Sounds and hearing

• Ear


  The body's main receiving system for acoustic wave information (i.e. sound)

  
• Parts of the ear

  • Outer ear
  • Middle ear
  • Inner ear

  
• Outer ear


  • Mechanical system, whose function is to catch and amplify sound

  
• Middle ear


  • Sensory (electrical) system, whose function is to convert mechanical pulses into electrical signals which are passed on by the auditory nerves to the brain

  
• Inner ear


  • Auditory system, whose function is to decode and analyze the electric nerve signals in the auditory cortex (brain)

  
• Main objective of the ear

  Receive the sound, amplify the intensity of the sound, analyze the frequency and intensity structure of the sound, and to reject random background noise

  
• Sound is a longitudinal wave

  
• Speed of sound


  Depends on medium and temperature

  
• Loudness


  How loud or soft a sound seems to a listener, determined by the intensity of sound

  
• Intensity


  Measure of the amount of energy in sound waves, unit is the decibel (dB)

  
• Intensity is a function of the amplitude of the sound waves and how far they have traveled from the source of the sound

  
• Pitch


  How high or low a sound seems to a listener, depends on the frequency of sound waves

  
• Pinna


  The visible part of the ear, serves as a sound reflector

  
• Resonance


  Natural amplification of a sound by the structures of the outer ear, the outer ear starts vibrating at the same frequency as the sound wave from the external source hitting the tympanic membrane

  
• Ear canal resonance


  Amplifies sound in the high frequencies (for an adult, typically in the region between 2000-4000 Hz), particular to the individual and depends on the length, volume and curvature of the canal

  
• Loudness


  Varies with intensity and frequency, described using the unit phon

  
• The solid lines on the graph indicate isophones, or curves of constant loudness as a function of intensity and frequency

  
• The dips in the isophones at frequencies 3,000 Hz and 8,000 Hz signify that lower intensities correspond to higher loudness because of the resonance effect in the outer ear canal

  
• Eardrum


  Approximately 0.5 mm thick membrane with a surface area of about 65 mm2, separates the outer ear canal from the middle ear cavity

  
• Purpose of eardrum


  • Absorb and transmit the pressure variations caused by acoustical waves in the outer ear canal

  
• The thinness of the eardrum means it is easily ruptured by large pressure variations between outer and middle ear (a pressure difference of about 8,000 Pa) or intense noise (about 160 dB)

  
• Eustachian tube


  Connects the middle ear and the mouth cavity for pressure equalization, normally closed but opens when swallowing

  
• Sound waves hitting the eardrum causes a partial reflection and transmission of the sound wave

  
• Middle ear


  Air-filled cavity connected by the Eustachian tube with the mouth cavity, dominated by the three middle bones (ossicles) - malleus (hammer), incus (anvil), and stapes (stirrup)

  
• Purpose of middle ear bones

  • Serve as mechanical impedance matching and amplifying system for the transmission of the eardrum vibrations towards the inner ear, act like a lever system to amplify the eardrum membrane vibrations

  
• The ossicle lever system in the middle ear is most efficient in the frequency range of 400 to 4,000 Hz, for higher and lower frequencies the stiffness and mass of the ossicle system limits its efficiency

  
• Inner ear


  Protected within the skull, consists of a spiral-shaped system of three parallel tubes (the cochlea) filled with an anionic (negatively charged) fluid

  
• Vibrations of the oval window


  1. Transmit pressure variations to the fluid in the closed vestibular and tympanic chambers of the cochlea

  2. Cause a vibration in the basilar membrane of the cochlear duct toward the tip of the cochlea

  3. Basilar membrane carries the organ of Corti, covered by fine hair sensors which are excited by the vibration and transmit the information to the brain

  
• Ear diseases


  • See provided link