Physiology and audiology

Dr. Ali A. Muttalib Mohammed Assistant Professor/ Consultant Otolaryngologist Head of ENT Dept, College of Medicine, University of Mosul In charge, Mosul Centre, Iraqi Board of Otolaryngology

Sound

Produced when an object or surface vibrates rapidly Transmitted through any elastic substance such as air, water, or bone. Density of the substance determines the speed at which the sound and pressure waves will travel.

The Hearing Mechanism

Outer Ear Acoustic Energy
Inner Ear Hydraulic Energy
Middle Ear Mechanical Energy
Auditory Nerve Electrical Energy
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Sound

Sound travels in air at 340 m/sec at 20 C and atmospheric pressure (sea level). Sound has two subjective physical properties; frequency (pitch) and intensity (loudness). The frequency is measured by Hz( cycle/sec) whereas the decibel dB is the unit of intensity.

Sound Terminology

Frequency The physical measurement of the number of vibrations per second a sound contains. The most common term used is Hertz (Hz). Intensity The physical measurement of the sound pressure level of a sound. Intensity is measured in decibels.


Relation of decibels to energy
10 dB 10 x increase in power 20 dB 100x increase in power 30 dB 1,000x increase in power 60dB 1,00,000x increase in power 120 dB 1 trillion x increase in power

Decibel (dB) Levels

0 dB - Threshold of hearing 65 dB - Average human conversation 85 dB - Damage-risk noise limit 120 dB - Threshold for discomfort 140 dB - Threshold of pain 160 dB - Ear drum rupture

Frequency Ranges

20 To 20,000 Hz
200 TO 6,800 Hz
Speech intelligibility 300 to 3,000 Hz

Hearing Mechanism

The auricle collects the sound waves and they pass along the EAM to the TM. The vibration of the TM is transmitted to the malleus, incus then to the stapes in the oval window causing vibration to set up in the endolymph and perilymph.

Hearing Mechanism

This stimulates the hair cells of the organ of Corti. Its these hair cells which convert the mechanical energy into electrical impulses which travel along the auditory nerve.

Perception of Sound

Auditory Nerve
Otolith Organs
Eustachian Tube
Opening to Throat
Ossicles
Middle Ear
Ear Drum
External Ear
Cochlea

Receptors of Sound

Detect fluid movement in the cochlea Transmit electrical impulses to the brain where sound is interpreted


The ratio of the functioning area of the TM to the footplate is 14:1. This is combined with an ossicular lever ratio of 1.3:1. The product of these (14 X 1.3 = 18:1) which represent the transformer mechanism of the middle ear.

Conversation voice A person with normal hearing can hear conversation voice with the opposite ear occluded in a quite room from a distance of 6 meters.

The tuning fork used should have a frequency of 512 Hz. The note of the higher frequency forks tends to decay quickly whereas, the lower frequency forks induce perception by vibration sensation.


The TF is struck against resilient surface and then held so that the acoustic axis is in line with the EAM. In this way the sound of the TF is heard by air conduction (AC). Bone conduction (BC) is heard by holding the TF with its base placed firmly against the mastoid bone, here the sound is transmitted through the bones of the skull to the cochlea.


It is done by comparing air conduction to bone conduction. More correctly, the test is done by requiring the subject to indicate as soon as the fork becomes inaudible by bone conduction then quickly transferring it close to the EAM. If it is then audible, the AC is said to be better than BC. If not then the BC is better than AC.

Normal subjects = AC > BC (Rinne +ve).CHL = BC > AC (Rinne –ve).SNHL = AC > BC (Rinne +ve) and often the BC is not heard.

This test compares the BC of the two ears. The TF is set in vibration and applied to the forehead or vertex of the skull in the midline and the patient is asked in which ear he hears the sound.


Weber Test


In normal subjects the sound is heard in the midline in both ears equally. In CHL the sound is heard in the affected ear (absence of environmental noise). In SNHL the sound is heard in the better (normal) ear.


In unilateral severe SNHL Rinne test will appear to give a –ve result. AC is absent but BC may be good because the sound is transmitted to the opposite cochlea through the skull.

This result may confuse the examiner in making a wrong diagnosis of CHL. In this situation Weber test is important and here it will not be lateralized to that ear as in conductive hearing loss.


This condition can be overcome by applying Barany,s noise box to the non-test ear. It is a clock work device which emits white noise (wide frequency) and raise the threshold of hearing in the non-test ear to such a level that the tuning fork can not be heard in that ear by cross hearing. It will then be found that the patient is unable to hear the TF by either AC or BC.


Is an instrument which delivers tones of variable frequency and intensity to the ear. Audiometry is done in a special room called isolated room. The frequencies usually tested are at octave steps i.e. 125, 250, 500, 1000, 2000, 4000 and 8000 Hz.


The intensity can be increased or decreased for each frequency and can vary from 10 dB to 120 dB till the patient threshold of hearing is assessed. AC is done by delivering pure tones to the ear under test through a suitable earphone whereas a vibrator is applied to the mastoid in assessing BC.


Normal hearing :Air conduction between 0 and 25 dB. Hearing is considered abnormal when there is < 10 dB difference between air and bone conduction thresholds

Isolated room

Normal Hearing

Low Frequency SNHLoss

Conductive Hearing Loss

High Frequency Sensorineural Hearing Loss

Mixed Hearing Loss

Determine Amount of Loss

From bass to treble, or low to high pitch
From faint to intense, or soft to loud



It is an objective test of the compliance and resistance of the middle ear mechanism. The compliance is maximal when air pressure in the external meatus equals that within the middle ear cavity and diminishes as the pressure increases or decreases, thus causing the tympanic membrane to be stretched..


By changing pressure in a sealed external auditory canal and then measuring the degree to which a tone is reflected from the ear drum, an indirect measure of middle ear pressure is obtained. .

Tympanometry

Auditory brainstem response (ABR) or brainstem evoked response audiometry (BERA) is an objective assessment of hearing to elicit brain stem responses to auditory stimulation by clicks or tone bursts. In this method, electrical potentials are generated by the cochlea, auditory nerve, brain stem and higher centres in response to auditory stimulus and picked up from the vertex by surface electrodes.

Auditory brainstem response (ABR)

Indications for ABR 1. To determine hearing threshold in infants, children, uncooperative adults and malingerers. 2. To differentiate between cochlear and retro cochlear pathology. 3. To diagnose brain stem pathology as multiple sclerosis or pontine tumours.

1. Rotation test: It is done by a rotating chair and has the disadvantage of stimulating both sides simultaneously.

2. Caloric test: In this test each labyrinth can be tested separately. Syringing the ear with hot or cold water induces convection currents within the lateral SCC and therefore stimulates them with resulting vertigo and nystagmus.

The patient lies with the head at an angle of 30 above the horizon, which brings the lateral SCC into a vertical plane. The ears are irrigated in turn with water at 30 C then at 44 C (7 C above and below body temperature).

This stimulation causes nystagmus with its quick component away from the ear on the cold testing and towards the ear on hot caloric testing (COWS). This nystagmus commonly lasts for about 2 minutes from the beginning of stimulation.


Canal paresis is present if the duration of nystagmus is reduced equally for both hot and cold tests. Canal paresis is suggestive of a lesion in the peripheral vestibular apparatus e.g. vestibule or vestibular nerve.