Sound and music are intricately linked with our everyday lives. We can perceive different sounds due to the elaborate human ear anatomy and specific characteristics of the sound itself (such as pitch and frequency). The external ear consisting of the pinna, the auditory canal, and the tympanic membrane, the ossicles of the middle ear, and the inner ear comprising of the cochlea, the semicircular canals, and the vestibule constitute the structure of the human ear. So, let us learn about the nature of sound waves and how the anatomy of the human ear enables the detection of sound.
Sound waves as mechanical waves: Sound is a disturbance that is transmitted through a medium via particle-to-particle interaction. Hence, sound waves are mechanical.
Sound waves as longitudinal waves: Sound waves are longitudinal, meaning that the direction of propagating sound waves is the same as the vibration of the medium particles.
Sound waves as pressure waves: The vibrating particles of the medium interact with the air particles. Hence, there are regions of high pressure (compressions) and areas of low pressure (rarefactions).
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Frequency: It indicates the rate of vibration of medium particles when a wave passes through it. Its unit is Hertz, where 1 Hertz = 1 vibration/second.
Pitch: It is the sensation of frequency. High pitch corresponds to high-frequency sounds, whereas low pitch relates to sounds of low frequency.
The human ear is capable of perceiving sound waves ranging between 20 Hz to 20,000 Hz.
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The human ear performs the functions of perception and interpretation of sound waves. The ear is divided into three parts:
The external ear consists of the following parts:
Pinna: It is a flap of cartilage located on the side of the head. Its peculiar shape, made up of thick folded cartilage, is well-suited for gathering the sound waves and directing them towards the auditory canal.
Auditory Canal: It is an inch long tunnel that ends at the tympanic membrane or the eardrum. The channel contains hairs and glands for secreting earwax.
Tympanic Membrane: It is a thin membrane, stretched tight like a drum head, located between the middle and the outer ear. When sound waves strike the membrane, it vibrates to produce sound.
The middle ear is an air-filled hollow cavity and is separated from the outer ear by the tympanum. The middle ear's anterior wall contains an opening that leads directly into the Eustachian tube; this tube connects the middle ear to the throat and equalizes the air pressure on either side of the eardrum.
Ear ossicles: The air space of the middle ear has an arch of three small bones - the malleus, the incus, and the stapes. These three bones are collectively termed as ear ossicles. The vibrating eardrum transmits the vibration to the ear ossicles, which relay the vibration to the inner ear.
The inner ear or the labyrinth is composed of the cochlea, the semicircular canals, and the vestibule. The bony labyrinth is filled with a fluid called the perilymph.
Cochlea: It is a bony canal with two-and-a-half spirals connected to the middle ear via the oval window. The cochlea is internally separated by membranes into three canals. The upper canal and the lower canal are filled with perilymph, while the central canal has endolymph.
The central canal also possesses sound receptors called organs of Corti; its hair cells transform sound vibrations to nerve impulses and are suited for the interpretation of sounds of different frequencies.
Semicircular canals: It is a set of three fluid-filled channels arranged perpendicular to each other and are responsible for the maintenance of body balance in motion.
Vestibule: The central portion of the labyrinth consists of two sacs - the utricle and the saccule. The vestibule has receptors for the maintenance of the body's static equilibrium.
Given below is a simple human ear diagram for class 9:
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1. What is the human ear's hearing mechanism?
The pinna collects the sound waves and transmits them through the auditory canal to the eardrum.
Upon striking the eardrum, the sound waves cause it to vibrate with a frequency precisely equal to that of the sound emitting source. The sound waves are converted to mechanical vibrations.
The vibrations are transmitted to the ear ossicles (malleus→incus→stapes), which amplify the vibrations.
The magnified vibrations are subsequently relayed to the oval window membrane, which results in vibrations of the fluid in the cochlea (inner ear).
Sensory cells are stimulated by the vibrations traveling in the fluids of the inner ear. The sensory receptors pick up the vibrations and convert them into electrical signals.
The electrical signals or nerve impulses travel via the auditory nerve to the brain.
The nerve cells present in the brain are stimulated, which results in the interpretation of the nerve impulses and subsequent hearing.
2. How do hearing aids work?
Hearing aids are small electronic tools worn in or behind the ear by people with hearing loss. It helps people to hear more by making some sounds louder so that people with hearing impairment can listen and communicate better. Hearing aids are required due to age, disease, or injury to the inner ear's sensory cells. Hearing aids magnify the incoming sound vibrations that get detected by the surviving sensory cells, converted into neural signals, and sent to the brain.
A hearing aid consists of three essential components - a microphone, an amplifier, and a speaker. The sound waves are received by the hearing aid via the microphone, which in turn converts the sound waves to electrical signals. The amplifier then receives these electrical signals and helps in increasing the power of the signals. The amplified signals are then sent to the ear through the speaker.