Anatomy of the ear

The external ear

The external ear consists of the (i) auricle or pinna, (ii) external acoustic canal and (iii) tympanic membrane (Figure
1.1A).
A. AURICLE OR PINNA
The entire pinna except its lobule and the outer part of external acoustic canal are made up of a framework of a single
piece of yellow elastic cartilage covered with skin. Figure 1.

B. External auditory canal

It extends from the bottom of the concha to the tympanic membrane and measures about 24 mm along its posterior wall.
The canal is divided into two parts: (i) cartilaginous and(ii) bony.
1. Cartilaginous part: It forms outer one-third (8 mm) of the canal. Cartilage is a continuation of the cartilage which
forms the framework of the pinna.
2. Bony part: It forms inner two-thirds (16 mm). Skin lining the bony canal is thin and continuous over the tympanic
membrane. It is devoid of hair and ceruminous glands. About 6 mm lateral to tympanic membrane.
C. Tympanic membrane: It forms the partition between the external acoustic canal and the middle ear

The middle ear

The middle ear together with the eustachian tube, aditus, antrum and mastoid air cells is called middle ear cleft. It is lined
by mucous membrane and filled with air (Figure 2).

Fig.2 Middle ear

Ossicles and middle ear: There are three ossicles in the middle ear the malleus, incus and stapes. The malleus has head,
neck, handle (manubrium), a lateral and an anterior process. The incus has a body and a short process, and a long process
which hangs vertically and attaches to the head of stapes. The stapes has a head, neck, anterior and posterior crura,and a

1
footplate. The footplate is held in the oval window by annular ligament. The ossicles conduct sound energy from the
tympanic membrane to the oval window and then to the inner ear fluid.

The inner ear

The internal ear or the labyrinth is an important organ of hearing and balance. It consists of a bony and a membranous
labyrinth. The membranous labyrinth is filled with a clear fluid called endolymph while the space between membranous
and bony labyrinths is filled with perilymph.

The physiology of the auditory system is complex, and our understanding of how the system functions is not yet complete.
Complete understanding of the auditory system also requires knowledge of the nature of sound and the function of the
structures.

Acoustics

Acoustics is the science that is concerned with the production, control, transmission, reception, and effects of sound.
Sound is energy: mechanical radiant energy that is transmitted through pressure waves in a material medium (e.g., air,
water, metal). In the case of hearing, sound is the sensation perceived by the ear.

This sound energy is captured by the outer ear, transformed by the middle ear, and transduced by the inner ear. Sound is
described in terms of its basic physical attributes: frequency, intensity, and time/phase of the vibration. These physical
attributes also have psychological correlates, which are pitch, loudness, and quality, respectively.

Frequency

A single-frequency sound, or pure tone, is the standard used in the assessment of auditory sensitivity (threshold).

Frequency is a physical attribute of a sound and is defined as the number of cycles per unit of time. Although measured
in cycles per second, frequency is reported in hertz (Hz) (Fig 3). Video1

2
Period refers to the amount of time it takes for one cycle to occur; therefore, period is the reciprocal of frequency (period
=1/frequency). This 1,000-Hz tone would have a period of 1/1,000 seconds. Wavelength (λ) is the distance sound travels
in one period and is reported in centimeters, feet, or miles, depending on how the velocity is recorded (Fig 4). Video2

Fig.3 Graphs of two sine waves of different frequencies plotted over time. The top graph is a low frequency (1,000 Hz)
and the bottom is a higher frequency (4,000 Hz) as denoted by the number of cycles in the same time window.

Table 1 Frequency, Period, and Wavelength of a 1,000-Hz Tone

Fig 4 Wavelength

The number of times an object vibrates determines its frequency, but how far the object moves determines its intensity.
Intensity or amplitude then becomes another physical attribute of sound. Intensity relates to the strength of a sound as
shown in Fig. 5; the distance a mass moves from the point of rest is the amplitude of the sound. Intensity is usually
measured in decibels (dB).

3
Fig. 5 Graphs of two sine waves of the same frequency (1,000 Hz) but different intensities plotted over time. The tones
have the same cycles per second but differ in the amplitude of the waves.

Phase
The phase of a sound refers to the relative timing of sound waves. It is simplest to refer to the starting phase of the signal.
Therefore, at time zero the point of the sine wave where the signal begins will be the starting phase.

Resonance refers to the phenomenon whereby one body can be set into motion by the vibration of another body. If a
given area has a “resonant frequency,” then that frequency is amplified when it is presented in that area. In other words,
there is an increase in the intensity of that signal because the surface of the area with the resonant frequency vibrates at the
particular frequency that has been presented and therefore increases its intensity. Video 3

Physiology of Hearing
The natural or usual manner by which humans detect sound is via an airborne or acoustic signal. Once the sound is
generated, it travels through the air in a disturbance called a sound wave. The flange and concha of the pinna collect,
amplify, and direct the sound wave to the tympanic membrane by the external auditory meatus.
At the tympanic membrane, several transformations of the signal occur: (1) the acoustic signal becomes mechanoacoustic;
(2) it is passed along to the ossicular chain.

Transmission and Natural Resonance of the External Ear

Natural resonance refers to inherent anatomic and physiological properties of the external and middle ear that allow
certain frequencies to pass more easily to the inner ear. The external ear serves to enhance the sound as it travels to the
cochlea and to protect the tympanic membrane. The concha and ear canal increase the intensity of the sound over the
frequency range 1,500 to 7,000 Hz by as much as 10 to 20 dB using only simple resonance. The natural resonance of the
external auditory canal is 2,700 Hz in the adult, and 5,300 to 7,200 Hz in newborns. This ear canal resonance is dependent
on the size and shape of the ear canal and is inversely related to its length.
The natural resonance of the middle ear is 800 Hz. The tympanic membrane is most efficient in transmitting sounds
between 800 and 1,600 Hz, whereas the ossicular chain is most efficient in transmitting sounds between 500 and 2,000
Hz.
These structures enhance sensitivity to sound between 500 and 3,000 Hz, which are approximately the frequencies that are
most important in human speech.

4
Transformation and the Middle Ear Mechanism

Acoustic Impedance (Fig. 6 B–D)

Acoustic impedance (Z) is defined as the resistance that must be overcome by a sound to pass through a medium. It is a
specific property of any substance that can be calculated multiplying the specific density by the acoustic velocity of the
respective substance (Fig. 6 B). Thus, Z changes with temperature, since acoustic velocity is temperature dependent.
Every time a sound wave passes from one medium to another (e.g., from air to perilymphatic fluid), a proportion of the
sound intensity is reflected. The greater the difference in acoustic impedance, the greater the reflected proportion (αr) of
the sound intensity (Fig. 6 C).
The acoustic impedance of water is more than 3000 times higher than that of air. When sound waves hit an interface
between air and water, more than 99% of the sound intensity is reflected (Fig. 6 D). This fact explains an observation
almost everyone has made during life: the babble of voices that can be heard in an indoor swimming pool disappears
immediately after dipping the head under water, since most of the sound intensity is reflected at the surface.
Impedance Conversion (Fig. 6 E, F)
The acoustic impedance of the inner ear is less than that of water alone. Despite this, only 2% of the sound intensity is
transmitted into the inner ear when ossicular continuity is interrupted.
The most important function of the middle ear is to bridge the impedance gap. Thus, first of all, the middle ear is an
impedance converter. Moreover, the middle ear is capable of applying the sound energy selectively to the oval window.
This is important, to ensure an inversely phased motion of the oval and the round windows.
The conversion of impedance is based on two principles:

1-Area Effect of the Tympanic Membrane

Most importantly, the area of the tympanic membrane is much greater than the area of the stapes footplate. Given that
force = pressure multiplied by effective area, a transmitted force can be calculated. At the stapes footplate, the same force
acts upon a smaller area, leading to a higher pressure (Fig. 6 E). The effective ratio of surface of the tympanic membrane
Atm versus surface of the stapes footplate Afp is 17:1.
The adult human tympanic membrane measures approximately 90mm2. Of this area, approximately 55mm2 are functional
in that primarily the lower two thirds of the membrane vibrates in response to sound. The tympanic membrane in turn is
connected to the stapes footplate by way of the ossicular chain. The stapes footplate has an area of 3.2mm2. The hydraulic
ratio created by the vibrating area of the tympanic membrane in comparison with that of the stapes footplate produces a
17:1 increase in sound energy transmission across the middle ear.
2- Lever Ratio of the Ossicular Chain
A further mechanism, postulated by von Helmholtz, is that there is a pressure gain caused by a leverage effect: The
manubrium of the malleus is longer than the long process of the incus.
This results in smaller movements of the stapes when compared with those of the tympanic membrane, but the acting
force increases reciprocally (Fig. 6 F). In total, the impedance conversion of the middle ear results in a transferred
proportion of 60% of the sound energy—much higher than the 2% without the middle ear.
The malleus handle is 1.3 times longer than the long process of the incus. This difference in length produces a 1.3:1 lever
ratio of the middle ear ossicles.

The overall middle ear transformer ratio is the product of 1.3 × 17, or a transformer ratio of 22:1 due to the combined area
effect of the drum and lever ratio of the ossicles. This equates to approximately 27 dBof gain and when combined with the
action of the external ear will compensate for the loss of energy due to the impedance mismatch.

5
Fig 6 Transformation and the Middle Ear Mechanism

6
ASSESSMENT OF HEARING

PURE TONE AUDIOMETRY

An audiometer is an electronic device which produces pure tones, the intensity of which can be increased or decreased in
5 dB steps (Figure 7). Usually, air conduction thresholds are measured for tones of 125, 250, 500, 1000, 2000, 4000 and
8000 Hz and bone conduction thresholds for 250, 500, 1000, 2000 and 4000 Hz. The amount of intensity that has to be
raised above the normal level is a measure of the degree of hearing impairment at that frequency. It is charted in the form
of a graph called audiogram (Figure 8). The threshold of bone conduction is a measure of cochlear function. The
difference in the thresholds of air and bone conduction (A–B gap) is a measure of the degree of conductive deafness. It
may be noted that audiometer is so calibrated that the hearing of a normal person, both for air and bone conduction, is at 0
dB and there is no A–B gap, while tuning fork tests normally show AC > BC.
When difference between the two ears is 40 dB or above in air conduction thresholds, the better ear is masked to avoid
getting a shadow curve from the non-test better ear. Similarly, masking is essential in all bone conduction studies.
Masking is done by employing narrow-band noise to the non-test ear.

Figure 7 Two-room audiometry setup. Audiometrician watches responses of the patient sitting across a glass partition.

Figure 8 Pure tone audiogram

Uses of Pure Tone Audiogram

(a) It is a measure of threshold of hearing by air and bone conduction and thus the degree and type of hearing loss.
(b) A record can be kept for future reference.
(c) Audiogram is essential for prescription of hearing aid.
(d) Helps to find degree of handicap for medicolegal purposes.
(e) Helps to predict speech reception threshold.

7
Clinical applications
Hearing loss can be of three types:
1. Conductive hearing loss. It is caused by any disease process interfering with the conduction of sound from the external
ear to the stapediovestibular joint. Thus the cause may lie in the external ear (obstructions), tympanic membrane
(perforation), middle ear (fluid), ossicles (fixation or disruption) or the eustachian tube (obstruction).
2. Sensorineural (SN) hearing loss. It results from lesions of the cochlea (sensory type) or VIIIth nerve and its central
connections (neural type). The term retrocochlear is used when hearing loss is due to lesions of VIIIth nerve, and central
deafness, when it is due to lesions of central auditory connections.
3. Mixed hearing loss. In this type, elements of both conductive and sensorineural deafness are present in the same ear.
There is air-bone gap indicating conductive element, and impairment of bone conduction indicating sensorineural loss.
Mixed hearing loss is seen in some cases of otosclerosis and chronic suppurative otitis media.

Objective test to assess the middle ear mechanism (Impedance and admittance)

Tympanometry. It is based on a simple principle, i.e. when a sound strikes tympanic membrane, some of the sound
energy is absorbed while the rest is reflected. A stiffer tympanic membrane would reflect more of sound energy than a
compliant one. By changing the pressures in a sealed external auditory canal and then measuring the reflected sound
energy, it is possible to find the compliance or stiffness of the tympano-ossicular system and thus find the healthy or
diseased status of the middle ear.
Essentially, the equipment consists of a probe which snugly fits into the external auditory canal and has three channels:
(i) to deliver a tone of 220 Hz, (ii) to pick up the reflected sound through a microphone and (iii) to bring about changes in
air pressure in the ear canal from positive to normal and then negative (Figure 9). By charting the compliance of tympano-
ossicular system against various pressure changes, different types of graphs called tympanograms are obtained which are
diagnostic of certain middle ear pathologies.

Figure 9 Principle of impedance audiometry. (A) Oscillator to produce a tone of 220 Hz. (B) Air pump to increase or
decrease air pressure in the air canal. (C) Microphone to pick up and measure sound level reflected from the tympanic
membrane.

Types of tympanograms (Figure 10)

Type A Normal tympanogram.
Type As Compliance is lower at or near ambient air pressure. Seen in fixation of ossicles, e.g. otosclerosis or malleus
fixation.
Type Ad High compliance at or near ambient pressure. Seen in ossicular discontinuity or thin and lax tympanic
membrane.
Type B A flat or dome-shaped graph. No change in compliance with pressure changes. Seen in middle ear fluid or thick
tympanic membrane.
Type C Maximum compliance occurs with negative pressure in excess of 100 mm H2O. Seen in retracted tympanic
membrane and may show some fluid in middle ear.

8
Figure 10 Types of tympanograms.
A—Normal.
As—Reduced compliance at ambient pressure (otosclerosis). ‘s’ stands for shallow tympanogram but remember for
stiffness.
Ad—Increased compliance at ambient pressure (ossicular discontinuity). ‘d’ stands for deep tympanogram. Remember
disruption of ossicular chain.
B—Flat or dome-shaped (fluid in middle ear).
C—Maximum compliance at pressures more than −200 mm H2O
(negative pressure in middle ear), e.g. eustachian tube obstruction or early stage of otitis media with effusion.

Dr. Sherko Saeed Zmnako

Assistant Professor (ORL. Head & Neck Surgery)
Consultant Otolaryngologist M.B.Ch.B. – H.D.L.O. – Ph.D.
College of Medicine - University of Sulaimani

12th November 2022