This document discusses hearing and assistive devices, from hearing aids to electronic ears. It begins by explaining the auditory system as an energy transformation system, where acoustic energy is transformed into mechanical, hydraulic, and bioelectric energy as it travels through the outer, middle, and inner ear. It then provides detailed descriptions of the anatomy and functions of the outer, middle, and inner ear. It also discusses various physical properties of sound including frequency, wavelength, amplitude, interference, resonance, and more. The document is an in-depth overview of the human auditory system and how assistive devices can help with hearing impairments.

1. 聽能障礙與輔具的應用—
從助聽器到電子耳
王智弘 醫師
三軍總醫院 耳鼻喉頭頸外科部
Chih-Hung Wang, MD, PhD
Department of Otolaryngology-Head and Neck Surgery
Tri-Service General Hospital, National Defense Medical Center
Taipei, TAIWAN, ROC

2. Auditory System = Energy Transformation System
acoustic energy---mechanical energy---hydraulic energy---bio-electric energy
→ Outer ear ←→ Middle ear ←→ Inner ear ←

3. How Do We Hear?

4. Sound waves

5. ++++
+++
++++
+++

6. Anatomy of the Outer Ear
The Outer Ear:
Auricle (pinna):
is composed of a sheet of cartilage, and this is
continous with the cartilage which forms the
framework of the outer 1/3 of the EAC.
Function:
☆ It protects the ear canal and eardrum by repelling any
objects that strike it
☆ collect and capture the acoustic energy and direct it to
the tympanic membrane
☆ the surface of the auricle can modify the
spectral composition of the incoming
sound
☆ localize the source of sound, using intensity and phase
differences of signal at two ears
☆ beautiful to look at

7. Anatomy of the Ext. Ear Canal
External auditory canal:
A narrow, slightly tortuous passage (from concha to the tympanic membrane,
approximately 2.5~3 cm in the adult).
The membrane is set obliquely in the depth of the canal, in such a way that the front
(anterior) wall is longer than the back (posterior) wall. (inferior wall of EAC is longer
than superior one)
.
sup.
pos. ant.
inf. 5 mm
ant.
sup inf.
pos

8. The ear canal reinforces sound
Sound is directed towards the eardrum in two ways:
► Intensity of sound at eardrum is increased by 20 dB because of resonances from the external
ear (concha, meatus, canal and eardrum)
► This increase in sound intensity is for high frequency sounds. In adults the peak freqeuncies
are found around 2500 Hz. For children the peak intensities are at higher frequencies

9. Resonance

10. 耳道的共振效應
波長: 4 x L (管長) 基礎共振頻率 first harmonic
4 x L x 1/3 第三共振頻率 third harmonic
4 x L x 1/5 第五共振頻率 third harmonic

11. Function of Ext. Ear Canal
In the adult, the outer cartilaginous portion
of the canal runs slightly upwards and
backwards, the inner bony portion
running slightly downwards and
forwards.
The skin covering the cartilaginous portion
of the canal contains short hairs,
ceruminous and sebaceous glands.
Two constrictions in the canal: one at the
junction of the cartilaginous and bony
portions; the other, the isthmus, 5 mm
from the tympanic membrane, in the
bony portion.
Function: cartilage bone
☆Cleaning: cilia Outer 1/3 Inner 2/3
☆Protection: cerumen, moisture
☆Resonance & sound amplification: Hair Yes NO
Natural frequency in EAC: Sebaceous gland Yes NO
343m/(4x0.03) = 2834 HZ Ceruminous gland Yes NO
2500~4000 Hz, 2x~4x intensity Sweat gland Yes NO
☆Sound localization:

12. The Eardrum & The Ossicles

13. The Middle ear as an Impedence Transformer
☆ The middle ear acts as an
impedence-matching device.
☆ Amplify sound pressure:
@ Lever action: d1p1=d2p2
malleal arm: incudal arm = 1.3: 1
@ Areal ratio of the tympanic membrane
to the oval window
a1p1=a2p2
tympanic memb.: oval window = 14: 1
→ x 18.2 → 25.25 db SPL
Exp.: Resonance in EAC: 2.5K~4k Hz: 2x ~ 4x

14. The Middle Ear Cleft:
The tympanic cavity is an air-containing cavity in the
air-
petrous part of the
temporal bone and is lined with mucous membrane.
It contains the auditory ossicles, and communicates in
ossicles,
front through the E-
E-
tube with the nasopharynx and behind with the mastoid
antrum.
antrum.
Roof:
A thin plate of bone, the tegmen tympani, which is part of
the petrous
temporal bone
It seprates the tympanic cavity from the meninges and
temporal lobe of the
brain in the middle cranial fossa.
fossa.
Floor:
A thin plate of bone, which may be deficient and partly
replaced by fibrous tissue.
It separates the tympanic cavity from the superior bulb of
the internal jugular vein.

15. Schematic diagram of
the middle ear
E-tube : 17-18 mm long at birth; 35 mm in adult

16. Structure of Inner Ear

17. Anatomical two parts:
1). Bony labyrinth: comprising a series of cavities within the bone
labyrinth
2). Membranous labyrinth: comprising a series of membranous
sacs and ducts contained within the bony labyrinth.

18. Anatomy of the Labyrinth
Bony Labyrinth:
Vestibule:
The central part of the bony labyrinth, lies posterior to
the cochlea and ant. to the semicircular canals.
Within the vestibule are the saccule and utricle of the
membranous labyrinth.
Semicircular canals:
Within the canals are the semicircular ducts. Each
canal has a swelling at one end called ampulla.
ampulla.
Cochlea:
Two and a half turns in the human, these coils(holly
bony tube) turning around a central ‘pillar’, the
pillar’
modiolus

23. Membranous Labyrinth:
Utricle: indirectly connected to the saccule by utricular duct.
Saccule: globular in shape, saccular duct
Semicircular ducts:
Cochlear duct: triangular in cross section, connected to the saccule by the ductus
reunions.

24. Perilymph:
• Primarily formed by filtration from blood vessels in the inner e ar
• Communicates with the cerebrospinal fluid (CSF) through the coch lear
aqueduct, a narrow channel 3- 4 mm long, with its inner ear opening at
the base of the scala tympani.
• Resembles the extracellular fluids:
( high Na+ :140 mEq/liter; low K+:10 mEq/liter; Protein: 200-400 mg%,
CSF: Na+ :152 mEq/liter; K+:4 mEq/liter; Protein: 20-50 mg% )
Endolymph:
• Produced by the secretory cells in the stria vascularis of the cochlea and the
dark cells of the vestibular labyrinth.
• Re-absorption of endolymph take place in the endolymphatic sac.
• Resenble the intracellular fluids:
( low Na+ :5 mEq/liter; high K+:144 mEq/liter; Protein: 126 mg%)

25. Chemical Composition of the
Cochlear fluids

26. Blood supply of the inner ear
Basilar artery—ant. inf. cerebellar artery—labyrinthine artery
↗ common cochlear a. --↗main cochlear a ----------------------------- ↓
↘vestibulo-cochlear a ↗cochlear ramus ↑
→post. vestibular a.↓
↘ ant. vestibular a. ----------------------------------------------------------↑

27. Blood supply of the inner ear
Basilar artery—ant. inf. cerebellar artery—labyrinthine artery
↗ common cochlear a. --↗main cochlear a ----------------------------- ↓
↘vestibulo-cochlear a ↗cochlear ramus ↑
→post. vestibular a.↓
↘ ant. vestibular a. ----------------------------------------------------------↑

32. Shearing is a particular form of bending

33. Inner Hair Cell Outer Hair Cell

34. Innervation and central connections
Two types: Afferent fibres: carrying sensory
fibres:
information to the brain.
Efferent fibres: Pass from the brain-stem to
fibres: brain-
the cochlea, perhaps 1000 in number
As many as 95% of all afferent fibres make
contact with the inner hair-cells, 5% with
hair-
outer hair cells, and each inner hair-cell has
hair-
terminals from about 10~20 afferent fibres.
fibres.
The vast majority of the fibres of the cochlear
nerve (about 30,000 of them) are afferent
and their cell bodies are in the spiral
ganglion in the modiolus.
modiolus.
The modiouls contains many small canal, the
most central of them carrying fibres from the
apex of the cochlea, while the outermost
canal carry fibres from the basal part of the
cochlea.

36. Travelling wave:

37. Travelling wave:

38. Response to a 150 Hz tone Response to a pulse sequence
Response to a 1.5 kHz tone
Low frequency traveling wave in 3-D
Response to a 15 kHz tone
High frequency traveling wave in 3-D

39. Tuning Curve
The frequency sensitivity of a hair cell can be displayed as a tuning curve

40. PHYSICAL PROPERTIES OF
SOUND
Chih-Hung Wang, MD, PhD
Tri-Service General Hospital, National Defense Medical Center
王智弘
三軍總醫院耳鼻喉部 國防醫學院耳鼻喉學系

41. Nature of sound
• The sensation of sound is determined by the
interaction of sound waves with the hearing system.
• The normal human ear has an auditory sensitivity
which ranges from 20-20,000 Hz.
Humans: 20- 20,000 Hz
Whales: 20- 100,000 Hz
Bats: 1500- 100,000 Hz
Frogs: 600- 3000 Hz
Fish: 20- 3000 Hz
Crickets: 500- 5000 Hz

42. Production of sound
► Sound is a form of energy
► The vibration of an elastic body (tuning fork, piano string, vocal cords) gives rise to
the propagation of pressure waves [ a sequence of increases (compressions)
and decreases (rarefactions) in pressure].
►When the vibrating object is a sound source, these pressure waves are known as
sound waves.
►Sound wave:
Energy transmission by media
Longitudinal wave = pressure wave
縱波: 波傳遞時，介質振動的方向與波行進方向平行
橫波:波傳遞時，介質振動的方向與傳遞方向互相垂直

43. At rest Compression Rarefaction

44. Characters of sound
1). Frequency: one double vibration is called a cycle. The
frequency of sound is determined by the number of complete
cycles per second and is expressed in hertz (Hz).
► Pitch: Psychoacoustics terms
► Octave: at twice the frequency:
► Fundamental frequency: the basic underlying sine wave
► Harmonics: the higher frequencies based on multiples of a
fundamental frequency.

46. •The lowest component of the waveform is known as the FUNDAMENTAL
•The second harmonic is twice the fundamental frequency, the third harmonic is
three times the fundamental frequency, and so forth.

47. 2). Period: the time for one double vibration= second/ per cycle
3). Wave length: the distance for one cycle = m/sec
4). Phase: the phase of a sine wave corresponds to its
distance form zero at the moment in which the
vibration begins (onset) and is usually defined in
terms of time (period) or angular measure.

48. interference
If you strike a tuning fork and rotate it next to your ear, you will note that the
sound alternates between loud and soft as you rotate through the angles where
the interference is constructive and destructive.

49. The arrow indicates one cycle of the sound. The time it takes to complete a
cycle is the period. Frequency is the inverse of this, the number of cycles in a
second. The distance sound travels during one period is the wavelength.
P = 1/f λ = S/f
The upside down y is λ (lambda): wavelength
P = period,
f = frequency
S = the speed of sound
 20℃、海平面上，聲音的速率: 343 m/s 溫度的變化:『v = (331 + 0.6T) m/s 』
在水中，20℃時聲音的速率: 1,482 m/s
 鋼鐵材質中則大約5,960 m/s。

50. Sine wave
• The waveform produced by simple harmonic
motion is the SINE WAVE
• the simplest type of sound wave is one which
has only one frequency and is constant in
amplitude

51. Beats
When two waves with the same amplitude but different
frequency are added together a phenomenon called "beating"
occurs.

52. Refraction 折射
▶In the first, the beam crosses the boundry between warm and cool at a right angle. All that
happens is the wavelength changes. It gets shorter, since the speed of sound is lower in
the cool air.
▶ Look at the beam that strikes the boundry at an angle.The wavefront that has just crossed
actually has two wavelengths; long for the part still in the warm air, short for the part in
the cold.This makes it skewed. All later waves just propagate off this crooked wavefront,
in a new direction.

53. Resonance

54. 5). Amplitude: The difference between normal (atmospheric)
pressure and the pressure in the presence of a sound wave. It varies
with time between positive and negative values and is expressed in
pascals (newtons/m2).
The lower limit to the hearing threshold at 1k Hz:
2 x 10-5 newton/m2= 20 µPa
► The measurement of amplitude of the sound wave:
Pressure: Newton/m2 or dyne/cm2
Intensity: power : watt/m2 or watt/cm2
Exp: 1k Hz: (power) 1 x10-16 watt/cm2 =(pressure) 0.0002 dyne/cm2
or 2 x 10-4 dyne/cm2
Intensity being proportional to the square of the amplitude of sound pressure
(I ∞ P2)

55. Pascal

56. Amplitude and waveform
•Amplitude of sounds as loudness.
•The shape of a sound is its waveform
•The shape of the curve is very important in establishing the
timbre of the sound..

57. Tuninig curve of single nerve fiber
100
dB 80
60
40
20
1 10 40
kHz
Normal Loss of OHC Loss of IHC & OHC

58. sound_zh_TW.jar

59. Measurement of Sound
MKS CGS
Length m cm
Mass Kg g
Time second second
sound wave
pressure Newton/m2 dyne/cm2
intensity watt/m2 watt/cm2
F=ma Newton: 1N = 105 dyne

60. Bel system
Bel system=Intensity (power) system, named
after the scientist
Alexander Graham Bell
BelIL = log I/I0
0 bel =1 x 10-12 watt/m2 (I0: a tone which is only just audible= a threshold sound)
1 bel =10 x 10-12 watt/m2
3 bel =10 x 10 x 10 x 10-12 watt/m2 (a whisper is 1000 times more powerful than
a threshold sound = raised to the power of three)

61. Decibel system
Decibel system: for clinical purposes, the bel has been broken down into
ten smaller units known as decibels (dB)
dBeL = 10 log I/I0 I: the intensity to be measured
I0: threshold sound = 1 x 10-12 watt/m2 = 1 x 10-16 watt/cm2
Exp: when noise is 1 x 10-14 watt/cm2
dBeL= 10 log 1 x 10-14 watt/cm2 / 1 x 10-16 watt/cm2 =20 dB
Similarly, a whisper (3 bels) = 30 dB = 103 x threshold sound
A conversational voice (6 bels) = 60 dB = 106 x threshold sound
A loud shout (9 bels) = 90 dB = 109 x threshold sound
Loudness: Psychoacoustics terms

62. (I (power)∞ p2 )
Intensity being proportional to the square of the amplitude of sound pressure
dBSPL = 20 log P/P0
P=power
Exp.:
A: 0.002 dyne/cm2: dBSPL=20 log 0.002/0.0002= 20
B: 0.02 dyne/cm2: = 40
C: 0.004 dyne/cm2: = 26
BelIL = dBelIL/10 = dBSPL/20

63. Auditory threshold
• 1). dB SPL (sound pressure level): a value in decibels which express
the pressure of sound in relation to a reference pressure which for
conventional purposes is the minimum pressure required for the perception
of a 1000-4000 Hz sine wave in the average adult.
• 2). dB HL (hearing level): the minimum intensity at which a person can
hear at a specific frequency (e.g. 1000 Hz ) in relation to a basic value (0 dB
HL) which represents minimum hearing in normal hearing subjects.
•
3). dB SL (sensation level): the level of hearing in dB HL above
threshold for the subject being tested.

64. 基本上我們使用音量來表示聲音的強弱，但是前述兩種計算音量的方法，只是
用數學的公式來逼近人耳的感覺，和人耳的感覺有時候會有相當大的落差，為
了區分，我們使用「主觀音量」來表示人耳所聽到的音量大小。例如，人耳對
於同樣振福但不同頻率的聲音，所產生的主觀音量就會非常不一樣。若把以人
耳為測試主體的「等主觀音量曲線」（Curves of Equal Loudness）畫出來，就可
以得到下面這一張圖：