EXTERNAL EAR PHISIOLOGY

EXTERNAL EAR PHISIOLOGY

The physical structures of the ear are deceptively simple, especially in the light of their exquisite functions. The ear is an energy transducer ,which means that it converts acoustic energy into electrochemical energy.

External  ear consists of

a. Auricle/Ear pinna

b. External Auditory Meatus

  AURICLE/ EAR PINNA

PHYSIOLOGY OF EXTERNAL EAR                                                

         The external ear carries out two different physiological functions. They are referred to as auditory and non-auditory functions.

The auditory functions allow efficient sound transmission from environment to tympanic membrane.

The non auditory functions of the canals are protection of tympanic membrane and deeper structures from injury and maintanence of a clear passage through which sound is conducted to middle ear.

AUDITORY FUNCTIONS

      There are lots of controversies regarding part played by external ear in hearing with lots of studies contradicting each other .Following are the functions of external ear in hearing.

[1]. Funneling

       Funneling is collecting and directing sound from outer  environment to the medial part of the ear . Pinna acts as a funnel ,which collects the sound and direct it to the ear canal. This function of pinna is not applicable to human beings as pinna in human being are immovable.(extrinsic ear muscles are vestigial).

      There have been studies to demonstrate that pinna does not significantly affect the auditory sensitivity to different sounds . Bekesy and Rosenblith conducted studies for this:

      1. They found out threshold in 2 conditions .First,after filling the depressions and second, without filling the depressions. They demonstrated no significant difference in the 2 thresholds.

      2. They also did another set of experiment in which they inserted a tube into the ear canal to avoid the participation of pinna and found out the threshold.They also did it without inserting the tube. They demonstrated that again there was no significant difference between the two conditions.

Thus, they concluded that there is no significant role played by pinna on hearing.

  

    Blavert (1969) suggested that the pinna acts as a filter that alternate or passes frequencies depending upon their direction of arrival.

[2] Localization

         It is the ability to identify the direction of arrival of sound or locating the sound source in free space.

   Basic concepts of localization

Azimuth:

      This is the angle between the sound source and the mid line. Positive azimuth denotes that the sound is coming from the same side with respect to the reference ear. Negative azimuth denotes that the sound source is on the opposite side of the reference ear.

Inter aural time difference:

   When the sound arise on one side of the head , then the time taken by it to arrive at the distant(remote) ear will be more than that of the ear that is near. Since the speed of sound is a constant independent of frequency, the inter aural time difference for any frequency will be same for a particular location.

Inter aural intensity difference:

   When the signal is reaching the far ear, there will be more amount of reduction in intensity for the higher frequency component of the sound wave, where as low frequency components will suffer only small amount of reduction. This reduction is attributed to the wavelength of the two types of components.i.e; wavelength of high frequency signal is shorter when compared to low frequency signals.

       The circumference of head is 17.5cm. High frequencies signal (4 kHz) having wavelength 8.6 cm has to cross 17.5 cm to reach the opposite ear. So there is intensity reduction taking place. Whereas low frequency signal (250Hz) wavelength is much bigger than head circumference. So no reduction in intensity.

Inter aural phase difference:

   The time difference between the sounds reaching the two ears creates the phase difference between two ears. It will vary according to frequency.

E.g.; sound of 1000Hz (period of 1 msec) arrive at right ear 0.5 msec after it has reached left ear. The tone at right ear is half period (180) out of phase with tone at left ear.

Head shadow effect:

   When sound comes from one side of the head rather than from front there is reduction in intensity for the high frequency sounds going to remote ear. This effect is significant for frequencies above 1500Hz. This is attributed to the fact that frequencies above 1500Hz have wavelength small enough to be attenuated by diffraction effect caused by the presence of the head.

NON-AUDITORY PHYSIOLOGY

[1] Prevention of physical injuries.

       This function is taken care mainly by the shape of the ear canal and the hairs of ear canal.

        The ear canal in adult is in S shape which prevents any sharp object from directly invading the middle ear and inner ear portions.

The other way of protection comes from the presence of hairs in the cartilaginous 1/3^rd of the ear canal. If studied closely under an otoscope the most medial hairs lie almost flat against the canal wall, whereas laterally they become more upright and more obstructive to anything entering the canal.

[2] Self cleaning property.

   This property of self cleaning, prevent the collection of debris in the medial portion of canal thus preventing blockage.

  There are 2 mechanisms in this; namely ACTIVE and PASSIVE mechanisms.

   In passive mechanism the shape of the ear canal is responsible for the cleaning action. Its course is superior-inferior-superior. So debris falls of because of the shape.

    Active mechanism is actually more responsible for this act of self cleaning. It is known that the lining on the outer surface of tympanic membrane and lining surface of ear canal are both epithelial layers. There are 2 types of growth and movement pattern with respect to umbo and manubrium. The lateral movement of epithelium with respect to umbo is called RADIAL PATTERNS of movement. The lateral movement of epithelium with respect to manubrium is called HORIZONTAL PATTERN. These two combination lead to lateral or external movement of old work torn layer of epithelium which is replaced by new layer. This movement when observed under    microscope appears like sea waves and thus also called as sea waves. This movement also helps in carrying a lot of debris along with the skin layer.

This movement of epithelium was studied by Alberti (1964).He used India ink to mark certain positions in the ear canal and on tympanic membrane and studied the movement pattern in 40 young adults. He reported about radial and horizontal movement of epithelium by as much as 0.07 mm per day. He referred them horizontal migration and radial migration.

    The actual mechanism underlying the skin migration is not understood. It is hypothesized that deeper part of epidermis start migrating outward detach themselves and desquamated materials then shed out of ear canal by certain mechanical activities such as chewing or other movements.(Johnson and Hawke,1988). This migration process is unique to ear canal skin. Possibly due to the fact that skin lining the EAC is enclosed and not subjected to usual surface contact that normally remove the dead skin layer from other skin surfaces.

[3] Properties of secretions of ear canal.

    The secretions of the ear canal are ceruman and sebum secreted by ceruminous and sebaceous glands respectively. These two combine to form earwax, which has anti bacterial and antifungal properties owing to the components which it has. (Cerotic acid, cholesterol, hexes bases, neurostzanic acid-a bitter substance, lysine, protein, amino acids etc.).The oily nature of earwax also does water proofing job thus preventing infections due to moisture. The bitterness in taste and its noxious nature also kills the insects which enters the ear canal somehow.

PRESSURE GAIN OF THE EAR

The ear canal may be considered as a tube, which is opened at one end and close at the other end by the presence of tympanic membrane. The ear canal contains air and it resonates in a frequency with wavelength 4 times the length of the tube.

So, Wavelength =4 times the length of the canal.

            גּ = 4l

From this we can understand that only Quarter of the wave can fit into the tube at any one pass .Thus the ear canal is called Quarter wave resonator.

       Resonance frequency of the ear canal can be calculated by using the following expression.

             גּ = c/f                         c = velocity of sound 34000 cm/s

Average length of ear canal is 2.5 cm. so,

            4×2.5 = 34000/f             f = 34000/4× 2.5 = 3400 Hz.

Wiener and Ross (1946)  In a study a  probe microphone was kept in two locations in the ear canal. One near the tympanic membrane and the other , one half-way between tympanic membrane and concha. At these locations they measured sound pressure distribution in the canal from free field sound presented at 3 angles of incidence(0º , 45º, and 90º). The pressure distribution in the ear canal at various frequencies varies, and most obvious pressure gain was reported in the region of 2-4 kHz(10-15 dB) with maximum increment.(17-22 dB at 3kHz).

The outer ear(concha or EAC ) act as resonator and amount of resonance provided is upto 20 dB between 2-5.

The sound source (distance) has a differential effect on the amount of pressure developed in the canal across frequencies.

Transfer function of the outer ear

 

·         The biggest change:

near 3 kHz

due to the resonance of the external auditory meatus

Reference:ZEMLIN,WILLIARD R.(1998)Speech and hearing science anatomy &physiology,3^rd eddition