Imran Ahmad Sajid,T.A., ISSG, UOP

Sense of Hearing

• Right now, as you are listening to my words, a miracle is taking place, which you have probably never noticed so far.

• Some vibrations emitted by my mouth are producing some waves in the air filling the room you are in.

• These vibrations in the air are entering your ear at a speed of 360 KM per second.

• These vibrations, which have been mere physical motions until now are being transformed into sound by incredibly complex operations taking place in your ears.

• Surprisingly, all these operations occur in less than 1,000th of a second.

• Thanks to this system you hear millions of different sounds, like the conversation you are listening to right now, you can hear thousands of different voices or music and distinguish sounds from one another.

• Moreover, you listen to them, without the interference of any hissing, humming, or rustling sounds, with a perfect quality much better than the world’s highest quality HiFi system.

• So how is all this accomplished? • What kind of structure do the sense of hearing

and do the ear have?• Over and above this to whom are we

indebted for this miracle of hearing?

• A scientific analysis of these questions leads us to conclude the following:

• The ear, the organ associated with hearing, is a marvel of biological design. It is a matchless device, built through the astounding collaboration of many sciences including

1. acoustics (the science of sound), 2. mechanics (the science of energy and forces), 3. hydraulics (study of fluids), and the 4. electronics (the branch of technology concerned with the

design, manufacture, and maintenance of electronic devices).

Before examining the structure of the ear some consideration of the concept of sound will be helpful.

Structure of the Presentation

A. Sound: Physical AspectsB. Structure of the Human Ear

a. Outer Earb. Middle Earc. Inner Ear

C. How do we sense Sounds? a. Place Theory b. Frequency Theory

D. Is listening the only thing our ears do? E. RecapF. Review Question

Does the Sound Exist in the External World?

• No such sounds exist outside. Absolute silence pervades the world.

• There are only vibrations outside. • These vibrations are converted into sounds by the ear

and the brain. • So long there is no ear to hear, there is no sound. • For this reason, if living beings have no sense of hearing, the concept of sound would

not exist. • It would never occur to anyone that the vibration corresponded to sounds. • Everyone would live and die in deep silence. • Yet, thanks to the sense of hearing given to us, we live in a world where we can hear

thousands of sounds. • Take pleasure from them, and communicate with one another.

Figure 1: Graphic representations of a sound wave. (A) Air at equilibrium, in the absence of a sound wave; (B) compressions and rarefactions that constitute a sound wave; (C) transverse representation of the wave, showing amplitude (A) and wavelength (λ).

Sound: Encyclopedia Britannica, 2009 DVD

A. Sound: Physical Aspects

1. Sound waves– Changes in pressure

caused by molecules of air moving

2. Frequency– Number of cycles per

second in a wave, measured in Hertz (Hz)

– Frequency determines pitch of the sound

Sound is the movement of air molecules brought about by the vibration of an object. Sounds travel in the air in wave patterns.

Normal human ear can hear 20 to 20,000 Hz

A.

Sound: Physical Aspects

3. Amplitude/Intensity– Magnitude (height) of sound

wave– Determines loudness,

measured in decibels (dB)

Practical Examples of Frequency and Intensity

A.

The Ear: The Organ for hearing

B.

The Ear

a. Outer Earb. Middle Earc. Inner Ear

Outer Ear Middle Ear Inner Ear

B.

a. Outer Ear

1. Auricle (Pinna)– the curved part of the ear

attached to the side of the head by small ligaments and muscles. It consists largely of elastic cartilage, and its shape helps collect sound waves from the air.

2. Auditory Canal– A tube like passage that leads to

the eardrum

3. Eardrum– Operates like a miniature drum,

vibrating when sound waves hit it.

– Part of the ear that vibrates when sound waves hit it.

Like a reverse mega phone, Designed to collect and bring sounds into the internal portion of the ear.

Eardrum Auricle / Pinna

Auditory Canal

B.

b. Middle Ear

• Contains three small bones; 1. the hammer

(Malleus), 2. Anvil (Incus), and 3. Stirrup (Stapes)

• These bones relay and amplify the incoming sound waves

Acts as a tiny mechanical amplifier.

Hammer Anvil Stirrup

B.

c. Inner Ear

• Oval window– Membrane between

middle ear and inner ear• Cochlea– Part of inner ear

containing fluid that vibrates

– This causes the basilar membrane to vibrate

– looks something like a snail

CochleaOval Window

Portion of the ear that changes the sound vibrations into a form that allows it to be transmitted to the brain.

B.

c. Inner Ear

• Basilar membrane– Membrane in the cochlea

which contains receptor cells, called hair cells

– When these hair cells are bent by the vibrations entering the cochlea, a neural message is transmitted to the brain

• Auditory nerve – Connection from ear to brain– Provides information to both

sides of brain

B.

B.

How are our brains able to sort out wavelengths of different

frequencies and intensities?

c.

a. Theories of Hearing

• Place theory– Pitch is determined by location of vibration along

the basilar membrane– Part of the basilar membrane nearest to oval

window is most sensitive to high frequency sounds

– Part of the basilar membrane nearest to cochlea’s inner end is most sensitive to low-frequency sounds

c.

b. Frequency Theory of Hearing

– Entire basilar membrane acts like a microphone, vibrating as a whole in response to a sound

– Pitch is determined by frequency hair cells produce action potentials

– Higher the pitch of a sound (number of wave crests per second) greater the number of nerve impulses that are transmitted up the auditory nerve to the brain

c.

Is listening the only thing our ears do?

D.

Balance: The Ups and Downs

• Several structures of the ear are related more to our sense of balance than to our hearing.

• Semicircular Canals – Structures of the inner ear consisting of three tubes

containing fluid that sloshes through them when the head moves, signaling rotational or angular movement to the brain

• Otoliths– Tiny, motion-sensitive crystals within the semicircular canals

that sense body acceleration– When we move, the crystals shift like sands on a windy beach.

D.

D.

Recap• The sense of hearing, motion, and balance are centered in the

ear. • The major parts of the ear are

1. the outer ear (which includes the auditory canal and eardrum), 2. the middle ear (which includes the hammer, anvil, and stirrup), and

the oval window leading to the inner ear. 3. The inner ear contains the cochlea, basilar membrane, and hair

cells. • The physical aspects of sound include frequency and intensity. • Place theory and frequency theory are believed to operate in

the transmition of sound waves into the experience of sound. • The sense of balance is located in the ear’s semicircular canals

and otoliths.

E.

Review Questions1. The tube-like passage leading from the outer ear is known as _____?

Auditory Canal2. The purpose of eardrum is to protect the sensitive nerves

underneath it. It serves no purpose in actual hearing… TRUE OR FALSE?

False: it vibrates when sound waves hit it, and transmits the sound3. To what part of ear do the three middle ear bones transmit their

sound??_____Oval window

4. What theory of hearing states that the entire basilar membrane responds to a sound, vibrating more or less, depending on the nature of the sound.____??

Frequency Theory5. The three fluid-filled tubes in the inner ear that are responsible for

our sense of balance are known as ______???Semicircular Canals

F.

THANKS Courtesy: Imran Ahmad Sajid, T.A., ISSG, University of Peshawar. imranahmad131@gmail.com

Questions?

Courtesy: Imran Ahmad Sajid, T.A., ISSG, University of Peshawar. imranahmad131@gmail.com