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How we hear sound is a part of our five senses – touch, smell, sight, taste, and hearing. Hearing is done through the sense organ called Ear. It connects us to our surroundings by catching every high and low-intensity sound coming towards us. The ear and brain play an instrumental role in the process of hearing.
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Key Terms: Sound Waves, Vibrations, Signals, Auditory Nerves, Wavelength, Frequency, Amplitude
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How Do We Hear?
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How do we hear? Hearing is made possible when the sound waves that reach our ear are converted into electrical signals which are carried to the brain through our auditory nerves. Sound waves need a medium to travel which is air. Sound is spread from the source to the surroundings in the form of vibrations.
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Structure of Ear
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The anatomy of the ear is divided into three parts, namely the outer ear, middle ear, and inner ear.
Outer Ear consists of the pinna and the ear canal. Pinna collects the sound from the surroundings and funnels them to the ear canal. The ear canal carries the sound waves to the middle ear.
Middle ear is separated from the outer ear by the eardrum. The eardrum is also referred to as the tympanic membrane which vibrates on being hit by the sound waves. The movement of the eardrum leads to the vibration of the ossicles, which consist of three tiny bones called - malleus, incus and stapes.
The inner ear consists of the cochlea and the semicircular canals. The vibrations received from the middle ear are converted into nerve signals in the snail-shaped cochlea. These signals are sent to the brain through the cochlear nerve or the auditory nerve.
- The hearing process occurs in the following manner:
- Sound waves first reach the outer ear and enter into the ear canal from where they travel up to the eardrum.
- The vibrations felt by the eardrum through these sound waves are carried forward to the middle ear.
- These bones pass on the amplified version of the vibrations received from the outer ear to the cochlea which is full of fluid. The vibrations cause ripples in the fluid inside the cochlea.
- These ripples make the hair cells move and transfer the signal to the auditory nerves.
- The auditory nerves carry these signals to the brain which analyses the signal by matching it to the already stored memory patterns of a variety of sounds already heard by the person and translates them into recognizable sounds.
Perception of Hearing
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When an object produces sound waves by vibrating, it pushes the air around it. These vibrations produced by the object compress and expand the air in the vicinity which results in areas of low and high pressures. The expansion of air molecules caused due to vibration of sound waves is termed as rarefaction. The waves create a pattern consisting of compression and rarefaction (as shown in the image) as they travel from the source object to our ears.
The frequency of sound waves is determined by the wavelength. In case of sound waves, one full cycle of rarefaction is equal to the wavelength. Short wavelengths result in high frequency and long wavelength results in low-frequency sounds.
The two important characteristic features of sound are frequency and amplitude.
- The frequency is nothing but the speed of vibration and it determines whether the sound is low pitch or high pitch. Examples of high pitch sounds are whistles, birds chirping or flute. Whereas, examples of low pitch sound are dog barking, other animal sounds, guitar, etc. Sound vibrations with frequency lower than 20 Hz are called infrasonic sounds.
- The amplitude is the size of the sound waves and it determines how loud the sound will be.
- Humans can hear sounds in the range starting from 20 Hz to 20,000 Hz.
- Sound vibrations with frequencies lower than 20 Hz are called infrasonic sounds.
- Sound vibrations with frequencies higher than 20,000 Hz are called ultrasonic sounds.
The Hair cells are the sensory cells that ride on the ripples caused in the cochlea fluid and detect the sound pitch. The hair cells at the wider end of the cochlea detect the high-pitched vibrations whereas the hair cells close to the central part of the cochlea pick up the low pitch vibrations.
The hair cells moving up and down bump with the hair-like projections called stereocilia and open up their tips. The chemicals rush into the cells through these openings to form an electrical signal which is carried forward by the auditory nerves to the brain for processing the sound. If the ear is exposed to very loud sounds, it damages the hair cells and results in hearing loss.
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Things to remember
- The ear is divided into three parts – outer ear, middle ear, and inner ear
- Hair cells play a very important role in the perception of hearing
- Sound needs a medium to travel from source or origin to the object.
- The loudness of sound is decided by the amplitude of the sound wave.
- The high or low pitch of the sound is decided by its frequency.
- Sound vibrations with frequencies lower than 20 Hz are called infrasonic sounds.
- Our brain stores the memory of different sound patterns like music, songs, natural sounds, etc. which makes it easy for someone hearing the sound to recognize it.
- Exposure to loud sounds, head injuries, neurological ailments, and old age are some of the reasons leading to hearing loss.
- Hearing loss due to old age results in a reduction of the range of frequencies a person can hear.
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Sample Questions
Question: Which characteristics of vibrations determine the pitch and loudness of sound? (3 Marks)
Answer: The two important characteristic features of sound vibrations are frequency and amplitude. The frequency is nothing but the speed of vibration and it determines whether the sound is low pitch or high pitch. Examples of high pitch sounds are whistles, birds chirping or flute. Whereas, examples of low pitch sounds are dog barking, other animal sounds, guitar, etc. The amplitude determines how loud the sound will be.
Question: How do we hear sound? (3 Marks)
Answer: Sound waves travel through the medium of air and reach our ears. The vibrations caused by the sound waves reach the eardrum which further transmits these vibrations to the brain through auditory nerves in the form of electrical signals. The brain processes this signal by translating it to a recognizable sound that we can understand.
Question: What role do hair cells play in perception of hearing? (3 Marks)
Answer: The Hair cells are the sensory cells that ride on the ripples caused in the cochlea fluid and detect the sound pitch. The hair cells at the wider end of cochlea detect the high pitched vibrations whereas the hair cells close to the central part of cochlea pick up the low pitch vibrations.
The hair cells moving up and down bump with the hair like projections called stereocilia and open up their tips. The chemicals rush into the cells through these openings to form an electrical signal which is carried forward by the auditory nerves to the brain for processing the sound.
Question: What causes hearing loss? (3 Marks)
Answer: Exposure to loud sounds, head injuries, neurological ailments, and old age are some of the reasons leading to hearing loss. If the ear is exposed to very loud sounds, it damages the hair cells and they can no longer transfer signals to the brain, which results in hearing loss.
Question: Give a detailed stepwise description of the hearing process. (3 Marks)
Answer: The hearing process occurs in the following manner:
- Sound waves first reach the outer ear and enter into the ear canal from where they travel up to the eardrum.
- The vibrations felt by the eardrum through these sound waves are carried forward to the middle ear which consists of three tiny bones called malleus, incus and stapes.
- These bones pass on the amplified version of the vibrations received from the outer ear to the cochlea which is full of fluid. The vibrations cause ripples in the fluid inside the cochlea.
- These ripples make the hair cells move and transfer the signal to the auditory nerves.
- The auditory nerves carry these signals to the brain which analyses the signal by matching it to the already stored memory patterns of a variety of sounds already heard by the person and translates into recognizable sound.
Question: Can a sound produced in vacuum be heard? (3 Marks)
Answer: Sound waves are mechanical waves that need a medium to travel from source or origin to the object. Sound is nothing but vibrating air. Vacuum has no air and so we cannot hear the sound produced in vacuum as it will have no medium to reach our ear.
Question: Differentiate between infrasonic and ultrasonic sounds (3 Marks)
Answer:
- Sound vibrations with frequency lower than 20 Hz are called infrasonic sounds. Animals which can hear these sounds are bats, dogs, monkeys, etc.
- Sound vibrations with frequency higher than 20,000 Hz are called ultrasonic sounds. Animals which can hear these sounds are giraffe, elephant, hippopotamus, and sea creatures like whales.
Question: What pattern do the sound waves form while traveling from source object to the ear? (3 Marks)
Answer: When an object produces sound waves by vibrating, it pushes the air around it. These vibrations produced by the object compress and expand the air in the vicinity which results in areas of low and high pressures. The expansion of air molecules caused due to vibration of sound waves is termed as rarefaction. The waves create a pattern consisting of compression and rarefaction as they travel from source object to our ears.
Question: Discuss the role of different parts of the ear in transmission of sound to the brain. (3 Marks)
Answer: The anatomy of the ear is divided into three parts, namely outer ear, middle ear and inner ear.
Outer Ear consists of the pinna and the ear canal. Pinna collects the sound from the surroundings and funnels them to the ear canal. The ear canal carries the sound waves to the middle ear.
Middle ear is separated from the outer ear by the eardrum. Eardrum is also referred to as the tympanic membrane which vibrates on being hit by the sound waves. The movement of the eardrum leads to the vibration of the ossicles, which consist of three tiny bones called - malleus, incus and stapes.
Inner ear consists of cochlea and the semicircular canals. The vibrations received from the middle ear are converted into nerve signals in the snail shaped cochlea. These signals are sent to the brain through the cochlear nerve or the auditory nerve.
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