Sound - Class 9 Science

  • Sound
  • Waves
  • Soundwave and its characteristics
  • Factors Affecting the Speed of Sound
  • Speed of Sound in Different Mediums
  • Reflection of Sound
  • Echo and Reverberation
  • Range of Hearing in Humans
  • Applications of Multiple Reflections
  • Human Ear
  • Solved Questions on Sound
  • Sound

    Sound is a type of energy that we perceive as auditory sensations, or simply put, as the sense of hearing. It's a result of vibrations or disturbances that propagate through a medium, usually air but also liquids and solids. These vibrations create changes in pressure within the medium, leading to the formation of waves known as sound waves.

    Key Aspects of Sound

    1. Vibration: Sound originates from a vibrating source, such as vocal cords, guitar strings, or even an object hitting a surface. These vibrations set molecules in the surrounding medium (like air) in motion.
    2. Medium: Sound requires a medium to travel through, as it relies on the interaction between particles. In space, where there's a near-vacuum, sound cannot travel because there are too few particles to transmit vibrations.
    3. Sound Waves: Vibrations cause molecules in the medium to compress (come closer together) and rarefy (move farther apart). This creates a repeating pattern of high-pressure areas called compressions and low-pressure areas called rarefactions, resulting in the formation of a sound wave.
    4. Propagation: The sound wave travels outward in all directions from the source. As it spreads, it carries the energy of the initial vibration with it.
    5. Speed: The speed of sound varies depending on the medium. For example, sound travels faster in solids than in liquids, and faster in liquids than in gases.
    6. Longitudinal Waves: Sound waves are a type of longitudinal wave. In a longitudinal wave, the particles of the medium move in a parallel direction to the direction of the wave propagation. This is in contrast to transverse waves where particles move perpendicular to the wave direction.


    Waves are a fundamental concept in physics that describes the transfer of energy and information through a medium without the actual physical movement of particles from one point to another. There are two main types of waves: transverse waves and longitudinal waves.

    Transverse Waves

    In a transverse wave, the motion of the particles in the medium is perpendicular to the direction in which the wave propagates. This means that the particles move up and down or side to side as the wave passes through them. A classic example of a transverse wave is a wave on a string. When you move one end of the string up and down, you create crests (high points) and troughs (low points) that travel along the string.

    Key characteristics of transverse waves

    1. The oscillation of particles is perpendicular to the wave's direction.
    2. They have crests (peaks) and troughs (valleys).
    3. Examples include electromagnetic waves (like light), water waves, and seismic S-waves.
    4. Explanation of Transverse Waves - CREST Olympiads

    Longitudinal Waves

    In a longitudinal wave, the particles of the medium oscillate back and forth in the same direction as the wave's propagation. This means that the particles experience compressions (where particles are close together) and rarefactions (where particles are spread out) as the wave moves through them. An example of a longitudinal wave is a sound wave travelling through air.

    Diagram of Longitudinal Waves - CREST Olympiads

    Key characteristics of longitudinal waves

    1. The oscillation of particles is parallel to the wave's direction.
    2. They have compressions and rarefactions.
    3. Examples include sound waves and seismic P waves.
    4. Explanation of Longitudinal Waves - CREST Olympiads

    Soundwave and its Characteristics

    A sound wave is a type of mechanical wave that carries energy through the vibration of particles in a medium, usually air. When an object vibrates, it creates compressions (regions of high pressure) and rarefactions (regions of low pressure) in the surrounding medium, propagating as a sound wave. These compressions and rarefactions travel outward in all directions, carrying the energy of the original vibration with them. When they reach our ears, they cause our eardrums to vibrate, and our brain processes these vibrations as sound.

    Soundwave and its Characteristics - CREST Olympiads

    Key characteristics of a sound wave are:


    1. The wavelength of a sound wave is the shortest distance over which the wave pattern repeats itself. It's represented by the Greek letter lambda (λ).
    2. In a sound wave, the combined length of a compression and an adjacent rarefaction forms its wavelength.
    3. The distance between the centres of two consecutive compressions or rarefactions also equals the wavelength.
    4. Notably, the distance between compression and an adjacent rarefaction's centre is half the wavelength. The SI unit for measuring wavelength is metres (m).


    1. Amplitude refers to the maximum displacement of particles in a medium from their undisturbed positions when a wave passes through. It describes the size of the wave.
    2. The amplitude is denoted by the letter "A." It's measured in metres (m) and signifies the same amplitude as the vibrating body producing the wave.
    3. Loudness is proportional to the square of the amplitude of the wave. This means that if the amplitude of a sound wave is doubled, its loudness becomes four times greater.
    4. A sound with a larger amplitude produces a louder sound, while a sound with a smaller amplitude produces a softer sound.
    5. The unit used to measure loudness is the decibel (dB).
    6. Explanation of Amplitude of Sound Waves - CREST Olympiads


    1. The time required for one complete wave (or cycle) is known as the time period of the wave. It can also be defined as the time taken for one full vibration of the vibrating source.
    2. The time period is represented by the letter "T" and is measured in seconds (s).


    1. Frequency indicates the rate at which waves are produced by their source. It's the number of complete waves (or cycles) generated in one second.
    2. Alternatively, it's the number of vibrations per second. Frequency is fixed and doesn't change as waves pass through different substances. It's measured in hertz (Hz), with the SI unit being the hertz.
      1 Hz = s-1
    3. A larger unit called kilohertz (kHz) is also used (1 kHz = 1000 Hz). The frequency is represented by the letter "f" or sometimes "v.
    4. Formula of frequency - CREST Olympiads

    Explanation of Time Period, Wavelength and Amplitude  of Sound - CREST Olympiads

    Velocity of Wave (Speed of Wave)

    1. The velocity or speed of a wave is the distance it travels in one second. The speed of sound refers to how quickly a sound wave travels through a medium. It's denoted by "v" and measured in metres per second (m/s or ms–1).
    2. Mathematically,
    3. Formula of Speed of Sound - CREST Olympiads

    4. Also, the velocity of a wave is the product of its frequency and wavelength. This can be expressed as speed = frequency × wavelength, known as the wave equation.
    5. Formula of Velocity of Wave - CREST Olympiads

    6. The speed at which sound waves travel through a medium depends on the properties of that medium, such as its density, elasticity, and temperature. In general, sound travels faster in denser and more elastic materials. For example, sound travels faster in water than in air and even faster in solids like metal.
      Generally, sound travels faster in solids than in liquids, and faster in liquids than in gases.
      The speed of sound increases with an increase in temperature. Warmer air, for example, allows sound to travel faster.


    1. Pitch refers to the perceived quality of how high or low a sound is.
    2. Sounds can have high or low pitch. High-pitched sounds are often described as shrill, while low-pitched sounds are described as deep or flat.
    3. The pitch of a sound is directly related to its frequency. Higher-frequency sounds have a higher pitch, and lower-frequency sounds have a lower pitch.
    4. Labelled Diagram of Pitch of a Sound - CREST Olympiads


    1. Quality, also known as timbre, describes the unique characteristic of a sound that distinguishes it from sounds with the same pitch and loudness.
    2. It is what allows you to differentiate between different musical instruments playing the same note.
    3. Quality is determined by the presence of overtones and harmonics in a sound wave, which gives it its distinct colour or richness.

    Factors Affecting the Speed of Sound

    The speed of sound is not constant and can vary based on several factors, including the properties of the medium through which the sound travels. Some of these factors are:

    1. Nature of the Medium: The speed of sound is influenced by the density and elasticity of the medium. In general, denser and more elastic materials transmit sound faster. Solids are denser and more elastic than liquids and gases, which is why sound travels fastest through solids, followed by liquids and then gases.

    2. State of Aggregation: The state in which a substance exists also affects the speed of sound. For example, sound travels faster in a solid state compared to a liquid or gas state of the same substance. This is because particles in solids are closely packed, leading to more efficient transmission of sound waves.

    3. Temperature: Temperature has a significant impact on the speed of sound. As temperature increases, the speed of sound generally increases in gases and liquids.
    In gases, this is because higher temperature leads to increased molecular motion, resulting in faster propagation of sound waves.
    However, the relationship between temperature and speed of sound in solids is more complex and may not always follow the same trend as in gases and liquids.

    4. Humidity: Humidity, or the moisture content in the air, can also affect the speed of sound in gases.
    Higher humidity levels can slightly increase the speed of sound due to the presence of water vapour molecules, which have a higher molecular mass compared to other gases in the air.

    5. Pressure: Pressure plays a minor role in affecting the speed of sound in gases. In practice, changes in pressure are often negligible compared to the effects of temperature and humidity.

    6. Composition of the Medium: The types of molecules present in the medium can influence the speed of sound. For example, sound travels differently in air, which is a mixture of various gases, compared to pure oxygen or nitrogen.

    7. Altitude and Elevation: Sound waves travel faster at higher altitudes because the air is less dense. At lower altitudes, where the air is denser, sound waves encounter more resistance and thus travel more slowly.

    8. Frequency of Sound: In some cases, the frequency of sound can affect its speed. This is particularly relevant in situations where the sound frequency approaches the resonant frequency of the medium, leading to a phenomenon called dispersion.

    Speed of Sound in Different Mediums

    The speed of sound varies depending on the medium through which it travels. The speed of sound is determined by factors such as the density, elasticity, and temperature of the medium.

    Speed of Sound in Different Mediums - CREST Olympiads

    Reflection of Sound

    Sound, like light, can undergo reflection. When sound waves encounter a surface, they can bounce off that surface and change their direction. This phenomenon is known as the reflection of sound. The reflection of sound waves is responsible for various acoustic effects and allows us to hear sounds even when the original source is not directly in our line of sight.

    Mechanism of Sound Reflection

    When sound waves strike a surface, they interact with the molecules of the medium (air, water, or solid) in such a way that the molecules are set into motion. This disturbance creates new sound waves that propagate away from the surface in various directions. However, a portion of these newly generated waves may travel back towards the original sound source, leading to sound reflection.

    Law of Reflection

    The reflection of sound follows the same principles as light reflection. When sound waves strike a surface, they obey the law of reflection, which states that the angle of incidence (the angle between the incident sound wave and the normal to the surface) is equal to the angle of reflection (the angle between the reflected sound wave and the normal).

    Angle Relationships

    Both the incident sound wave and the reflected sound wave make equal angles with the normal to the surface. All three elements – incident wave, reflected wave, and the normal – lie in the same plane.

    Diagram of Reflection of Sound - CREST Olympiads

    Echo and Reverberation


    An echo is a notable example of the reflection of sound. It refers to the phenomenon in which a sound is heard again after it reflects off a surface and travels back to the listener's ears. It occurs due to the reflection of sound waves from a distant object or surface, creating a delayed repetition of the original sound.

    Key Points about Echo

    1. Reflection and Delay: Echo is a result of the reflection of sound waves. When a sound wave encounters a surface, a portion of the sound energy bounces off the surface and travels back towards the source. This reflected sound wave takes a certain amount of time to reach the listener's ears, causing a noticeable time delay between the original sound and its echo.
    2. Minimum Distance: For a distinct echo to be heard, the time interval between the original sound and the echo must be at least 0.1 seconds. This corresponds to a minimum distance between the listener and the reflecting surface. The formula that relates the distance between the listener and the surface, the speed of sound, and the time interval after which the echo is heard is:
    3. Formula of Distance between listner and the surface - CREST Olympiads

      Diagram of Echo - CREST Olympiads

    4. Surface Reflection: Echoes occur most commonly when sound waves encounter large and distant surfaces, such as cliffs, buildings, or mountains. These surfaces reflect the sound waves back to the listener's location.
    5. Echo Location: Echoes are used as a means of determining distances in various applications. For example, bats use echoes to locate prey through a process called echolocation. Similarly, sonar technology uses echoes of sound waves to map the depths of oceans and to detect underwater objects.
    6. Echo Chambers in Entertainment: Echo chambers are designed spaces with controlled multiple reflections that are used in entertainment, particularly in sound recording and music production. Musicians and sound engineers can manipulate these reflections to create specific reverberation effects in recordings.


    1. Reverberation refers to the phenomenon where sound persists even after the source of the sound has stopped emitting it. This phenomenon occurs due to multiple reflections of sound waves within an enclosed space. When sound waves encounter surfaces within a room, they reflect off those surfaces multiple times before eventually losing energy and fading away. This gives rise to a prolonged persistence of sound, creating a 'reverberating' effect.
    2. Reverberation can enhance the richness and fullness of sound, particularly in musical performances. It gives a sense of spaciousness and can make music and voices sound more immersive. However, excessive reverberation can also cause problems, such as making speech less intelligible or creating a "muddy" sound in a room.
    3. In various settings, such as concert halls or recording studios, engineers use acoustic treatments to control and manage reverberation. Techniques like adding sound-absorbing materials (such as curtains, panels, or foam) to surfaces can help reduce excessive reverberation and improve the clarity of sound.

    Range of Hearing in Humans

    Audible Range

    The normal range of human hearing covers frequencies from approximately 20 Hz to 20,000 Hz (20 kHz). This range is not equally sensitive across all frequencies; our hearing is most sensitive to frequencies between 2 kHz and 4 kHz.

    Inaudible Range

    1. Infrasound: Frequencies below 20 Hz are known as infrasound. Humans cannot hear these frequencies, but they can still have physiological effects on the body. Infrasound is sometimes produced by natural events like earthquakes or by man-made sources.
    2. Ultrasound: Frequencies above the audible range (above 20 kHz) are referred to as ultrasound. Humans cannot hear ultrasound, but some animals, such as bats and certain marine mammals, have the ability to hear and use ultrasonic frequencies for various purposes like navigation and communication.
     Learn more about Sound

    Applications of Multiple Reflections

    Multiple reflections of sound can be harnessed for various practical purposes due to their unique properties. Some of the uses of multiple reflections of sound are:

    Sonar (Sound Navigation and Ranging)

    Sonar is a technology that uses sound waves to navigate, communicate, and detect objects underwater. It's widely used in maritime and naval applications for tasks such as underwater navigation, mapping the seafloor, locating underwater objects, and detecting submarines.

    This is the way sonar functions:

    1. Emission of Sound Waves: A sonar system emits a sound signal, usually in the form of a pulse of sound waves, into the water. These sound waves travel through the water and interact with objects and surfaces in their path.
    2. Reflection of Sound Waves: When the emitted sound waves encounter an object or the seabed, they reflect off these surfaces just like light reflects off a mirror. These reflections create echoes that travel back towards the sonar system.
    3. Reception and Analysis: The sonar system's receiver detects the echoes as they return. By analysing the time it takes for the echoes to return and the characteristics of those echoes, the system can determine the distance, size, shape, and even material properties of the objects or surfaces that reflected the sound waves.
    4. Mapping and Navigation: Sonar systems can create detailed underwater maps by measuring the depth of the water and the shape of the seabed. They are used for safe navigation in shallow or poorly charted waters, as they provide real-time information about the underwater environment.
    5. Submarine Detection: In military applications, sonar is used to detect submarines. By listening to the sound of a submarine's engines, sonar operators can identify the presence and location of submarines even when they're submerged.


    Ultrasound is a type of sound wave that has a frequency higher than the upper limit of human hearing (typically above 20,000 hertz or 20 kHz). Ultrasound technology has a wide range of applications, particularly in medical imaging, where it's used to visualise the internal structures of the body.

    1. Generation of Ultrasound Waves: In medical ultrasound, a transducer emits a pulse of high-frequency ultrasound waves into the body. These waves travel through the body and interact with different tissues and organs.
    2. Reflection of Ultrasound Waves: When ultrasound waves encounter boundaries between tissues of different densities or structures, they reflect back as echoes. The density and composition of tissues affect the way the ultrasound waves are reflected.
    3. Reception and Image Formation: The transducer also acts as a receiver, detecting the returning echoes. By analysing the time it takes for the echoes to return and their intensity, the ultrasound machine constructs real-time images of the internal structures, such as organs, blood vessels, and developing foetuses during pregnancy.
    4. Diagnostic Imaging: Medical professionals use ultrasound to diagnose and monitor various conditions. Obstetricians use it to monitor foetal development, cardiologists use it to assess heart function, and radiologists use it for imaging abdominal organs, muscles, and joints.
    5. Non-Invasive Procedure: Ultrasound imaging is non-invasive and doesn't involve ionising radiation like X-rays. It's safe and widely used for routine medical imaging, especially during pregnancy.

    Human Ear

    The human ear is a complex sensory organ responsible for detecting and interpreting sound waves. It consists of three main parts: the outer ear, the middle ear, and the inner ear. Each part plays a crucial role in the process of hearing.

    Labelled Diagram of Human Ear - CREST Olympiads

    Outer Ear

    The outer ear is the visible portion of the ear that collects and channels sound waves towards the middle ear. It consists of two main components:

    1. Pinna (Auricle): The pinna is the external, fleshy part of the ear that captures sound waves from the environment. It helps in funnelling sound waves into the ear canal.
    2. Ear Canal (Auditory Canal): The ear canal is a tube-like structure that carries sound waves from the pinna to the eardrum (tympanic membrane). It also plays a role in amplifying certain frequencies of sound.

    Middle Ear

    The middle ear is a small air-filled cavity located between the eardrum and the oval window (a membrane-covered opening to the inner ear). It contains three tiny bones known as the ossicles:

    1. Malleus (Hammer): The malleus is the first ossicle and is connected to the eardrum. When sound waves cause the eardrum to vibrate, the malleus also vibrates.
    2. Incus (Anvil): The incus is the second ossicle and is situated between the malleus and the stapes. It transmits vibrations from the malleus to the stapes.
    3. Stapes (Stirrup): The stapes is the third ossicle and is connected to the oval window of the inner ear. As the stapes vibrates, it transfers the sound vibrations from the middle ear to the fluid-filled inner ear.

    The middle ear also contains the Eustachian tube, which helps equalise the pressure between the middle ear and the outside environment. This is important for maintaining proper hearing and preventing discomfort.

    Inner Ear

    The inner ear is a complex and intricate structure responsible for converting sound vibrations into electrical signals that can be interpreted by the brain. It consists of two main components:

    1. Cochlea: The cochlea is a spiral-shaped, fluid-filled structure that resembles a snail's shell. It contains specialised hair cells that are responsible for converting mechanical vibrations into electrical signals. Different sections of the cochlea respond to different frequencies of sound.
    2. Vestibular System: This system is responsible for maintaining balance and spatial orientation. It includes the semicircular canals and the utricle and saccule, which are filled with fluid and detect head movement and changes in position.

    Hearing Process

    1. Sound waves are collected by the pinna and directed into the ear canal.
    2. The sound waves cause the eardrum to vibrate.
    3. The vibrations of the eardrum are transmitted to the ossicles (malleus, incus, and stapes) in the middle ear.
    4. The stapes vibrate against the oval window, transmitting vibrations into the fluid-filled cochlea of the inner ear.
    5. Inside the cochlea, hair cells are stimulated by the fluid vibrations. These hair cells convert mechanical energy into electrical signals (nerve impulses).
    6. These electrical signals travel along the auditory nerve to the brainstem, where they are further processed and interpreted as sound.
    7. The brain then processes these signals, allowing us to perceive and interpret various aspects of sound, including its pitch, volume, and location.

     Learn more about Reflection of Light

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