of the tube
 B.   ½ the length of the tube
 C.   The length of the tube
 D.   2 times the length of the tube
                        Transfer functions
            This goes in.                       This comes out.


            Frequency                            Frequency

(What would you call this sound?)
             Which one is transfer function of a
                     bandpass filter?
A.                           B.

                 Frequency                      Frequency


                 Frequency                      Frequency
             Head-related transfer function

From Gelfand (1998)
             Transfer functions of different
            components of the external ear

From Gelfand (1998)
       Positions in the azimuthal plane

From Gelfand (1998)
      Dependence of HRTF on position of
               sound source

From Pickles (1988)
     Differences in the inputs to the two

From Gelfand (1998)
Possible functions of the external ear?
                (pick 2)

A. “boosting” certain sound frequencies
B. Helping us to figure out where a sound is
   coming from
C. “catching” sound
D. Keeping loud sound out
              Function of the middle ear
The ear canal is filled with ______; the
   inner ear is filled with _______.
A.   Fluid, air
B.   Air, fluid
C.   Fluid, fluid
D.   Air, air
 True or false? Fluid opposes the flow
    of sound energy more than air.
A. True
B. False
Opposition to the flow of sound
        energy is called
 Transfer function of the middle ear

From Pickles (1988)
                 Mechanisms of middle ear
                   impedance matching

                              • Areal ratio of the
                                tympanic membrane
                                to the stapes footplate
                              • Lever action of the

From Pickles (1988)
                            Ossicular Motion

                 Mechanisms of middle ear
                   impedance matching
                             • Areal ratio of the
                               tympanic membrane
                               to the stapes footplate
                             • Lever action of the
                             • Buckling of tympanic

From Pickles (1988)
     How big of a hearing loss would we
        have without a middle ear?
A.   20 dB
B.   30 dB
C.   45 dB
D.   60 dB
     Acoustic Reflex
The “active” middle ear function
                      Middle ear muscles

From Gelfand (1998)
Stimuli that elicit the acoustic reflex

       • Intense sound
         – ipsilateral or contralateral
       • Motor activity
       • Speaking
  Characteristics of the acoustic reflex
• Threshold 85-100 dB      • More effective in
  SPL for 250-4000 Hz        reducing low frequency
  tones, 20 dB lower for     transmission
  broadband noise          • Reflex “decay” occurs,
• Ipsilateral reflex         especially at high
  threshold lower than       frequencies
  contralateral            • Latency of 40-150 ms.
    Functions of acoustic reflex?
• Inner ear protection
• Ongoing adjustments in input to inner ear so
  input is constant
• Smoothing out the middle ear transfer
• Improving hearing in the presence of low-
  frequency competing sounds
• The passive properties of the external and
  middle ear modify the spectrum of sounds
  entering the ear.
• These modifications depend on the
  direction from which a sound arrives.
• The middle ear functions to overcome the
  impedance mismatch between air and
  inner ear fluid.
• The auditory system regulates middle ear
  stiffness to modify the sound delivered to
  the inner ear.
               Text sources
• Gelfand, S.A. (1998) Hearing: An introduction
  to psychological and physiological acoustics.
  New York: Marcel Dekker.
• Pickles, J.O. (1988) An introduction to the
  physiology of hearing. Berkeley: Academic