The passage of sound in the ear. How do we hear

Many of us are sometimes interested in a simple physiological question regarding how we hear. Let's look at what our hearing organ consists of and how it works.

First of all, we note that the auditory analyzer has four parts:

1. Outer ear. It includes the auditory drive, the auricle, and the eardrum. The latter serves to isolate the inner end of the auditory wire from the environment. As for the ear canal, it has a completely curved shape, about 2.5 centimeters long. On the surface of the ear canal there are glands, and it is also covered with hairs. It is these glands that secrete ear wax, which we clean out in the morning. Also, the ear canal is necessary to maintain the necessary humidity and temperature inside the ear.
2. Middle ear. That component of the auditory analyzer, which is located behind the eardrum and is filled with air, is called the middle ear. It is connected by the Eustachian tube to the nasopharynx. The Eustachian tube is a fairly narrow cartilaginous canal that is normally closed. When we make swallowing movements, it opens and air enters the cavity through it. Inside the middle ear are three small auditory ossicles: the anvil, malleus, and stirrup. The hammer, with the help of one end, is connected to the stirrup, and it is already with a casting in the inner ear. Under the influence of sounds, the tympanic membrane is in constant motion, and the auditory ossicles further transmit its vibrations inward. It is one of the most important elements that must be studied when considering what structure of the human ear
3. Inner ear. In this part of the auditory ensemble, there are several structures at once, but only one of them, the cochlea, controls hearing. It got its name because of its spiral shape. It has three channels that are filled with lymph fluids. In the middle channel, the liquid differs significantly in composition from the rest. The organ responsible for hearing is called the organ of Corti and is located in the middle canal. It consists of several thousand hairs that pick up the vibrations created by the fluid moving through the channel. It also generates electrical impulses, which are then transmitted to the cerebral cortex. A particular hair cell responds to a particular kind of sound. If it happens that the hair cell dies, then the person ceases to perceive this or that sound. Also, in order to understand how a person hears, one should also consider the auditory pathways.

auditory pathways

They are a collection of fibers that conduct nerve impulses from the cochlea itself to the auditory centers of your head. It is through the pathways that our brain perceives a particular sound. The auditory centers are located in the temporal lobes of the brain. The sound that travels through the outer ear to the brain lasts about ten milliseconds.

How do we perceive sound?

The human ear processes the sounds received from the environment into special mechanical vibrations, which then convert the fluid movements in the cochlea into electrical impulses. They pass along the pathways of the central auditory system to the temporal parts of the brain, so that they can then be recognized and processed. Now the intermediate nodes and the brain itself extracts some information regarding the volume and pitch of the sound, as well as other characteristics, such as the time of picking up the sound, the direction of the sound, and others. Thus, the brain can perceive the received information from each ear in turn or jointly, receiving a single sensation.

It is known that inside our ear there are some “templates” of already studied sounds that our brain has recognized. They help the brain to correctly sort and identify the primary source of information. If the sound is reduced, then the brain accordingly begins to receive incorrect information, which can lead to misinterpretation of sounds. But not only sounds can be distorted, over time the brain is also subjected to incorrect interpretation of certain sounds. The result may be an incorrect reaction of a person or an incorrect interpretation of information. In order to hear correctly and reliably interpret what we hear, we need synchronous work of both the brain and the auditory analyzer. That is why it can be noted that a person hears not only with the ears, but also with the brain.

Thus, the structure of the human ear is quite complex. Only the coordinated work of all parts of the hearing organ and the brain will allow us to correctly understand and interpret what we hear.

The sense of hearing is one of the most important things in human life. Hearing and speech together constitute an important means of communication between people, serve as the basis for the relationship of people in society. Hearing loss can lead to behavioral problems. Deaf children cannot learn full speech.

With the help of hearing, a person picks up various sounds that signal what is happening in the outside world, the sounds of the nature around us - the rustles of the forest, the singing of birds, the sounds of the sea, as well as various musical works. With the help of hearing, the perception of the world becomes brighter and richer.

The ear and its function. Sound, or a sound wave, is an alternating rarefaction and condensation of air, propagating in all directions from the sound source. A sound source can be any vibrating body. Sound vibrations are perceived by our organ of hearing.

The organ of hearing is built very complex and consists of the outer, middle and inner ear. The outer ear consists of the pinna and the ear canal. The auricles of many animals can move. This helps the animal to catch where even the quietest sound comes from. Human auricles also serve to determine the direction of sound, although they are immobile. The ear canal connects the outer ear with the next section - the middle ear.

The ear canal is blocked at the inner end by a tightly stretched tympanic membrane. A sound wave striking the eardrum causes it to oscillate, vibrate. The vibration frequency of the tympanic membrane is greater, the higher the sound. The stronger the sound, the more the membrane vibrates. But if the sound is very weak, barely audible, then these vibrations are very small. The minimum audibility of a trained ear is almost on the border of those vibrations that are created by the random movement of air molecules. This means that the human ear is a unique hearing instrument in terms of sensitivity.

Behind the tympanic membrane lies the air-filled cavity of the middle ear. This cavity is connected to the nasopharynx by a narrow passage - the auditory tube. When swallowing, air is exchanged between the pharynx and the middle ear. A change in the pressure of the outside air, for example, in an airplane, causes an unpleasant sensation - it "stuffs the ears." It is explained by the deflection of the tympanic membrane due to the difference between atmospheric pressure and the pressure in the middle ear cavity. When swallowing, the auditory tube opens and the pressure on both sides of the eardrum equalizes.

In the middle ear are three small, successively interconnected bones: the hammer, anvil, and stirrup. The hammer connected to the tympanic membrane transmits its vibrations first to the anvil, and then the enhanced vibrations are transmitted to the stirrup. In the plate separating the cavity of the middle ear from the cavity of the inner ear, there are two windows covered with thin membranes. One window is oval, a stirrup “knocks” at it, the other is round.

The inner ear begins behind the middle ear. It is located deep in the temporal bone of the skull. The inner ear is a system of labyrinth and convoluted canals filled with fluid. There are two organs in the labyrinth at once: the organ of hearing - the cochlea and the organ of balance - the vestibular apparatus. The cochlea is a spirally twisted bone canal that has two and a half turns in humans. Vibrations of the membrane of the foramen ovale are transmitted to the fluid that fills the inner ear. And it, in turn, begins to oscillate with the same frequency. Vibrating, the liquid irritates the auditory receptors located in the cochlea. The canal of the cochlea along its entire length is divided in half by a membranous septum. Part of this partition consists of a thin membrane - a membrane. On the membrane are perceiving cells - auditory receptors. Vibrations of the fluid filling the cochlea irritate individual auditory receptors. They generate impulses that are transmitted along the auditory nerve to the brain. The diagram shows all the successive processes of the transformation of a sound wave into a nervous signaling. Auditory perception. In the brain, there is a distinction between the strength, height and nature of the sound, its location in space. We hear with two ears, and this is of great importance in determining the direction of sound. If sound waves arrive at the same time in both ears, then we perceive the sound in the middle (front and back). If sound waves arrive a little earlier in one ear than in the other, then we perceive the sound either on the right or on the left.

1. Sound-conducting and sound-receiving parts of the hearing aid.

2. The role of the outer ear.

3. The role of the middle ear.

4. The role of the inner ear.

5. Determining the localization of the sound source in the horizontal plane - binaural effect.

6. Determining the localization of the sound source in the vertical plane.

7. Hearing aids and prostheses. Tympanometry.

8. Tasks.

Rumor - perception of sound vibrations, which is carried out by the organs of hearing.

4.1. The sound-conducting and sound-receiving parts of the hearing aid

The human hearing organ is a complex system consisting of the following elements:

1 - auricle; 2 - external auditory meatus; 3 - eardrum; 4 - hammer; 5 - anvil; 6 - stirrup; 7 - oval window; 8 - vestibular ladder; 9 - round window; 10 - drum stairs; 11 - cochlear canal; 12 - the main (basilar) membrane.

The structure of the hearing aid is shown in fig. 4.1.

According to the anatomical feature, the external ear (1-3), the middle ear (3-7) and the inner ear (7-13) are distinguished in the human hearing aid. According to the functions performed in the human hearing aid, sound-conducting and sound-receiving parts are distinguished. This division is shown in Fig. 4.2.

Rice. 4.1. The structure of the hearing aid (a) and the elements of the organ of hearing (b)

Rice. 4.2. Schematic representation of the main elements of the human hearing aid

4.2. The role of the outer ear

Function of the outer ear

The outer ear consists of the auricle, the auditory canal (in the form of a narrow tube), and the tympanic membrane. The auricle plays the role of a sound collector, concentrating sound

waves on the ear canal, as a result of which the sound pressure on the eardrum increases compared to the sound pressure in the incident wave by about 3 times. The external auditory canal, together with the auricle, can be compared to a tube-type resonator. The tympanic membrane, which separates the outer ear from the middle ear, is a plate consisting of two layers of collagen fibers oriented in different ways. The thickness of the membrane is about 0.1 mm.

The reason for the greatest sensitivity of the ear in the 3 kHz region

Sound enters the system through the external auditory canal, which is an acoustic tube closed on one side with a length L = 2.5 cm. The sound wave passes through the auditory canal and is partially reflected from the eardrum. As a result, the incident and reflected waves interfere and form a standing wave. Acoustic resonance occurs. Conditions for its manifestation: the wavelength is 4 times the length of the air column in the ear canal. In this case, the air column inside the channel will resonate to sound with a wavelength equal to four of its lengths. In the auditory canal, as in a pipe, a wave of length λ = 4L = 4x0.025 = 0.1 m will resonate. The frequency at which acoustic resonance occurs is determined as follows: ν = v /λ = 340/(4x0.025) = 3.4 kHz. This resonant effect explains the fact that the human ear is most sensitive at about 3 kHz (see Equal Loudness Curves in Lecture 3).

4.3. The role of the middle ear

The structure of the middle ear

The middle ear is a device designed to transmit sound vibrations from the air of the outer ear to the liquid medium of the inner ear. The middle ear (see Figure 4.1) contains the tympanic membrane, the oval and round windows, and the auditory ossicles (hammer, anvil, stirrup). It is a kind of drum (0.8 cm 3 in volume), which is separated from the outer ear by the tympanic membrane, and from the inner ear by oval and round windows. The middle ear is filled with air. Any difference

pressure between the outer and middle ear leads to deformation of the tympanic membrane. The tympanic membrane is a funnel-shaped membrane pressed into the middle ear. From it, sound information is transmitted to the bones of the middle ear (the shape of the tympanic membrane ensures the absence of natural vibrations, which is very important, since the natural vibrations of the membrane would create a noise background).

Sound wave penetration through the air-liquid interface

To understand the purpose of the middle ear, consider direct the transition of sound from air to liquid. At the interface between two media, one part of the incident wave is reflected, and the other part passes into the second medium. The share of energy transferred from one medium to another depends on the value of the transmittance β (see formula 3.10).

That is, when moving from air to water, the sound intensity level decreases by 29 dB. From an energetic point of view, such a transition is absolutely inefficient. For this reason, there is a special transmission mechanism - a system of auditory ossicles, which perform the function of matching the wave resistances of air and liquid media to reduce energy losses.

The physical basis of the functioning of the auditory ossicles

The ossicular system is a sequential link, the beginning of which (hammer) connected with the tympanic membrane of the outer ear, and the end (stapes)- with an oval window of the inner ear (Fig. 4.3).

Rice. 4.3. Diagram of sound wave propagation from the outer ear through the middle ear to the inner ear:

1 - eardrum; 2 - hammer; 3 - anvil; 4 - stirrup; 5 - oval window; 6 - round window; 7 - drum stroke; 8 - snail move; 9 - vestibular course

Rice. 4.4. Schematic representation of the location of the tympanic membrane and the oval window: S bp - the area of ​​the tympanic membrane; S oo - area of ​​the oval window

The area of ​​the tympanic membrane is equal to Bbp = 64 mm 2, and the area of ​​the oval window S oo = 3 mm 2. Schematically them

the mutual arrangement is shown in fig. 4.4.

Sound pressure P 1 acts on the eardrum, creating a force

Bones system works like leverage with shoulder ratio

L 1 /L 2 \u003d 1.3, which gives a gain in strength from the side of the inner ear by 1.3 times (Fig. 4.5).

Rice. 4.5. Schematic representation of the operation of the ossicular system as a lever

Therefore, a force F 2 \u003d 1.3F 1 acts on the oval window, creating a sound pressure P 2 in the liquid medium of the inner ear, which is equal to

The performed calculations show that when sound passes through the middle ear, its intensity level increases by 28 dB. The loss of the sound intensity level during the transition from air to liquid is 29 dB. The total intensity loss is only 1 dB instead of 29 dB, which would be the case in the absence of the middle ear.

Another function of the middle ear is to reduce the transmission of vibrations in the case of sound of great intensity. With the help of muscles, the connection between the bones can be reflexively weakened at too high sound intensities.

A large change in pressure in the environment (for example, associated with a change in altitude) can cause the eardrum to stretch, accompanied by pain, or even rupture. To protect against such pressure drops, a small Eustachian tube, which connects the middle ear cavity to the upper part of the pharynx (to the atmosphere).

4.4. The role of the inner ear

The sound-perceiving system of the hearing aid is the inner ear and the cochlea that enters it.

The inner ear is a closed cavity. This cavity, called the labyrinth, has a complex shape and is filled with a fluid - perilymph. It consists of two main parts: the cochlea, which converts mechanical vibrations into an electrical signal, and the semicircle of the vestibular apparatus, which ensures the balance of the body in the field of gravity.

The structure of the snail

The cochlea is a hollow bone formation 35 mm long and has the shape of a cone-shaped spiral containing 2.5 curls.

The section of the cochlea is shown in fig. 4.6.

Along the entire length of the cochlea, two membranous septa run along it, one of which is called vestibular membrane, and the other - main membrane. space between

Rice. 4.6. Schematic structure of the cochlea containing channels: B - vestibular; B - drum; U - cochlear; RM - vestibular (Reissner) membrane; PM - cover plate; OM - main (basilar) membrane; KO - organ of Corti

them - the cochlear passage - is filled with a fluid called endolymph.

The vestibular and tympanic canals are filled with a special fluid called perilymph. At the top of the cochlea, they are interconnected. The vibrations of the stirrup are transmitted to the membrane of the oval window, from it to the perilymph of the vestibular passage, and then through the thin vestibular membrane to the endolymph of the cochlear passage. Endolymph vibrations are transmitted to the main membrane, on which the organ of Corti is located, containing sensitive hair cells (about 24,000), in which electrical potentials arise, transmitted through the auditory nerve to the brain.

The tympanic passage ends with a round window membrane, which compensates for the movement of the relymph.

The length of the main membrane is approximately 32 mm. It is very heterogeneous in its shape: it expands and thins in the direction from the oval window to the top of the cochlea. As a result, the modulus of elasticity of the main membrane near the base of the cochlea is about 100 times greater than at the top.

Frequency-selective properties of the main membrane of the cochlea

The main membrane is a heterogeneous transmission line of mechanical excitation. Under the action of an acoustic stimulus, a wave propagates along the main membrane, the degree of attenuation of which depends on the frequency: the lower the frequency of stimulation, the farther from the oval window the wave propagates along the main membrane. So, for example, a wave with a frequency of 300 Hz will propagate approximately 25 mm from the oval window before attenuation, and a wave with a frequency of 100 Hz will propagate approximately 30 mm.

It is currently believed that the perception of pitch is determined by the position of the maximum vibration of the main membrane.

Oscillations of the main membrane stimulate receptor cells located in the organ of Corti, resulting in action potentials transmitted by the auditory nerve to the cerebral cortex.

4.5. Determining the localization of the sound source in the horizontal plane - binaural effect

binaural effect- the ability to set the direction to the sound source in the horizontal plane. The essence of the effect is illustrated in Fig. 4.7.

Let the sound source be alternately placed at points A, B and C. From point A, which is directly in front of the face, the sound wave hits both ears equally, while the path of the sound wave to the auricles is the same, i.e. for both ears, the path difference δ and the phase difference Δφ of sound waves are equal to zero: δ = 0, Δφ = 0. Therefore, the incoming waves have the same phase and intensity.

From point B, the sound wave arrives at the left and right auricles in different phases and with different intensities, since it travels different distances to the ears.

If the source is located at point C, opposite one of the auricles, then in this case the path difference δ can be taken equal to the distance between the auricles: δ ≈ L ≈ 17 cm = 0.17 m. In this case, the phase difference Δφ can be calculated by the formula: Δφ = (2π/λ) δ. For frequency ν = 1000 Hz and v« 340 m/s λ = v/ν = 0.34 m. From here we get: Δφ = (2π/λ) δ = (2π/0.340)*0.17 = π. In this example, the waves arrive in antiphase.

All real directions to the sound source in the horizontal plane will correspond to a phase difference from 0 to π (from 0

Thus, the phase difference and the unevenness of the intensities of sound waves entering different ears provide a binaural effect. The man with the

Rice. 4.7. Different localization of the sound source (A, B, C) in the horizontal plane: L - the distance between the auricles

with limited hearing, it can fix the direction to the sound source with a phase difference of 6 °, which corresponds to fixing the direction to the sound source with an accuracy of 3 °.

4.6. Determining the localization of the sound source in the vertical plane

Let us now consider the case when the sound source is located in a vertical plane oriented perpendicular to the straight line connecting both ears. In this case, it is equally removed from both ears and there is no phase difference. The intensity values ​​of the sound entering the right and left ears are the same. Figure 4.8 shows two such sources (A and C). Will the hearing aid distinguish between these sources? Yes. In this case, this will happen due to the special shape of the auricle, which (shape) helps to determine the localization of the sound source.

The sound coming from these sources falls on the auricles at different angles. This leads to the fact that the diffraction of sound waves on the auricles occurs in different ways. As a result, the spectrum of the sound signal entering the external auditory canal is superimposed with diffraction maxima and minima, depending on the position of the sound source. These differences make it possible to determine the position of the sound source in the vertical plane. Apparently, as a result of the vast experience of listening, people have learned to associate different spectral characteristics with the corresponding directions. This is confirmed by experimental data. In particular, it has been established that the ear can be "deceived" by a special selection of the spectral composition of sound. So, a person perceives sound waves containing the main part of the energy in the 1 kHz region,

Rice. 4.8. Different localization of the sound source in the vertical plane

localized "behind" regardless of the actual direction. A sound wave with frequencies below 500 Hz and in the region of 3 kHz is perceived as being localized "in front". Sound sources containing most of the energy in the 8 kHz region are recognized as being localized "from above".

4.7. Hearing aids and prostheses. Tympanometry

Hearing loss due to impaired conduction of sound or partial impairment of sound perception can be compensated with the help of hearing aids-amplifiers. In recent years, great progress has been made in this area, associated with the development of audiology and the rapid introduction of achievements in electroacoustic equipment based on microelectronics. Miniature hearing aids operating in a wide frequency range have been created.

However, in some severe forms of hearing loss and deafness, hearing aids do not help patients. This occurs, for example, when deafness is associated with damage to the receptor apparatus of the cochlea. In this case, the cochlea does not generate electrical signals when subjected to mechanical vibrations. Such lesions can be caused by an incorrect dosage of drugs used to treat diseases that are not at all related to ENT diseases. At present, partial rehabilitation of hearing is possible in such patients. To do this, it is necessary to implant electrodes in the cochlea and apply electrical signals to them corresponding to those that arise when exposed to a mechanical stimulus. Such prosthetics of the main function of the cochlea is carried out with the help of cochlear prostheses.

Tympanometry - a method for measuring the compliance of the sound-conducting apparatus of the auditory system under the influence of hardware changes in air pressure in the ear canal.

This method allows you to evaluate the functional state of the tympanic membrane, the mobility of the ossicular chain, pressure in the middle ear and the function of the auditory tube.

Rice. 4.9. Determination of compliance of the sound-conducting apparatus by tympanometry

The study begins with the installation of a probe with an ear mold put on it, which tightly covers the ear canal at the beginning of the external auditory canal. Excessive (+) or insufficient (-) pressure is created through the probe in the ear canal, and then a sound wave of a certain intensity is applied. Having reached the eardrum, the wave is partially reflected and returns to the probe (Fig. 4.9).

Measuring the intensity of the reflected wave allows you to judge the sound-conducting capabilities of the middle ear. The greater the intensity of the reflected sound wave, the less the mobility of the sound-conducting system. A measure of the mechanical compliance of the middle ear is mobility parameter, measured in arbitrary units.

During the study, the pressure in the middle ear is changed from +200 to -200 dPa. At each pressure value, the mobility parameter is determined. The result of the study is a tympanogram that reflects the dependence of the mobility parameter on the amount of excess pressure in the ear canal. In the absence of middle ear pathology, the maximum mobility is observed in the absence of excess pressure (P = 0) (Fig. 4.10).

Rice. 4.10. Tympanograms with varying degrees of system mobility

Increased mobility indicates insufficient elasticity of the tympanic membrane or a dislocation of the auditory ossicles. Decreased mobility indicates excessive stiffness of the middle ear associated, for example, with the presence of fluid.

With the pathology of the middle ear, the appearance of the tympanogram changes

4.8. Tasks

1. The size of the auricle is d = 3.4 cm. At what frequency will diffraction phenomena be observed on the auricle? Solution

The phenomenon of diffraction becomes noticeable when the wavelength is comparable to the size of the obstacle or gap: λ ≤ d. At shorter lengths waves or high frequencies diffraction becomes negligible.

λ \u003d v / ν \u003d 3.34, ν \u003d v / d \u003d 334 / 3.34 * 10 -2 \u003d 10 4 Hz. Answer: less than 10 4 Hz.

Rice. 4.11. The main types of tympanograms in pathologies of the middle ear: A - no pathology; B - exudative otitis media; C - violation of the patency of the auditory tube; D - atrophic changes in the tympanic membrane; E - rupture of the auditory ossicles

2. Determine the maximum force acting on the human eardrum (area S = 64 mm 2) for two cases: a) hearing threshold; b) pain threshold. The sound frequency is taken equal to 1 kHz.

Solution

The sound pressures corresponding to the thresholds of hearing and pain are ΔΡ 0 = 3?10 -5 Pa and ΔP m = 100 Pa, respectively. F = ΔΡ*S. Substituting the threshold values, we get: F 0 \u003d 310 -5? 64? 10 -6 \u003d 1.9-10 -9 H; F m = 100? 64-10 -6 \u003d 6.410 -3 H.

Answer: a) F 0 = 1.9 nN; b) F m = 6.4 mN.

3. The difference in the path of sound waves arriving in the left and right ear of a person is χ \u003d 1 cm. Determine the phase shift between both sound sensations for a tone with a frequency of 1000 Hz.

Solution

The phase difference resulting from the path difference is: Δφ = 2πνχ/ν = 6.28x1000x0.01/340 = 0.18. Answer:Δφ = 0.18.

The sound wave is a double oscillation of the medium, in which a phase of pressure increase and a phase of pressure decrease are distinguished. Sound vibrations enter the external auditory canal, reach the eardrum and cause it to vibrate. In the phase of pressure increase or thickening, the tympanic membrane, together with the handle of the malleus, moves inwards. In this case, the body of the anvil, connected to the head of the hammer, due to the suspension ligaments, is displaced outward, and the long sprout of the anvil is inward, thus displacing the inward and stirrup. Pressing into the window of the vestibule, the stirrup jerkily leads to a displacement of the perilymph of the vestibule. Further propagation of the wave along the scala vestibule transmits oscillatory movements to the Reissner membrane, which, in turn, sets in motion the endolymph and, through the main membrane, the perilymph of the scala tympani. As a result of this movement of the perilymph, oscillations of the main and Reissner membranes occur. With each movement of the stirrup towards the vestibule, the perilymph eventually leads to a displacement towards the tympanic cavity of the membrane of the vestibule window. In the pressure reduction phase, the transmission system returns to its original position.

The air way of delivering sounds to the inner ear is the main one. Another way of conducting sounds to the spiral organ is bone (tissue) conduction. In this case, a mechanism comes into play, in which the sound vibrations of the air fall on the bones of the skull, propagate in them and reach the cochlea. However, the mechanism of bone tissue sound transmission can be twofold. In one case, a sound wave in the form of two phases, propagating along the bone to the liquid media of the inner ear, in the pressure phase will protrude the membrane of the round window and, to a lesser extent, the base of the stirrup (taking into account the practical incompressibility of the liquid). Simultaneously with such a compression mechanism, another one can be observed - an inertial variant. In this case, when sound is transmitted through the bone, the vibration of the sound-conducting system will not coincide with the vibrations of the bones of the skull and, consequently, the main and Reissner membranes will vibrate and excite the spiral organ in the usual way. Vibration of the bones of the skull can be caused by touching it with a sounding tuning fork or telephone. Thus, the bone transmission path in case of violation of sound transmission through the air becomes of great importance.

Auricle. The role of the auricle in the physiology of human hearing is small. It has some significance in ototopics and as collectors of sound waves.

External auditory meatus. It is a tube shape, due to which it is a good conductor of sounds in depth. The width and shape of the ear canal does not play a special role in sound conduction. At the same time, its mechanical blockage prevents the propagation of sound waves to the eardrum and leads to a noticeable hearing impairment. In the ear canal near the tympanic membrane, a constant level of temperature and humidity is maintained, regardless of fluctuations in temperature and humidity in the external environment, which ensures the stability of the elastic media of the tympanic cavity. Due to the special structure of the outer ear, the pressure of a sound wave in the external auditory canal is twice as high as in a free sound field.

Tympanic membrane and auditory ossicles. The main role of the tympanic membrane and the auditory ossicles is to transform sound vibrations of high amplitude and low strength into vibrations of the fluids of the inner ear with low amplitude and high strength (pressure). The vibrations of the tympanic membrane bring the movement of the hammer, anvil and stirrup into subordination. In turn, the stirrup transmits vibrations to the perilymph, which causes displacement of the membranes of the cochlear duct. The movement of the main membrane causes irritation of the sensitive, hair cells of the spiral organ, as a result of which nerve impulses arise that follow the auditory pathway to the cerebral cortex.

The tympanic membrane vibrates primarily in its lower quadrant with the synchronous movement of the malleus attached to it. Closer to the periphery, its fluctuations decrease. At maximum sound intensity, the oscillations of the tympanic membrane can vary from 0.05 to 0.5 mm, and the amplitude of oscillations is greater for low-frequency tones, and less for high-frequency tones.

The transformational effect is achieved due to the difference in the area of ​​the tympanic membrane and the area of ​​the base of the stirrup, the ratio of which is approximately 55:3 (area ratio 18:1), as well as due to the lever system of the auditory ossicles. When converted to dB, the lever action of the ossicular system is 2 dB, and the increase in sound pressure due to the difference in the ratio of the useful areas of the tympanic membrane to the base of the stirrup provides sound amplification by 23 - 24 dB.

According to Bekeshi /I960/, the total acoustic gain of the sound pressure transformer is 25 - 26 dB. This increase in pressure compensates for the natural loss of sound energy resulting from the reflection of a sound wave during its transition from air to liquid, especially for low and medium frequencies (Vulshtein JL, 1972).

In addition to the transformation of sound pressure, the eardrum; also performs the function of sound protection (shielding) of the snail window. Normally, sound pressure transmitted through the ossicular system to the cochlear media reaches the vestibule window somewhat earlier than it reaches the cochlear window through the air. Due to the pressure difference and phase shift, perilymph movement occurs, causing bending of the main membrane and irritation of the receptor apparatus. In this case, the membrane of the cochlear window oscillates synchronously with the base of the stirrup, but in the opposite direction. In the absence of the tympanic membrane, this sound transmission mechanism is disrupted: the sound wave following the external auditory canal simultaneously reaches the window of the vestibule and the cochlea in phase, as a result of which the action of the wave cancels out. Theoretically, there should be no shift in the perilymph and irritation of sensitive hair cells. In fact, with a complete defect of the tympanic membrane, when both windows are equally accessible to sound waves, hearing is reduced to 45 - 50. Destruction of the ossicular chain is accompanied by a significant loss of hearing (up to 50-60 dB).

The design features of the lever system make it possible not only to amplify weak sounds, but also to perform a protective function to a certain extent - to weaken the transmission of strong sounds. With weak sounds, the base of the stirrup oscillates mainly around the vertical axis. With strong sounds, sliding occurs in the anvil-malleolar joint, mainly with low-frequency tones, as a result of which the movement of the long process of the malleus is limited. Along with this, the base of the stirrup begins to oscillate mainly in the horizontal plane, which also weakens the transmission of sound energy.

In addition to the tympanic membrane and the auditory ossicles, the protection of the inner ear from excess sound energy is carried out as a result of contraction of the muscles of the tympanic cavity. With the contraction of the stirrup muscle, when the acoustic impedance of the middle ear increases sharply, the sensitivity of the inner ear to sounds, mainly of low frequency, decreases to 45 dB. Based on this, there is an opinion that the stapes muscle protects the inner ear from excess energy of low-frequency sounds (Undrits V.F. et al., 1962; Moroz B.S., 1978)

The function of the tensor tympanic membrane muscle remains poorly understood. It is believed to have more to do with ventilation of the middle ear and maintaining normal pressure in the tympanic cavity than with protection of the inner ear. Both intra-ear muscles also contract when opening the mouth, swallowing. At this point, the sensitivity of the cochlea to the perception of low sounds decreases.

The sound-conducting system of the middle ear functions optimally when the air pressure in the tympanic cavity and mastoid cells is equal to atmospheric pressure. Normally, the air pressure in the middle ear system is balanced with the pressure of the external environment, this is achieved due to the auditory tube, which, opening into the nasopharynx, provides air flow into the tympanic cavity. However, the continuous absorption of air by the mucous membrane of the tympanic cavity creates a slightly negative pressure in it, which requires constant alignment with atmospheric pressure. At rest, the auditory tube is usually closed. It opens when swallowing or yawning as a result of contraction of the muscles of the soft palate (stretching and lifting the soft palate). When the auditory tube is closed as a result of a pathological process, when air does not enter the tympanic cavity, a sharply negative pressure arises. This leads to a decrease in auditory sensitivity, as well as extravasation of serous fluid from the mucous membrane of the middle ear. Hearing loss in this case, mainly tones of low and medium frequencies, reaches 20 - 30 dB. Violation of the ventilation function of the auditory tube also affects the intralabyrinthine pressure of the fluids of the inner ear, which in turn impairs the conduction of low-frequency sounds.

Sound waves, causing the movement of the labyrinth fluid, vibrate the main membrane, on which the sensitive hair cells of the spiral organ are located. Irritation of hair cells is accompanied by a nerve impulse that enters the spiral ganglion, and then along the auditory nerve to the central sections of the analyzer.

The singing of birds, a pleasant melody, the happy laughter of a cheerful child... What would our life be like without sounds? Not many people think about what complex mechanisms we carry in our body. Our ability to hear depends on an extremely complex, interconnected, and detailed system. “The hearing ear and the seeing eye, the Lord made both” (Proverbs 20:12). He does not want us to have any doubts about the authorship of this system. Quite the opposite, God wants man to walk firmly in the realization of the truth of Creation: “Know that the Lord is God, and that He created us, and we are His” (Psalm 99:3).

Human hearing designed to capture a wide range of sound waves, turn them into millions of electrical impulses, sending them further to the brain for deep and fast analysis. All sounds are actually "listened" by the brain and then presented to us as coming from an external source. How does the hearing system work?

The process begins with sound - the oscillatory movement of air - vibration, in which pulses of air pressure propagate towards the listener, eventually reaching the eardrum. Our ear is extremely sensitive and is able to perceive pressure changes as small as 0.0000000001 atmospheres.

The ear consists of 3 parts: outer, middle and inner. The sound first reaches the outer ear through the air, then hitting the eardrum. The membrane transmits vibration to the bones. Here there is a change in the way sound is conducted - from air to the bones. The sound then travels to the inner ear, where it is transmitted by fluid. Thus, in the process of hearing, 3 methods of sound transmission are involved: air, bone, liquid. Let's take a closer look at them.

Human Hearing: The Journey of Sound

The sound first reaches the ears, which act like satellite dishes. (Fig.1) The human auricle has its own unique relief of bulges, concavities and grooves, due to which the sound comes from the auricle to the auditory canal in two ways. This is necessary for the finest acoustic and three-dimensional analysis, allowing you to recognize the direction and source of sound, which is important for language communication.

Figure 1 Source: APP, www.apologeticspress.org

The auricle also amplifies sound waves, which then enter the auditory canal - the space from the shell to the eardrum is about 2.5 cm long and about 0.7 cm in diameter. The design of the Lord is already directly visible here - our finger is thicker than the auditory canal! Otherwise we would hurt hearing still in infancy. This passage is shaped to create an optimal range resonance.

Another interesting characteristic is the presence of wax (earwax), which is constantly secreted from 4000 glands. It has antiseptic properties, protecting the ear from bacteria and insects. But how then is this narrow passage continually cleared? The Lord took care of this detail, creating a cleansing mechanism.

It turns out that inside the passage, any particles move in a spiral, since the cells on the surface of the auditory canal line up in the form of a spiral directed outward. In addition, the epidermis (the top layer of the skin) grows there to the sides, and not up, as it usually happens on the skin. Falling off, it moves in a spiral outward to the auricle, constantly taking the wax with it. Without such a cleaning system, our ear would quickly clog up.

Human hearing: the middle ear masterfully solves the most difficult problem in physics

Have you ever tried to yell at a person underwater? This is practically impossible, since 99.9% of the sound traveling through the air is reflected by the water. But in our ear, sound travels to the sensitive cells of the cochlea through the liquid, since these cells cannot be in the air. How is this most difficult task of sound transition from air to liquid solved in our ear? We need a matching device. This role is played by the middle ear, which consists of a membrane, special bones, muscles and nerves. (See Fig. 2)

Upon reaching the eardrum, the sound causes it to vibrate. Swinging, she sets in motion a hammer, whose handle is attached to the membrane. The hammer, in turn, forces the next bone, called the anvil, to move. Between them is a cartilaginous joint, which, like all other joints, must be constantly lubricated to maintain operation. The Lord took care of this too - everything is done automatically without our participation, so we have nothing to worry about.

The lower part of the anvil, which looks like an axis, transmits the movement to the next bone, called the stirrup (it resembles a stirrup in shape). As a result of the transmission of motion, the stirrup is constantly pushed. The lower oval base of the stirrup resembles a piston and enters the oval window of the cochlea. This piston is connected to the oval window by a special fixture, strong but movable, so that the piston moves back and forth in the oval window.

The eardrum is amazingly sensitive. It is able to respond to vibration with a diameter of only one hydrogen atom! Even more surprising is that the membrane is a living tissue with blood vessels and nerves. Blood cells are thousands of times larger than a hydrogen atom and while moving in the vessels constantly vibrate the membrane, but at the same time it is still able to catch a sound vibration the size of one hydrogen atom. This is possible thanks to an extremely efficient noise filtering system. After determining even the weakest vibration, the membrane can return to its original position in 5 thousandths of a second. If she couldn't return to her regular state so quickly, every sound that entered her ear would echo.

The hammer, anvil and stirrup are the smallest bones in our body. And these bones have muscles and nerves! One muscle is attached by a tendon to the handle of the malleus, the other to the stirrup. What are they doing? With a loud sound, you need to lower the sensitivity of the entire system so as not to damage it. With a sharp loud sound, the brain reacts much faster than we have time to realize what we heard, while it instantly forces the muscles to contract and dull the sensitivity. The response time to loud sound is only about 0.15 seconds.

Certainly, genetic mutations or random stepwise changes proposed by evolutionists cannot be responsible for the development of such a complex mechanism. The air pressure inside the middle ear should be the same as the pressure outside the eardrum. The problem is that the air inside is absorbed by the body. This results in a decrease in pressure in the middle ear and a decrease in the sensitivity of the eardrum due to the fact that it is pressed inward by higher external air pressure.

To solve this problem, the ear is equipped with a special channel known as the Eustachian tube. It is an empty tube 3.5 cm long that runs from the inner ear to the back of the nose and throat. It provides air exchange between the middle ear and the environment. When swallowing, yawning and chewing, special muscles open the Eustichean tube, letting in outside air. This ensures the pressure balance. A malfunction of the tube leads to pain, prolonged blockage, and even bleeding in the ear. But how did it originate in the first place, and which parts of the middle ear appeared first? How did they function one without the other? An analysis of all the parts of the ear and the importance of each to human hearing demonstrates the presence of irreducible complexity (the whole organ must have come into existence as one or it could not function), which is powerful evidence of creation.

Human Hearing: The Inner Ear: A System of Incredible Complexity

So, the sound passed through the air to the eardrum, and in the form of vibration was transmitted to the bones. What's next? And then these mechanical movements should turn into electrical signals. This miracle of transformation takes place in the inner ear. The inner ear consists of the cochlea and the nerves attached to it. Here we also observe a very complex structure.

Having two ears helps us calculate the location of the sound. The difference in time the sound reaches the ears may be as little as 20 millionths of a second, but this delay is enough to determine the source of the sound.

The cochlea is a special organ of the inner ear, which is arranged in the form of a labyrinth and filled with a special fluid (perilymph). See Fig.1 and Fig.3. triple coated for durability and tightness. This is necessary for the subtle processes taking place in it. We remember that the last bone (stapes) enters the oval window of the cochlea (Fig. 2 and Fig. 3). Having received vibration from the eardrum, the stirrup moves its piston back and forth in this window, creating pressure fluctuations inside the liquid. In other words, the stirrup transmits sound vibration to the cochlea.

This vibration propagates in the fluid of the cochlea and reaches there a special organ of hearing, the organ of Corti. It turns the vibrations of the liquid into electrical signals that go through the nerves to the brain. Since the snail is completely filled with liquid, how does the piston manage to enter it? Remember how nearly impossible it is to put a cork in a fully filled bottle. Due to the high density of the liquid, it is difficult to compress it.

It turned out that at the bottom of the cochlea there is a round window (like a rear exit), covered with a flexible membrane. As the stapes plunger enters the oval window, the membrane of the round window below bulges out under pressure in the fluid. It's like a bottle with a rubber bottom that sags every time the cork is pushed in. With this ingenious pressure relief device, the stirrup can transmit sound vibration to the cochlear fluid.

However, pressure pulses do not propagate in a liquid in a simple manner. To understand how they spread, let's look inside the snail's labyrinth (See Figure 3 and Figure 4). The labyrinth canal consists of three canals - the upper one (scala vestibularis), the lower one (scala tympani) and the canal in the middle (cochlear duct). They are not interconnected and go in parallel in the labyrinth.

From the piston, the pressure goes up in the labyrinth to the top of the cochlea only through the upper channel (and not through all three). There, through a special connecting hole, the pressure passes into the lower channel, which goes back down the labyrinth and exits in a round window. In Figure 3, the red arrow indicates the pressure path from the oval window up the circle in the labyrinth. At the top, the pressure passes into another channel, indicated by a blue arrow, and is directed along it down to the round window. But why all this? How does this help us hear?

The fact is that in the middle of the two channels of the labyrinth there is a third channel (cochlear duct), also filled with liquid, but different from the liquid in the other two channels. This middle channel is not connected to the other two. It is separated from the upper channel by a flexible plate (Reissner's membrane), and from the lower channel by an elastic plate (basilar membrane). Passing along the upper channel up the labyrinth, the sound in the liquid vibrates the upper plate. Going back down the cochlea along the lower channel, the sound in the liquid vibrates the lower plate. Thus, as sound travels through the fluid of the labyrinth up the cochlea and back down, the plates of the middle channel vibrate. After the passage of sound, their vibration gradually fades away. How does the vibration of the plates of the middle channel provide us with hearing?

Between them is the most important part of the auditory system - the organ of Corti. He is extremely small, but without him we would be deaf. The nerve cells of the organ of Corti convert the oscillatory movements of the plates into electrical signals. They are called hair cells and play a huge role. How do the hair cells of the organ of Corti manage to convert the vibrations of the plates into electrical signals?

Look at Figures 4 and 5. The fact is that these cells are in contact from above with a special integumentary membrane of the organ of Corti, which looks like a hard jelly. At the top of the hair cells are 50 to 200 cilia called stereocilia. They enter the integumentary membrane.

Fig.7

As sound travels through the labyrinth of the cochlea, the laminae of the middle canal vibrate, and this causes the jelly-like integumentary membrane to vibrate. And its movement causes the oscillation of the steriocilia of the hair cells. The swaying of the steriocilia causes the hair cells to produce electrical signals that are sent further to the brain. Amazing, isn't it? The organ of Corti has about 20,000 hair cells, which are divided into internal and external (Fig. 5 and Fig. 6). But how does the vibration of cilia produce electrical signals?

It turns out that the movement of steriocilia causes the opening and closing of special ion channels on their surface (Fig. 7). Channels, opening, let ions inside, which changes the electrical charge inside the hair cell. Changes in electrical charge enable the hair cell to send electrical signals to the brain. These signals are interpreted by the brain as sound. The problem is that we have to open and close the ion channel at speeds up to the highest sound frequency we can detect - up to 20,000 times per second. Something must open and close millions of these channels on the surface of the cilia at up to 20,000 times per second. Scientists have discovered that for this purpose, a molecular spring is attached to the surfaces of sterociliums!!! (Fig. 7.) Rapidly stretching and contracting as the cilia vibrate, it provides such a high speed of opening and closing of the channels. Brilliant design!

Human hearing: we actually listen with the brain

The snail is able to pick up every instrument in the orchestra and notice the missing note, hear every breath and hear whispers - all at an astounding sampling rate of up to 20,000 times per second. The brain interprets the signals and determines the frequency, strength, and meaning of the signals. While a large piano has 240 strings and 88 keys, the inner ear has 24,000 "strings" and 20,000 "keys" that allow us to hear an incredible amount and variety of sounds.

The above is only half the way, as the hardest part happens in the brain, which is what we actually "hear". Our ears are sensitive enough to hear a feather gliding over clothes, but we can't hear blood flowing through capillaries a few millimeters from our ears. If we were constantly listening to our breathing, saliva swallowing, every heartbeat, joint movement, etc., we would never be able to focus on anything. Our brain automatically muffles some sounds, in some cases it blocks them altogether. Breathe in the air and see if you can hear it. Of course you can, but you usually don't hear. You have inhaled approximately 21,000 times in the last 24 hours. The auditory part of the human brain works like a security guard, listening to every sound and telling us what we need to hear and what not. Sounds can also evoke memories.

Conclusion

It is obvious that all parts of the ear are necessary for human hearing. For example, if all the components are in place, but there is no eardrum, then how will the sound get to the bones and the cochlea? What is the point then of having a labyrinth, an organ of Corti and nerve cells, if the sound does not even reach them? If everything is in place, including the membrane, but "only" the oval window or, say, fluid in the cochlea is missing, then there will be no hearing, since the sound cannot reach the nerve cells.

The absence of the smallest detail will make us deaf, and the presence of the rest of the system - useless. Moreover, every "small detail" in this chain is in fact itself a system of many components. The tympanic membrane, for example, is made up of specialized living tissue, malleus attachments, nerves, blood vessels, and so on. The cochlea is a labyrinth, triple coating, three separate channels, different fluids, flexible duct plates, etc.

It is foolish to believe that such amazing complexity happened by chance as a result of stepwise evolution. The observed complexity of the human hearing system points to the historical reality of God's creation of Adam, as the Word of God says. “The hearing ear and the seeing eye, the Lord made both” (Proverbs 20:12).

In future issues, we will continue to explore God's design for the human body. I hope this article has helped you understand more deeply His wisdom and His love for you. “I will praise you, for I am wonderfully built, and my soul is fully aware of this” (Psalm 139:13). Give God praise and gratitude, because He is worthy!