Sound - Class 8 Notes & Olympiad Questions

Sound - Class 8 Science

  • Sound
  • Sound Produced by Humans
  • Human Hearing
  • Some Important Terms
  • Frequency, Time Period, Wavelength and Amplitude
  • Characteristics of Sound
  • Audible and Inaudible Sound
  • Reflection of Sound
  • Absorption of Sound
  • Noise and Music
  • Noise Pollution
  • Solved Questions on Sound
  • Sound

    Sound is an integral part of our everyday experience, enriching our perception of the world around us. It's a phenomenon that allows us to communicate, enjoy music, and understand our environment. At its core, sound is the result of vibrations or oscillations in objects that create waves, which then propagate through a medium, like air or water, and are eventually detected by our ears.

    Key Points:

    a) Vibration and Oscillation: Sound is produced when an object vibrates or oscillates, meaning it moves back and forth around a central position. This motion creates a disturbance in the air molecules around it.

    b) Propagation: When an object vibrates, it causes neighbouring air molecules to vibrate as well. This sets off a chain reaction where the disturbance is passed from one molecule to the next, creating a wave-like motion that moves away from the source.

    c) Medium for Propagation: Sound requires a material medium to travel through, such as air (gas), water (liquid), or solids. In the absence of a medium, such as in a vacuum, sound cannot propagate because there are no particles to transmit the vibrations.

    d) Speed of Sound: The speed at which sound travels depends on factors such as the density of the medium. Sound travels faster in denser mediums. In a solid, the molecules are tightly packed, allowing sound to travel quickly. In liquids, the molecules are less tightly packed, and in gases, they are even more spread out, resulting in slower sound transmission.

    e) Propagation and Reflection: Sound waves can travel through a medium and bounce off obstacles. When sound waves encounter a boundary between two media, they can be partially reflected and partially absorbed. This phenomenon is responsible for echoes.

    f) Examples:

    1. Speaking and Conversation: When people talk, their vocal cords vibrate, producing sound waves that carry the information of their words to the ears of the listeners.
    2. Musical Instruments: Instruments like guitars, pianos, drums, and violins produce sound when their strings, keys, or surfaces are struck, plucked, or otherwise manipulated.
    3. Nature Sounds: Birds chirping, leaves rustling in the wind, waves crashing on the shore, and rain falling are all examples of sounds found in nature.
    4. Echoes: When a sound wave reflects off a surface and returns to its source, it creates an echo. This can happen in open spaces, caves, or even in buildings with reflective surfaces.
    5. Movies and Media: Sound effects, background music, and dialogue in movies, TV shows, and video games enhance our immersive experience.
    6. Industrial Sounds: Machinery, construction work, factories, and industrial processes generate a wide range of sounds.

    Sound Produced by Humans

    The process of producing sound in humans involves the intricate workings of the voice box, also known as the larynx.
    A detailed explanation of how sound is produced by humans is given below:

    Larynx and Vocal Cords: The larynx, commonly referred to as the "voice box," is a part of the human throat situated at the top of the windpipe (trachea). It contains a pair of folds known as the vocal cords or vocal folds. These vocal cords are made of delicate and flexible tissue.

    Air Flow: When we speak or sing, air from the lungs is pushed upward through the trachea and into the larynx. The air passes through the gap between the vocal cords.

    Vibration: As the air flows through the gap between the vocal cords, it causes the vocal cords to vibrate due to the pressure and the airflow. These vibrations are similar to the way a guitar string vibrates when plucked.

    Sound Generation: The vibrations of the vocal cords produce sound waves. The frequency and pattern of these vibrations determine the pitch of the sound produced.

    Human Hearing

    The process of hearing involves the collection of sound waves by the outer ear, the conversion of sound waves into vibrations in the middle ear, and the transformation of these vibrations into nerve impulses in the inner ear. These nerve impulses are then sent to the brain, which interprets them as the sounds we perceive.

    A detailed explanation of the human ear and how sound is perceived by humans is given below:

    Human Hearing System - Labelled Diagram of Human Ear

    Outer Ear

    a) The outer ear consists of two main parts: the pinna (also known as the auricle) and the ear canal (auditory canal).
    b) The pinna is the visible part of the ear that collects sound waves from the surrounding environment. It helps to funnel sound waves into the ear canal.
    c) The ear canal is a tube-like structure that extends from the pinna to the eardrum (tympanic membrane). Sound waves travel through the ear canal, causing vibrations in the eardrum.

    Middle Ear

    a) The middle ear is located between the eardrum and the inner ear. It contains three small bones known as the ossicles: the malleus (hammer), incus (anvil), and stapes (stirrup).
    b) When sound waves reach the eardrum, they cause the eardrum to vibrate. These vibrations are transmitted to the ossicles in the middle ear.
    c) The ossicles amplify the vibrations as they pass through them. The malleus receives the vibrations from the eardrum, and these vibrations are then transmitted to the incus and, finally, to the stapes.
    d) The stapes bone is attached to the oval window, which is a membrane-covered opening that leads to the inner ear.

    Inner Ear

    a) The inner ear is a complex structure containing the cochlea and other components responsible for hearing and balance.
    b) The cochlea is a spiral-shaped, fluid-filled structure that plays a crucial role in converting vibrations into electrical signals that the brain can interpret as sound.
    c) As the stapes bone presses against the oval window, it creates pressure waves in the fluid inside the cochlea. These pressure waves travel through the cochlear fluid, causing the basilar membrane (a membrane inside the cochlea) to move.
    d) Hair cells located on the basilar membrane are responsible for transducing these mechanical movements into electrical signals. These hair cells are connected to auditory nerve fibres.

    Nerve Impulses and Interpretation

    a) The electrical signals generated by the hair cells are transformed into nerve impulses.
    b) The auditory nerve, also known as the cochlear nerve, carries these nerve impulses from the inner ear to the brain.
    c) The brain's auditory processing centres receive these nerve impulses and interpret them as specific sounds. Different frequencies and amplitudes of sound waves are translated into various pitches and volumes, respectively.

    Some Important Terms

    Sound Wave

    a) A sound wave is like a ripple in the air. When something makes a sound, like a guitar string vibrating or someone talking, it creates a disturbance in the air.
    b) This disturbance travels in all directions, similar to how ripples spread across a pond when you drop a stone in it. These air disturbances are what we call sound waves.


    a) An oscillator is something that moves back and forth in a repeating pattern. Imagine a pendulum swinging back and forth or a guitar string vibrating. These are examples of oscillators.
    b) When an oscillator moves back and forth, it creates sound waves, which are the ripples of energy that travel through the air and reach our ears as sound.

    Frequency, Time Period, Wavelength and Amplitude of a Sound


    a) Frequency refers to the number of complete oscillations or vibrations that a sound wave undergoes in a unit of time. In other words, it tells us how many cycles of the wave occur per second.
    b) Frequency is measured in Hertz (Hz), where 1 Hz is equivalent to 1 cycle per second.
    c) High-frequency sounds have a lot of vibrations in a short time, giving us high-pitched sounds like a whistle.
    d) Low-frequency sounds have fewer vibrations, resulting in low-pitched sounds like a deep voice.

    Time Period

    a) The time period of a sound wave is the amount of time it takes for one complete cycle of the wave to pass a given point. It is measured in seconds (s).
    b) The time period is the reciprocal of the frequency, meaning that the time period (T) and frequency (f) are related by the equation:

    T = 1 / f

    A sound wave with a shorter time period will have a higher frequency, and vice versa. For instance, if a sound wave completes one cycle in 0.01 seconds, its frequency is 100 Hz.


    a) Wavelength refers to the distance between two consecutive points that are in phase with each other on a sound wave. In simpler terms, it's the length of one complete cycle of the wave.
    b) For sound waves, the wavelength is the physical distance between two consecutive compressions or rarefactions.
    c) Wavelength is denoted by the Greek letter lambda (λ) and is usually measured in meters (m).
    d) The relationship between frequency, wavelength, and the speed of sound (v) is given by the equation:

    v = f x λ

    This equation indicates that as the frequency of a sound wave increases, its wavelength decreases, and vice versa, while the speed of sound remains constant in a given medium.


    a) Amplitude in the context of sound represents the maximum displacement of air particles from their resting position as the sound wave passes through the medium. In simpler terms, it's the height or intensity of the wave.
    b) A higher amplitude corresponds to a more intense or louder sound, while a lower amplitude corresponds to a quieter sound.
    c) For example, when you pluck a guitar string harder, it vibrates with a greater amplitude, producing a louder sound. Amplitude is often measured in units like decibels (dB) for sound intensity.

    Diagram of Time Period, Wavelength and Amplitude of a Sound - Frequency

    Characteristics of sound

    1. Loudness

    a) Loudness is how loud or quiet a sound is. It's related to the amplitude of the sound wave. Amplitude is like the strength of the wave – the bigger the vibrations (amplitude), the louder the sound.
    b) Louder sounds have larger amplitudes, and quieter sounds have smaller amplitudes.
    c) The loudness is often measured in units called decibels (dB). Interestingly, the relationship between loudness and amplitude isn't just direct – it's proportional to the square of the amplitude. So, if you double the amplitude, the loudness becomes four times as intense.

    Labelled Diagram of Loudness and Amplitude - Characteristics of Sound

    2. Pitch

    a) Pitch refers to how high or low a sound is. It's determined by the frequency of the sound wave.
    b) High-pitched sounds have high frequencies, meaning the source of the sound is vibrating rapidly. Low-pitched sounds have low frequencies, meaning the vibrations are slower.
    c) Imagine a violin string and a bass guitar string being plucked – the violin string vibrates quickly, creating a high-pitched sound, while the bass guitar string vibrates more slowly, producing a low-pitched sound. Interestingly, our brain interprets higher-frequency vibrations as higher-pitched sounds.

    Labelled Diagram of Pitch and Frequency - Science Grade 8

    3. Quality (Timbre)

    a) Quality, also known as timbre, is what makes different sounds unique. It's what allows you to distinguish between different musical instruments or even the voices of different people.
    b) Even if two instruments play the same note at the same loudness, you can usually tell them apart because of their unique qualities. For example, a guitar sounds different from a flute, even when they play the same note. This is because the sound waves produced by different instruments have different patterns and combinations of frequencies. These patterns create the unique character of the sound, which we perceive as quality or timbre.

    Audible and Inaudible Sounds

    Audible Sounds

    a) Audible sounds are the sounds that humans can hear.
    b) Our ears are sensitive to a specific range of frequencies, which is the number of vibrations or cycles per second. This range of frequencies that our ears can perceive as sound is typically between 20 Hz (hertz) and 20,000 Hz.
    c) For example, a low-pitched sound like a deep bass note might have a frequency of 50 Hz, while a high-pitched sound like a bird chirping might have a frequency of around 5,000 Hz. These are all within the audible range, and our ears can detect and interpret them as sounds.

    Inaudible Sounds

    a) Inaudible sounds are sounds that cannot be heard by the human ear due to their frequency being outside the range of human hearing.
    b) There are two types of inaudible sounds: infrasonic and ultrasonic sounds.

    1. Infrasonic Sounds

    a) Infrasonic sounds are sounds with frequencies below the range of human hearing, meaning they have frequencies lower than 20 Hz.
    b) Even though we can't hear these sounds, some animals, like elephants and certain whales, are known to communicate using infrasonic frequencies.
    c) Infrasound can also be produced by natural events like earthquakes and volcanic eruptions.
    d) Some researchers believe that our bodies might still be able to sense infrasound even if we can't consciously hear it, which could explain why we sometimes feel uneasy during thunderstorms or volcanic activity.

    2. Ultrasonic Sounds

    a) Ultrasonic sounds, on the other hand, have frequencies higher than the upper limit of human hearing, which is 20,000 Hz.
    b) Bats are a classic example of animals that use ultrasonic frequencies for echolocation – they emit high-frequency sounds that bounce off objects and return to their ears, helping them navigate and locate prey in the dark.
    c) Ultrasonic frequencies also find applications in various technologies, such as medical imaging (ultrasound) and cleaning (ultrasonic cleaners).

    Reflection of Sound

    a) When a sound wave encounters a boundary that separates two different media, a part of the sound is bounced back, or reflected, from the surface. This phenomenon is similar to how light reflects off a mirror. The angle at which the sound wave strikes the boundary determines the angle at which it reflects.
    b) When you shout in front of a wall, for example, you might notice that you can hear your own voice coming back to you, especially in a large open area. This is because the sound waves you produced are reflecting off the wall and reaching your ears.
    c) An echo is a distinct type of sound reflection. It occurs when a sound wave reflects off a surface and returns to the listener's ears with a noticeable time delay after the original sound was produced. To hear an echo, there must be a significant gap between the original sound and the reflected sound.
    d) For example, if you shout across a canyon or a large empty room, you might hear your shout coming back to you a moment later. The minimum distance required for a distinct echo to be heard is about 17.2 meters, assuming the sound takes more than 0.1 seconds to travel to the obstacle and back. The actual time delay needed for an echo to be perceived depends on the speed of sound in the medium.

    Diagram of Reflection of Sound - Science Grade 8

    Absorption of Sound

    a) When a sound wave encounters a surface, some of its energy is absorbed by the surface and converted into other forms of energy, such as heat. The ability of a material or surface to absorb sound is influenced by its texture, density, and composition.
    b) Soft and porous materials, like carpets, curtains, and foam panels, tend to absorb more sound energy than hard and smooth surfaces. This is why you might notice that rooms with soft furnishings tend to have less echoing and reverberation compared to rooms with mostly hard surfaces.

     Explore more about Sound

    Noise and Music


    a) Noise refers to any unwanted or unpleasant sound that is characterized by irregular vibrations. It is often a jumble of different frequencies and amplitudes that lack a pleasing or organized pattern.
    b) Noise can be disruptive, and annoying, and can interfere with our ability to hear and communicate effectively.
    c) Examples of noise include the sounds of traffic, construction, machinery, and other chaotic and unpleasant sounds. Noise pollution, which is the presence of excessive and harmful noise in the environment, can have negative effects on our health, well-being, and concentration.


    a) Music, on the other hand, is a deliberate arrangement of sounds in a way that produces a pleasing and harmonious experience for the listener.
    b) Unlike noise, music is created with intention and often follows patterns of rhythm, melody, harmony, and structure.
    c) Musical sounds are produced by regular vibrations that are organized in a meaningful and expressive manner. Music has the power to evoke emotions, tell stories, and convey ideas. It is a form of artistic expression that can be enjoyed for its aesthetic and emotional qualities.

    Different Types of Soundwaves - Science Grade 8

    Noise Pollution

    Noise pollution refers to the excessive and unwanted presence of loud and disruptive sounds in the environment, which can have negative effects on human health, well-being, and overall quality of life. These sounds are often caused by human activities such as industrial processes, transportation, construction, and recreational activities.

    Effects of Noise Pollution

    1. Health Problems: Prolonged exposure to high levels of noise can lead to health issues such as insomnia, stress, anxiety, and even cardiovascular problems like hypertension.
    2. Hearing Loss: Continuous exposure to loud noises, especially in industrial settings, can result in permanent hearing damage or loss over time.
    3. Interference with Communication: Excessive noise can make it difficult for people to communicate effectively, leading to misunderstandings and reduced productivity.
    4. Disturbance of Peace: Noise pollution disrupts the peaceful atmosphere of residential areas and natural environments, affecting people's comfort and relaxation.

    Measures to Control Noise Pollution

    1. Plantation of Trees: Planting trees along roadsides and in urban areas acts as a natural barrier that absorbs and reduces sound, helping to create quieter surroundings.
    2. Soundproofing Systems: Industries and noisy facilities can use soundproofing techniques to minimize the transmission of noise to the surrounding areas.
    3. Volume Control: Setting limits on the volume of loudspeakers, music systems, and public announcements can prevent excessive noise in public spaces.
    4. Vehicle Noise Reduction: Encouraging the use of quieter vehicle engines, minimizing horn usage, and enforcing regulations on vehicle noise can help reduce noise pollution from transportation.
    5. Urban Planning: Designing cities with noise-reducing features such as green spaces, buffer zones, and acoustic barriers can mitigate noise pollution.
    6. Regulations and Enforcement: Governments can establish and enforce noise regulations to ensure that noise levels in residential, commercial, and industrial areas are within acceptable limits.
    7. Public Awareness: Educating people about the harmful effects of noise pollution and promoting responsible noise behaviour can encourage individuals and communities to contribute to noise reduction efforts.

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