CHAPTER (6)

THE PERCEPTION OF

SOUND Dr.khitam Y. Elwasife

• The study of the physical structure of the ear is a study in physiology. The study of human perception of sound is one of psychology and psychoacoustics. Psychoacoustics is an inclusive science embracing the physical structure of the ear, the sound pathways and their function, the human perception of sound, and their inter relationships.

• the ear develops into three different structures: the inner ear the middle ear and the outer ear The inner ear consists of the vestibule ear organ .The middle ear consists of the tympanicطبلاني cavity . The external ear consists of the auricle and the external acoustic .Each structure originates form different layers or tissues: the ectoderm , endoderm. The ear begins to appear during the 22nd day of embryo tic development

Ear Anatomy

Sensitivity of the Ear

• The sensitive nature of our hearing can be determine by a thought experiment. The bulky door of an chamber is opened, revealing extremely thick walls, and 3-ft wedges of glass fiber, pointing inward, lining all walls, ceiling, and what could be called the floor, except that you walk on an open steel grillwork.

• You sit on a chair. This experiment takes time, and you lean back, patiently waiting. It is very eerie in here. The sea of sound and noises of life and activity in which we are normally immersed and of which we are ordinarily scarcely conscious is now conspicuous by its absence

• Understanding how humans hear is a complex subject involving the fields of physiology, psychology and acoustics. In this part of Lesson 2, we will focus on the acoustics (the branch of physics pertaining to sound) of hearing.

• We will attempt to understand how the human ear serves as an good transducer, converting sound energy to mechanical energy to a nerve impulse that is transmitted to the brain.

• The ear's ability to do this allows us to perceive the pitch of sounds by detection of the wave's frequencies, the loudness of sound by detection of the wave's amplitude and the timbre of the sound by the detection of the various frequencies that make up a complex sound wave.

The ear consists of three basic parts - the outer ear, the

middle ear, and the inner ear. Each part of the ear serves a

specific purpose to do detecting and interpreting sound. The

outer ear serves to collect and channel sound to the middle

ear. The middle ear serves to transform the energy of a

sound wave into the internal vibrations of the bone structure

of the middle ear and finaly transform these vibrations into a

compression wave in the inner ear. The inner ear serves to

transform the energy of a compressional wave within the

inner ear fluid into nerve impulses that can be transmitted to

the brain. The three parts of the ear are shown below

Anatomy of the Ear

Major Divisions of the Ear

Peripheral Mechanism Central Mechanism

Outer

Ear

Middl

e Ear

Inner

Ear

Cranial

Nerve Brain

• consists of an earflap and an approximately 2-cm long ear canal. The earflap provides protection for the middle ear in order to prevent damage to the eardrum. The outer ear also channels sound waves that reach the ear through the ear canal to the eardrum of the middle ear. Because of the length of the ear canal, it is capable of amplifying sounds with frequencies of approximately 3000 Hz. As sound travels through the outer ear,

•the sound is still in the form of apressure wave, with an alternating pattern of high and low pressure regions. It is not until the sound reaches the eardrum at the interface of the outer and the middle ear that the energy of the mechanical wave becomes converted into vibrations of the inner bone structure of the ear.

The outer ear is the external portion of the ear ,which consists of

the pinna and external auditory meatus .It gathers sound energy and focuses it

on the eardrum

Pinna

Pinna • The visible portion that is commonly referred to as "the ear"

• Helps localize sound sources

• Directs sound into the ear

• Each individual's pinna creates a distinctive imprint on the acoustic wave traveling into the auditory canal

• Extends from the pinna to the tympanic membrane طبلاني

– About 26 millimeters (mm) in length and 7 mm in diameter in adult ear.

– Size and shape vary among individuals.

• Protects the eardrum

• Flexible

– Sensitivity to sounds greatest in this frequency region

– Noises in this range are the most hazardous to hearing

Function of Outer Ear

• Collect sound

• Localization

• Resonator

• Protection

• Sensitive (earlobe)

• Other?

The outer ear includes the pinna (also called auricle), the ear canal, and the very

most superficial layer of the ear drum (also called the tympanic membrane). In

humans, and almost all vertebrates, the only visible portion of the ear is the outer

ear. Although the word "ear" may properly refer to the pinna (the flesh covered

cartilage appendage on either side of the head), this portion of the ear is not vital

for hearing .

Function: The pinna helps direct sound through

the ear canal to the tympanic membrane (eardrum (

• The ear canal also increases the loudness of the sounds traversing it. the ear canal, with an average diameter of about 0.7 cm and length of about 2.5 cm, is idealized by straightening and giving it a uniform diameter throughout its length. Acoustically, this is a reasonable approximation, closed at the inner end by the eardrum.

• The middle ear

• is an air-filled cavity that consists of an eardrum and three tiny, interconnected

bones - the hammer, anvil, and stirrup. The eardrum is a very durable and tightly

stretched membrane that vibrates as the incoming pressure waves reach it.

• a compression forces the eardrum inward and a rarefaction forces the eardrum

outward, thus vibrating the eardrum at the same frequency of the sound wave.

• The opening of the Eustachian tube is also within the middle ear .

• Transmitting sound energy from a low dense medium such as air into a high dense

medium like water. Without some transfer mechanism, sound originating in air

bounces off water like light off a mirror. For efficient energy transfer, the two

impedances must be matched; in this case the impedance ratio is about 4,000:1, the

ear must provide a way for energy in air to enter the ear’s fluid interior

The Middle Ear

• Being connected to the hammer, the movements of the eardrum will set the hammer, anvil, and stirrup into motion at the same frequency of the sound wave.

• The stirrup is connected to the inner ear; and thus the vibrations of the stirrup are transmitted to the fluid of the inner ear and create a compression wave within the fluid. The three tiny bones of the middle ear act as levers to amplify the vibrations of the sound wave.

• Due to a mechanical advantage, the displacements of the stirrup are greater than that of the hammer. Furthermore, since the pressure wave striking the large area of the eardrum is concentrated into the smaller area of the stirrup, the force of the vibrating stirrup is nearly 15 times larger than that of the eardrum.

• This feature enhances our ability of hear the faintest of sounds. The middle ear is an air-filled cavity that is connected by the Eustachian tube to the mouth. This connection allows for the equalization of pressure within the air-filled cavities of the ear.

• When this tube becomes clogged during a cold, the ear cavity is unable to equalize its pressure; this will often lead to earaches and other pains.

• The inner ear consists of a cochlea, the semicircular canals, and the auditory nerve. The cochlea and the semicircular canals are filled with a water-like fluid.

• The fluid and nerve cells of the semicircular canals provide no role in hearing; they merely serve as accelerometers for detecting accelerated movements and assisting in the task of maintaining balance. The cochlea is a snail-shaped organ that would stretch to approximately 3 cm being filled with fluid

• the inner surface of the cochlea is lined with over 20 000 hair-like nerve cells that perform one of the most critical roles in our ability to hear

The eustachian tube connects the middle ear cavity with the nasopharynx.

It aerates the middle ear system and clears mucus

from the middle ear into

The nasopharynx. Opening and closing. Normal

opening of the eustachian tube equalizes atmospheric

pressure in the middle ear; closing of the eustachian

tube protects the middle ear from unwanted pressure

fluctuations and loud sounds. Mucociliary clearance

drains mucus away from the middle ear into the

nasopharynx, thus preventing infection from

ascending to the middle ear.

The Cochlea

The inner ear structure called the

cochlea is a snail-shell like structure

divided into three fluid-filled parts.

Two are canals for the transmission

of pressure and in the third is the

sensitive organ of Corti which

detects pressure impulses and

responds with electrical impulses

which travel along the auditory

nerve to the brain

• These nerve cells differ in length by small amounts; they also have different degrees of elasticity to the fluid that passes over them. As a compression wave moves from the interface between the hammer of the middle ear and the oval window of the inner ear through the cochlea, the small hair-like nerve cells will be set in motion.

• Each hair cell has a natural sensitivity to a particular frequency of vibration. When the frequency of the compressional wave matches the natural frequency of the nerve cell, that nerve cell will resonate with a larger amplitude of vibration. This increased vibration amplitude induces the cell to release an electrical impulse that passes along the auditory nerve towards the brain.

• In a process that is not clearly understood, the brain is capable of interpreting the qualities of the sound upon reception of these electric nerve impulses.

ear wax))Cerumen

• Earwax, also known by the medical

term cerumen, is a yellowish waxy substance

secreted in the ear canal of humans and other

mammals. It protects the skin of the human ear

canal, assists in cleaning and lubrication, and

also provides some protection from bacteria

, insects and water. Excess or impacted cerumen

can press against the eardrum and/or occlude

(block) the external auditory canal or hearing

aids, prevent hearing

• The purpose of wax:

– Repel water

– Trap dust, sand particles, micro-organisms,

and other debris

– Moisturize epithelium in ear canal

– Odor discourages insects

– Antibiotic, antiviral, antifungal properties

– Cleanse ear canal

• Eardrum:

• The object is to get the energy represented by the vibratory motion of an eardrum diaphragm, transferred with maximum efficiency to the fluid of the inner ear. The three ossicles (hammer, anvil, and stirrup, as

• shown in Figure form a mechanical linkage between the eardrum and the oval window, which is in intimate contact with the fluid of the inner ear. The first of the three bones, the malleus, is fastened to the eardrum. The third, the stapes, is actually a part of the oval window. There is a lever action in this linkage with a ratio leverage ranging from 1.3:1 to 3.1:1. That is, the eardrum motion is reduced by this amount at the oval window of the inner ear.

Tympanic Membrane Thin membrane - Forms boundary

between outer and middle ear Vibrates in response to

sound waves- Changes acoustical energy into mechanical

energy

How Sound Travels Through The Ear

Acoustic energy, in the form of sound waves, is channeled into

the ear canal by the pinna.

Sound waves hit the Eardrum membrane and cause it to vibrate,

like a drum, changing it into mechanical energy.

The hammer, which is attached to the Eardrum membrane, starts

the ossicles into motion.

The stapes moves in and out of the oval window of the cochlea

creating a fluid motion, or hydraulic energy.

The fluid movement causes membranes in the Organ of Corti to

shear against the hair cells.

This creates an electrical signal which is sent up the Auditory

Nerve to the brain. The brain interprets it as sound!

Stapedius Muscle

• Attaches to stapes

• Contracts in response to loud sounds; (the Acoustic Reflex)

• Changes stapes mode of vibration; makes it less efficient and reduce loudness perceived

• Built-in earplugs

• Absent acoustic reflex could signal conductive loss or marked sensor neural loss

Central Auditory System

• Auditory Nerve – band of nerve fibers (25-30K)

– Travels from cochlea through internal auditory meatus to skull cavity and brain stem

– Carry signals from cochlea to primary auditory cortex, with continuous processing along the way

Vestibular system • The vestibular system, which contributes

• to balance and to the sense of spatial orientation,

is the sensory system that provides the leading

contribution about movement and sense of

balance. Together with the cochlea, a part of

• the auditory system, it constitutes the labyrinth of

the inner ear in most mammals, situated in

the vestibulum in the inner ear . As movements

consist of rotations and translations, the

vestibular system comprises two components:

the semicircular canal system, which indicate rotational movements; and

the otoliths, which indicate linear accelerations. The vestibular system sends signals

primarily to the neural structures that control eye movements, and to the muscles

that keep a creature upright. The projections to the former provide the anatomical

basis of the vestibule-ocular reflex , which is required for clear vision; and the

projections to the muscles that control posture are necessary to keep a creature

upright.

The brain uses information from the vestibular system in the head and

from proprioception throughout the body to understand the

body's dynamics and kinematics ) from moment to moment

equilibrium - The hair cells within the vestibule and semicircular canals of the cochlea

are responsible for equilibrium

- the parts of the ear function in dynamics equilibrium is cochlea

- ear structure function in static equilibrium is Saccule and utricle غدة كيسية

otolithic membrane

their movements in response to changes in the position of the head with

reference to gravity stimulate the hair cells to send nerve impulses to the

CNS(Central Nervous System )which are interpreted as information about

static equilibrium

dynamic equilibrium -

The special sense which interprets

balance when one is moving, or at

least the head is moving; the

semicircular canals contain the

receptors for dynamic equilibrium;

within each semicircular canal is a

complex mechanoreceptor called a

crista ampullaris which contains the

(Hair cells) for dynamic equilibrium;

when the per lymph in one of the

semicircular canals moves, the hair

cells in the crista ampullaris are

stimulated to send nerve impulses to

the brain; this advises the brain of

whether or not a person has their

balance during body movements or if

their body is in motion.

The Franssen Effect

• The ear is relatively adept at identifying the locations of sound sources. However, it also uses a memory that can sometimes confuse direction.

• The Franssen effect demonstrates this. Two loudspeakers are placed to the left and right of a listener in a live room.

• The loudspeakers are about 3 ft from the listener at about 45° angles. A sine wave is played through the left loudspeaker, and the signal is immediately faded out and simultaneously faded in at the right loudspeaker, so there is no appreciable change in overall level.

• Most listeners will continue to locate the signal in the left loudspeaker,

• even though it is silent and the sound location has changed to the right loudspeaker.

• They are often surprised when the cable to the left loudspeaker is disconnected, and they continue to “hear” the signal coming from the left loudspeaker. This demonstrates the role of auditory memory in sound localization.

Perception of Reflected Sound

• In the preceding section, “reflected” sound was considered in a rather limited way.

• A more general approach is taken in this section. The loudspeaker arrangement used by Haas was also used by other researchers, and this is the familiar stereo setup: two separated loudspeakers with the listener located symmetrically between the two loudspeakers.

• The sound from one loudspeaker is designated as the direct sound, that from the other loudspeaker, the delayed sound (the reflection). The delay injected between the two signals and their relative levels is adjustable.

• With the sound of the direct loudspeaker set at a comfortable level, and with a delay of, say 10 msec, the level of the reflected, or delayed, loudspeaker sound is slowly increased from a very low level.

• The sound level of the reflection at which the observer first detects a difference in the sound is the threshold of reflection detection. For levels less than this, the reflection is inaudible; for levels greater than this, the reflection is clearly audible

The Cocktail-Party Effect

• effect” or “auditory scene analysis.

• ” Imagine yourself at a busy party with many talkers . You are able to listen to one talker while excluding many other conversations and sounds.

• But if someone from across the room calls your name, you will be alert to that. There is evidence that musicians and conductors are highly skilled at this auditory segregation; they can independently follow the sounds of multiple musical instruments simultaneously.

• This ability to distinguish particular sounds is greatly assisted by our localization abilities.

•

• If the sounds of two talkers are played over one loudspeaker, it can be difficult to differentiate them. However, if the sounds of two talkers are each played over two physically separated loudspeakers, it can be quite easy to follow both (factors such as relative language, gender, and pitch of the talkers also play a role).

• While humans function well at differentiating sources at cocktail parties, electronic signal processing systems have a more difficult time. This field of signal processing is referred to as source separation, or blind source separation.

Aural Nonlinearity

• When multiple frequencies are input to a linear system, the same frequencies are output.

• The ear is a nonlinear system. When multiple frequencies are input, the output can contain additional frequencies. This is a form of distortion that is introduced by the auditory system and it cannot be measured by ordinary instruments. It is a subjective effect requiring a different approach.

• This experiment demonstrates the nonlinearity of the ear and the output of aural harmonics. It can be performed with a playback system and two audio oscillators.

• Plug one oscillator into the left channel and the other into the right channel, and adjust both channels for an equal and comfortable volume level at some midband frequency. Set one oscillator to 23 kHz and the other to 24 kHz without changing the level settings. With either oscillator alone, nothing is heard because the signal is outside the range of the ear. However, if the tweeters are good enough, you might hear a distinct 1-kHz tone

Sound Pressure Level

• Sound waves are energy produced by vibrating objects

• The larynx vibrates to produce the voice

• The vibrations create a pattern, which the ear translates into sound

• As you double the distance from a noise source, the source loudness decreases by half

• Strong vibrations from very loud noises can damage the ear

How We Hear Sounds

• Sound waves enter the ear canal striking the eardrum.

• When eardrum vibrates, ossicles conducts vibrations to the cochlea.

• Tiny hair like cells in cochlea respond to vibrations by generating nerve impulses.

• Brain interprets nerve impulses as sound.

Note: Healthy hair cells are the key to good hearing. Although, some die off naturally as you age, many more are killed early, from unprotected exposure to hazardous noise.

How Hearing is Damaged • Hair like cells are flattened.

• You do not get used to noise; you gradually loose your

hearing

• Once haring is damaged, it cannot be repaired or replaced.

• Conductive – Sound is not conducted from outer ear to inner ear

– Reduction in sound level

– Condition results from fluid in middle ear, foreign bodies, infection in ear canal, impacted ear wax, malformation of ear

• Sensorineural – Results from damage to the inner ear or nerve pathways from ear to brain

– Corrected through surgery

– Caused by birth injury, diseases, noise exposure, head trauma, aging

• Mixed – Hearing loss resulting from both conductive and sensorineural

Types of Hearing Loss

Occupational/Non-Occupational

Hearing Loss

• Occupational Hearing Loss – Results from constant exposure to sound levels above 85 dB

– Damage to hair cells in cochlea

• Non-Occupational Hearing Loss – Results from constant exposure to sound levels above 85dB

– Results from damage to outer, middle, or inner ear, hereditary,

– Damage to hair cells in cochlea, damage to nerve cells relaying sound message to brain, damage to structure of ear

Prism of sound

• The ear is exquisitely sensitive to sound. We can hear vibrations of the

eardrum of less than a tenth the width of a hydrogen atom! The ear is also

very good at separating out the different frequency components of a sound

(e.g., the different harmonics that make up a complex tone). Each place on

the basilar membrane is tuned to a different frequency as in Figure, so

that low-frequency sounds cause the membrane to vibrate near the top

(apex) of the spiral, and high-frequency sounds cause the membrane to

vibrate near the bottom (base) of the spiral. Each nerve cell or neuron in

the auditory nerve is connected to a single place on the basilar membrane,

so that information about different frequencies travels to the brain along

different neurons.

• Similarly, the ear separates out the different frequencies of sound to

produce an acoustic spectrum. Actually, the human eye can distinguish

just three basic colors. The ear, on the other hand, can separate up to

a hundred different sound frequencies, corresponding to the number of

frequencies that can be separated by the basilar membrane. We get a

much more detailed experience of the “color” of sounds )timbre( than we

do of the color of light. This is how we can tell the difference between two

different instruments playing the same note, for example, a French horn

and a cello both playing C3. Although the pitch of the two instruments is

the same, the timbre—which is determined by the relative levels of

the harmonics —is different . By separating out the different harmonics

on the basilar membrane, the ear can distinguish between the two

sounds.

The ear acts a bit like a prism for sound.

A prism separates out the different

frequencies of light (red, yellow, green,

blue etc.) to produce a spectrum