Sound Wave Transmission in Ear: Perception of Sounds

How does our ear detect sound waves?

Video source: StudentHelp4AP

How hearing occurs?

Almost all sound waves are unique and thus contribute to the authenticity of each sound.

To understand how we hear, you first need to know that sounds are invisible vibrations circulating in the air. When a person speaks, the leaves of the tree rustle, the phone rings or something else produces 'sound', vibrations are sent in all directions in the air. We know them as sound waves.

Nearly all sound waves are unique. That's why everyone or everything has a different voice - and a person or thing doesn't always sound the same. Some sound waves may be thin or thick, high or soft.

When our ears catch sound waves, they turn them into messages that our brains can understand. How well the sounds are captured and how clearly they are sent to the brain depends on how well our ears work.

Parts of our ears

The ear has three main parts:

• External ear - captures sound waves and directs them to the middle ear.

The outer (external) ear is composed of 2 parts: auricle and ear canal. Due to its specific shape, the auricle is responsible for collecting sound waves. This structure also allows us to distinguish the direction of sound coming. Waves entering the pinna pass through the ear canal, which is about 2 cm long, into the eardrum. During this transfer, due to the structure of the ear canal and the auricle, especially at 3000 Hz, sounds are transmitted to the middle ear by increasing their intensity (transfer function of the outer ear). Sound waves, which are mainly pressure waves, strike the eardrum and cause it to vibrate.

• Middle ear - the common ear transmits sound waves in the air to mechanical pressure waves to be transferred to the fluids of the inner ear.

The middle ear is an air-filled space consisting of the eardrum and 3 ossicles attached to each other. These 3 bones are hammers (malleus), incus and stapes, respectively. The eardrum is a highly tense structure and the vibration frequency of the membrane is the same as the frequency of the incoming sound wave. Since the eardrum is associated with the malleus bone, the sound waves that vibrate the membrane also vibrate the malleus bone and the movement of the malleus moves the other two bones (incus and stapes). As the stirrup base is connected to the oval window in the inner ear, these vibrations are transmitted to the inner ear.

• inner ear (cochlea) - converts pressure waves into sound signals that our brains can understand.

The inner ear is composed of two parts: the est vestibular system işlevsel which is functionally related to balance and the ‘cochlear system işitme which is the hearing center.

In order to be able to hear naturally, each department must work properly.

How does natural hearing occur?

How does our ear perceive sound and how does it work? - Sound wabve transmission

• Sound enters the ear canal
• Sound waves travel through the ear canal and hit the eardrum.
• Eardrum and hearing bones vibrate
• These sound waves vibrate the eardrum and the three bones in the middle ear.
• Liquid flows in the middle ear
• The resulting vibrations - known as cochlea - move in the fluid in the spiral-shaped inner ear and move the hairy cells in the cochlea. Hairy cells detect movement and convert it into chemical signals for the auditory nerve. The sound entering through the oval window, watching in the scale vestibuli, passes the scale tympania in the helicotrema and stimulates the appropriate region of the organ of Corti depending on its frequency
• Hearing nerves communicate with the brain
• The hearing nerve then sends the information it receives to the brain with electrical pulses, which are perceived as sound in the brain.

The cochlea's triple channel system

In the video, the cochlea's triple channel system and helix structure are clearly seen. The nerves connected to the scala media and the corti organ located between the scala vestibuli and the scala tympani are separated from the cochlea by the so-called 'spiral ganglion' throughout the cochlea. The extensions of these ganglia then form the cochlear nerve (the auditory nerve), a thick bundle of yellow nerve fibers in the middle of the cochlea. This nerve carries both the sensory signals from the cochlea to the brain and related centers, as well as the nerve fibers responsible for transmitting signals from the central nervous system to the cochlea to regulate cochlea activity. The top part of the cochlea is called 'helicotrema' and the scala tympani and the scala vestibuli merge with each other.

Transmitted energy: hearing of sound waves

The base of the stapes is in contact with the ‘round window bulunan at the base of the cochlea, which forms the entrance to the cochlea. The movement of the stapes base towards the oval window moves the perilymph fluid along the scale vestibuli. This movement passes from the helicotrema to the scale tympania and pushes the round window at the bottom of the oval window outwards. This action of perilymph causes vibration of both endolymph and basilar membrane.

The basilar membrane is a fibrous membrane that separates the scale media from the scala tympani. The membrane, which has an expanding structure towards helicotrema, contains about 20.000-30.000 basilar fibers. While these fibers are short and thick at the beginning of the membrane, they grow in length towards the top of the cochlea and their diameter decreases. Therefore, the basilar membrane is narrow and rigid near the oval window, while at the apical end it is wider and flexible. This variation in the membrane allows the basilar membrane to vibrate in different positions according to the frequency of the sound. Considering that each substance has a certain natural frequency depending on its composition, the natural frequency of the narrow and stiff head of the basilar membrane is higher than the natural frequency of the larger and flexible apical tip. Each sound frequency therefore vibrates a particular region of the basilar membrane more than others, and the amount of vibration is proportional to the amplitude of the incoming wave.

Hearing of different sound frequencies

A high-frequency sound wave causes resonance in the first parts of the membrane where the natural frequency is high and causes the highest vibration. low-frequency sounds also produce the most vibration in the last part of the membrane, which has a low natural frequency. As a result of this mechanical volume adjustment, the hair cells on the head of the cochlea and therefore the neurons they synapse are more stimulated in high wavelength sounds; the opposite is the case for low frequency sounds. In short, the frequency analysis of the audible slats is first performed in the cochlea by the basilar membrane.

The main method used by the nervous system to determine different sound frequencies is to determine the most stimulated positions on the basilar membrane.

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Source links >> Engineering Acoustics/The Human Ear and Sound Perception / Hearing and Perception

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