HearingVivian Lam

Estefhani ChangJessy Feng Han

Jenny Chen

Aim: How do we hear?

Why can we hear?Audition: sense or act of hearing which is highly adaptive

● Humans are social creatures● Allows for communication and connections between people● We are most attuned to hear human voices:

- Recognize specific voices by the stimulation of ear receptors which send impulses to the brain

and compares past experiences in order to recognize the voice.

Sound WavesJostling air molecules leads to waves of compressed and expanded air. Ears detect these changes in air pressure and transforms them into neural impulses, when becomes sound. Combination of air and bone conduction (vibrations).

Amplitude of sound waves determines their loudness while frequency determines pitch:

● Frequency: number of complete wavelengths that pass a point in a given time

● Pitch: a tone’s experienced highness or lowness; depends on frequency

● Long waves have low frequency and low pitch● Short waves have high frequency and high pitch

Sound Waves cont’dSound is measured in decibels (dB) with zero decibels representing the absolute threshold for hearing

Every 10 dB corresponds to a tenfold increase in sound intensity. So a normal conversation (60 dB) is 10,000 more intense than a 20 dB whisper

Structure of the Ear Outer Ear: visible portion of the ear and is responsible for collecting and sending sounds into the eardrum

Middle Ear: chamber between the eardrum and cochlea containing three tiny bones (hammer, anvil, stirrup) that concentrate the vibrations of the eardrum on the cochlea’s oval window

Inner Ear: innermost part of the ear, containing the cochlea, semicircular canals, and vestibular sacs

Cochlea: a coiled bony, fluid-filled tube in the inner ear; sound waves traveling through the cochlear fluid trigger nerve impulses

Auditory canal

Eardrum Hammer

Anvil

Stirrup

Cochlea

Auditory nerve

Outer Ear Middle Ear Inner Ear

Oval window

Semicircular canals

How do we Hear?

Process1. Sound waves enters the outer ear and it channels the waves through auditory

canal to eardrum, causing it to vibrate.2. The middle ear contains the hammer, anvil, and stirrup which picks up the

vibrations and transmits them to the cochlea in the inner ear3. The incoming vibrations cause the oval window to vibrate, jostling the fluid that

fills the cochlea.4. The motion causes ripples in basilar membrane, bending the hair cells lining its

surface5. Hair cell movement triggers impulses in the adjacent nerve cells6. Axons of these cells converge to form the auditory nerve which sends neural

messages (via the thalamus) to the auditory cortex

Hearing LossHair Cells: ~16,000 in the cochlea. “Extreme sensitivity and extreme speed.”

Sensorineural hearing loss (nerve deafness) : hearing loss caused by damage to the cochlea’s hair cell receptors or their associated nerves

-Disease but most likely heredity, aging, and prolonged exposure to loud noises

Conduction hearing loss: hearing loss caused by damage to the mechanical system that conducts sound waves to the cochlea

-Ringing indicates mistreatment of hair cells

Hearing LossSounds above 85 dBA can cause permanent damage.

The more sound pressure, the less time it takes to cause damage. (85 dBA 8 hrs, 100 dBA 30 min)

The impact of noise also adds up. Risk increases over time.

- A mobile device at its loudest is ~105 dB, 100 times more intense than 85 dBA.

Teen hearing loss is 1 in 5, up by a third since early 1990s

Test Your Hearing!

TreatmentsA hearing aid has a microphone to receive sound, it then converts the sound wave to electrical signals and sends it to the amplifier to increase the signal’s power. The signal is then sent to the ear through a speaker.

Cochlear implant: a device for nerve deafness which converts sounds into electrical signals and stimulates the auditory nerve through electrodes threaded into the cochlea

- Does not enable hearing for those born deaf

How do we detect loudness? Loudness is interpreted by the brain from the number of activated hair cells.

- the louder the sound, the more hair cells are activated

If a hair cell loses sensitivity to soft sounds, it may still respond to loud

sounds.

Hearing differs in our sensation of soft sounds.

Hearing aids compress sounds; sounds that are harder to hear are amplified

more than loud sounds.

Perceiving PitchPlace theory: in hearing, the theory that links the pitch we hear with the place where the cochlea’s membrane is stimulated

But… neural signals generated by low-pitched sounds are not as neatly localized as those generated by high-pitched sounds

Frequency theory: in hearing, the theory that the rate of nerve impulses traveling up to the auditory nerve matches the frequency of a tone, thus enabling us to sense its pitch (low pitches)

But… an individual neuron cannot fire more than 1000 times per second

Volley principle: by firing in rapid succession, neural cells can achieve a combined frequency above 1000 waves per second

Locating SoundsSound waves strike one ear sooner and more intensely than the other. The

brain analyzes the minute differences in the sounds received by the two

ears and computes the sound’s source

EX: When a car to the right honks, the right ear receives a more intense

sound sooner than the left ear.

This is because sound travels at 750 miles per hour and our ears are only 6

inches apart so the differences are very small