Auditory Theory: Acoustics

Lecture 006 Hearing I

Reading Assignment for Lecture 007

Before next class please read Sections

  • 2.3.n Frequency and Pressure Sensitivity ranges
  • 2.4.n Loudness perception

pages 79 to 91 of Acoustics and Psychoacoustics.

We may have a brief quiz on these sections at the beginning of the next class.

 

Brain Bullets

  • The ear consists of three parts
    • the outer ear
    • the middle ear
    • the inner ear
  • Outer Ear
    • Pinna
    • Gathers and amplifies audio
    • Concha assists us in identifying the location of sounds that are in front of or behind us and to some extent above and below us
    • Transfers energy from the air to the eardrum (tympanic membrane) via the Auditory Canal
    • the Auditory Canal has a resonant frequency around 4 kHx
  • Middle ear
    • To transfer the movements of the tympanic membrane to the fluid which fills the cochlea without significant loss of energy
    • To protect the hearing system to some extent from the effects of loud sounds.
    • Ossicles multiuplay gain by 3
    • Tensor timpani and the stapedius muscles contract automatically in response to sounds with levels greater than approximately 75 db and they have the effect of increasing the impedence of the middle ear by stiffening the ossicular chain
    • This is called the acoustic reflex: approximately 12 to 14 db of attenuation is provided
  • Inner Ear
    • Cochlea
      • tube coiled into a spiral with approximately 2.75 turns
      • outer channels, the scala vestibuli and scala tympani , are filled with an incompressible fluid known as perilymph
      • the inner channel is the scala media.
      • scala vestibuli terminates at the oval window and the scala tympani at the round window
      • travelling waves in the scala vestibuli displace the basilar membrane.
    • Organ of Corti
      • is the sensitive element in the inner ear and can be thought of as the body's microphone
      • contains four rows of hair cells which protrude from its surface
      • Above them is the tectoral membrane which can move in response to pressure variations in the fluid- filled tympanic and vestibular canals.
      • There are some 16,000 -20,000 of the hair cells distributed along the basilar membrane which follows the spiral of the cochlea.
    • Basilar Membrane
      • Triangular membrane taper in thicknes to the thin end
      • Resonates at a location proportional to the frequency
  • The Place Theory
    • High frequency sounds selectively vibrate the basilar membrane of the inner ear near the entrance port (the oval window).
    • Lower frequencies travel further along the membrane before causing appreciable excitation of the membrane.
    • The basic pitch determining mechanism is based on the location along the membrane where the hair cells are stimulated.
    • Cochlea = 2 3/4 turns,
    • about 3.2 cm length.
    • Resolves about 1500 separate pitches
    • with 16,000-20,000 hair cells.
  • Sharpening of Pitch Perception
    • One way to sharpen the pitch perception would be bring the peak of the excitation pattern on the basilar membrane into greater relief by inhibiting the firing of those hair cells which are adjacent to the peak.
  • Critical Bands
    • The critical bandwidth is that bandwidth at which subjective responses rather abruptly change.
  • ERB
    • Equivalent Rectangular Bandwidth
    • a filter with an ideal rectangular frequency response curve which passes the same power as the filter in the ear
    • ERB = ([6.23 x 10-6 x fc2] + [93.39 x 10-3 x fc] + 28.52] Hz
    • The critical bandwidth varies from a little less than 100 Hz at low frequency to between two and three musical semitones (12 to 19%) at high frequency