sound, speech, ear, and hearing - university of denvermysite.du.edu/~balzar/imaging-4.pdfthe vocal...

Medical Imaging Physics 4April 3, 2008

Medical Imaging Physics Spring Quarter

Week 2-2

Sound, Speech, Ear, and Hearing

Davor [email protected]

www.du.edu/~balzar

mailto:[email protected]

http://www.du.edu/~balzar


Outline•

Field trips?

MRI (UCHSC)Radiology (Porter Hospital)

•

Sound, Speech, Ear, and Hearing

•

Reading assignment:CSG 9-11; D 12-14http://www.sprawls.org/ppmi2/USPRO/

•

HomeworkCSG 10: 1,3,4,15; 11:

4,6Due Tuesday, Apr 8

•

QuizTuesday, Apr 8

http://www.sprawls.org/ppmi2/USPRO/


Sound and Speech

•

We hear only certain frequencies:16-20 Hz –

20 kHzLower and higher frequencies are important:

•

Infrasound: earthquakes, ventilator systems –

headaches, psychological problems,...

•

Ultrasound: Imaging of the body, examining the fetus, ...

•

Sound is a wave:Longitudinal waveNeeds a medium to propagate

Δp = Δpmax

sin(kx -

ωt)

s(x,t) = Acos(kx -

ωt)

wave speed wavelength frequency2 angular frequencywave numberamplitude (maximum displacement)mass density

v

ff

kA

v f

λ

ω π

ρ

λ−−−= −−−−

=


Speed of Sound in Matter

•

How does speed of sound vary in different materials?

However, the elastic bulk modulus (B) of gases is much smaller

Bvρ

= ( ) ( )condensed matter 1000 airρ ρ≈

Medium Speed of sound (m/s)

Air (20 °C) 343

Water (20 °C) 1482

Granite 6000


Sound Intensity and Level

•

Sound intensityAcoustic impedance:

•

Sound level

Range:

•

An order of magnitude increase => 10 dB increase

( )0

10 dB log II

β = [ ]decibel (dB) 12 20 10 W/mI −=

0I I= 0 dBβ =

12010I I= 120 dBβ =

2 2 2 21 12 2

I v A Z Aρ ω ω= =

Z vρ=


Standing Waves in Pipes

•

Closed at one end

1,3,5,...4nvf n nL

= =Only odd harmonics exist

1

11

4

4

Lv vf

L

λ

λ

=

= =FundamentalFirst Harmonic

2

2 12

4 / 33 34

Lv vf f

L

λ

λ

=

= = =Third Harmonic

3

3 13

4 / 55 54

Lv vf f

L

λ

λ

=

= = =

Fifth Harmonic

4

3 13

4 / 77 74

Lv vf f

L

λ

λ

=

= = =Seventh Harmonic


Hearing the Body

•

Percussion or tapping used with very different thingsIn 1761, Auenbrugger

published On Percussion of the Chest

•

Stethoscope –

auscultation (Auscultation Assistant) “To view the chest”

used first time in 1816Both open and closed (with a diaphragm) bell stethoscopes are used, which changes the resonant frequency

http://www.wilkes.med.ucla.edu/inex.htm


The Production of Voiced Sounds•

The vocal cords are located within the larynx•

During the formation of sounds, vocal cords are drawn together

•

As air is exhaled, the pressure below the cords is increased

•

When air passes between the close cords, the pressure decreases as speed increases (Bernoulli’s effect)

•

The process is repeated, which results in a glottal sound wave

•

The fundamental frequency depends on the mass and tension of the vocal cords

In men, it’s about 125 Hz and double this in womenLowest frequency is 64 Hz (low C) and the highest is 2048 Hz (five octaves higher)


Voice Power

•

Spoken words and resultant sound carry relatively low energy (30-40 μJ)

Specific heat of water c = 1.00 cal/(g·K)

•

For an average word, this translates into tens of μW in power –

compare it to the power generated by the heart

•

The vowel sounds contain much more power than consonant vowels (up to 680:1 => 28 dB)


The Ear and Hearing

•

The sense of hearing involves:The mechanical system that gathers and transmits the signal to the hair cells in the cochleaThe sensors that produce action potentials in the auditory nervesThe auditory cortex in the brain that decodes and interprets the

signal

•

The ear:Converts mechanical sound waves into electrical pulses in the auditory nerveThe outer, middle and inner ear


The Ear and Hearing

•

The outer ear:Ear canal and the eardrum (tympanic membrane)

•

The middle earThree small bones (ossicles) and a connection to the mouth (Eustachian tube)

•

The inner earFluid-filled, spiral-shaped cochlea containing the organ of Corti, where hair cells convert vibrations of sound waves into nerve pulsesLabyrinth including the sensors of the vestibular (sense of balance) system


The Outer Ear

•

The visible part of the ear (external auricle or pinna) is not considered as a part of the outer ear

•

External auditory canal:2.5 cm longEquivalent to an organ pipe closed at one end (λ/4 standing wave)With λ

= 10 cm, it gives a resonant frequency of 3300 HzThe sensitivity of the ear is best around that frequency


Eardrum

•

The eardrum0.1 mm thick and about 65 mm2

in areaBecause malleus

is attached off-center, it doesn’t vibrate like a drumheadThe movements of eardrum must be smaller than the movements of air molecules in the sound wave

•

At the threshold of hearing (3000 Hz) about 10-11

m, which is less than the diameter of the hydrogen atom!

•

Sound pressures above 160 dB may rupture the eardrum


The Middle Ear

•

Three bones (ossicles):The malleus

(hammer), the incus

(anvil), and the stapes (stirrup)Lever action amplifies the movements of the eardrum (factor of 20, equivalent to 26 dB gain)Impedance match (efficient transmission) of sound energy between the eardrum and cochlea –

optimal in the 400-4000 Hz range

•

The Eustachian tubesConnect each middle ear to the back of the mouth

•

Drainage path for fluids•

Pressure equalizers for the middle ear (8 kPa

or 1/12 atm

causes pain)•

Can be blocked by fluids from a cold and swelling of tissues)


The Inner Ear

•

The best protected sensing organ in the body•

Spiral-shaped fluid-filled cochlea•

The stapes push the flexible membrane covering the oval window of the cochlea

•

Pressure variations are thus transmitted to the fluid, which causes motion in the flexible basilar membrane

•

This stimulates the hair cells in the organ of Corti•

The hair cells produce action potentials (electrical pulses)•

These signals travel to the brain via auditory nerve28,000 conductors

•

The cochlear nerve provides information on both the sound frequency and intensity


The Cochlea

•

A sound wave of particular frequency produces a wave-like ripple in the basilar membrane

•

Hair cells with the greatest movement send signals to the brain

•

The stiffness of the basilar membrane changes 10,000 times from the oval window to the tip

Most rigid close to the oval window –

most sensitive to high frequenciesLow frequencies cause the largest movement at the tipThis allows for a frequency identification


The Organ of Corti

•

The hair cell100 closely packed hairs a few microns in lengthExposed to the fluid in the organ of CortiAs the basilar membrane moves, hair cells follow itFriction with the fluid causes bending of hairs, which can produce a voltage and action potentialA motion of 1 nm can produce a signalMany things are still being studied


Sensitivity of the Ears

•

The ear is not uniformly sensitive over the frequency range2-5 kHz optimalSensitivity changes with ageThe loudness of sound is mental response to intensity –

it depends on frequency

•

The unit is phon: 1 phon

= loudness of 1 dB 1000 Hz sound


Testing Hearing

•

Tests:Tone (or speech) audiometryImpedance (immitance) evaluation of the middle ear

•

Tone audiometryEars tested separately (250-8000 Hz)Hearing loss (noise induced) due to partial nerve damage of the cochlea


Testing Hearing

•

Impedance (immitance) evaluationAcoustic impedance describes opposition to the flow of soundAdmittance is the opposite -> immitance

describes bothEar accomplishes about 26 dB gain in pressure at the oval windowImpedance will change if ossicular

chain is broken, eardrum perforated or middle ear filled with fluidMeasuring reflected percentage of the sound signal

•

Stiffness of the eardrum strongly defines the reflection percentage•

Stiffness is estimated by changes in air pressure


Tympanograms

Normal

Stiff eardrum

Compliant eardrum (ossicular

chain broken)


Hearing Problems

•

21 million in US deaf (1985)•

The definition of deaf:

Average hearing threshold at 500, 1000, and 2000 Hz of 90 dBAbove 55 dB

a hearing aid needed

•

Sound level of speech between 45 and 90 dB•

Hearing loss causes:

Conduction (sound does not reach the inner ear)•

Wax or fluid (temporary)•

Solidification of ossicles

(surgery possible)Nerve (signals do not reach the brain)Cohlear

implants available


Hearing Problems•

Hearing aidsFor hearing losses in the range 40-85 dB

•

Above 85 dB technically possible but not useful because of the pain thresholdDigital technology these days helps suppress noise

•

Vestibular Sense system5 motion sensorsTotal of 134,000 hair cells3 semicircular canals for angular and 2 for linear acceleration sensing

•

During acceleration fluid pushes on the septum to cause motion of the hair cells

Motion sickness:•

Ocean ride and weightlessness•

Alcohol?–

Density of the fluid affected!

sound, speech, ear, and hearing - university of denvermysite.du.edu/~balzar/imaging-4.pdfthe vocal...

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