SPA 127 - Ch. 12: Hearing Science

Sound Waves

• Sound is vibration of particles created by a source

• These vibrations travel in waves

• Waves repeat in cycles of compression and rarefaction

• Waves travel through a medium

  • E.g., air, water, bone

Vibration Source

Examples:

• A guitar strong

• The head of a snare drum

• The diaphragm of a loudspeaker

• The vocal cords

Measuring Sound: Frequency

• Frequency

  • Number of cycles (vibrations) per second

  • Perceived as pitch

  • Unit of measurement: Hertz (Hz)

  • For example, 1,000 Hz means there are 1,000 cycles in one second

Human Hearing Range (Frequency)

• Humans hear from 20 Hz to 20,000 Hz

• The most important speech frequencies are from 250 Hz to 8,000 Hz

  • Middle C on a piano = 262 Hz

Measuring Sound: Amplitude

• Amplitude

  • Maximum displacement in the positive and negative direction

  • Perceived as loudness

  • Unit of measurement: decibel (dB)

Human Hearing Range (Amplitude)

• 140 decibels: sound becomes painful

• ≥ 90 decibels: hearing protection required in industrial jobs

• 60 decibels: average conversation

• 0 decibels: softest amplitude the average person can perceive

Simple vs. Complex Sounds

• Simple sounds

  • Vibrate at a single frequency

  • Ex. Pure tone

  • Almost simple: Whistling, tuning fork

• Complex sounds

  • Vibrations that contain two are more frequencies

  • Nearly all sounds are complex

  • Ex. Speech, music, noise

Human Auditory System

• Peripheral Auditory System

• Central Auditory System

Peripheral Auditory Systen

• Outer Ear

• Middle Ear

• Inner Ear

Outer Ear

• Pinna (auricle)

  • Cartilaginous structure

  • Collets and directs sound waves into external auditory canal

• Ear canal (external auditory canal)

  • 2.5 cm in length for adults

  • Outer 1/3 is cartilage, Inner 2/3 is bony

  • Funnel sound waves to the eardrum

  • Contains cerumen glands (earwax)

Outer Ear: How Does Hearing Work?

• The pinna collects sound waves and works like a funnel to send them through the ear canal

• The pinna helps us determine the direction of incoming sound (localization)

• The ear canal amplifies sound waves from 2,000 to 5,000 Hz

Middle Ear

• Tympanic Membrane

  • Separates the outer ear and the middle ear

  • Sound causes it to vibrate and transmit sound to the ossicles

• Ossicles

  • Transmits vibration into fluid movement in inner ear

  • Malleus: attached to tympanic membrane

  • Incus: between malleus and stapes

  • Stapes: footplate of stapes inserts into the inner ear

Middle Ear: How Does Hearing Work?

• The tympanic membrane is a thin membrane that vibrates when sound waves strike it

• The stapes pushes into the inner ear, causing movement of fluid

• Middle ear amplifies incoming sound by about 30 decibels

Inner Ear

• Consists of:

  • Cochlea: Sensory organ for hearing

  • Vestibule & semicircular canals: Sensory organs for balance

Inner Ear

• Cochlea is a spiral-shaped organ with 3 fluid-filled chambers

• Contains thousands of hair cells that pick up fluid vibrations and cause impulses in the auditory nerve

Inner Ear: How Does Hearing Work?

• Tiny sensory receptors called hair cells lie on top of the basilar membrane

• Fluid movement in the cochlea causes basilar membrane to move up and down

• Hair cells are deflected, allowing fluid to enter them

• Fluid entering the inner hair cells causes release neurotransmitter that stimulates the auditory nerve

• Outer hair cells can change their shape, amplifying basilar membrane motion for quiet sounds

Central Auditory Pathway

• 8th cranial nerve (vestibulocochlear) transmits impulses to the brainstem

• Information from the two ears is combined in the brainstem

• Brainstem transmits signals to the auditory cortex where it is perceived as sound

Two Mechanisms of Hearing

• Air conduction: Sound waves travel through air and pass through outer ear, middle ear, and inner ear

• Bone conduction: Vibrations are sent through skull to directly stimulate the inner ear