Sound Short Notes Class 9 Science Chapter 12 NCERT Notes

Sound Short Notes | Class 9 NCERT

Introduction

Sound is a form of energy that produces a sensation of hearing in our ears. It is created by vibrating objects.

Production of Sound

The vibration of an object produces sound. In the surrounding media (air, water, or solids), these vibrations produce waves that reach our ears and are interpreted as sound.

Examples of Sound Production:
  • Tuning Fork: A tuning fork's prongs vibrate when you strike it, creating sound. A hung table tennis ball will move when you contact it with a vibrating tuning fork, illustrating the vibrations.
  • Musical Instruments: Different kinds of musical instruments  produce sound through vibration. For instance, the vibrating strings of a guitar and the vibrating membrane of a drum both make music.

Propagation of Sound

  1. A medium, such as air, water, or solids, is necessary for sound to travel. It is unable to move in a vacuum.
  2. Wave Movement: Sound travels as a wave through the medium by creating compressions (high-pressure areas) and rarefactions (low-pressure areas).
  3. Sound waves need a medium to travel through because they are mechanical waves.
  4. Sound speed varies depending on the medium: solids have the fastest sound speed, liquids have the slowest sound speed, and gases have the slowest sound speed.

Sound Waves as Longitudinal Waves

  1. Sound waves are longitudinal waves because the particles in the medium through which they travel vibrate in the same direction as the wave propagation. This indicates that the particles travel back and forth in the same direction as the wave.
  2. Compressions and Rarefactions: Sound waves produce low-pressure areas known as rarefactions and high-pressure areas known as compressions as they move. The sound energy is carried through the medium by these alternating rarefactions and compressions.
  3. Propagation: Energy is transferred from one particle in the medium—which could be solids, liquids, or air—to another during the propagation of sound. The medium's particle vibrations cause this transfer to happen.

Characteristics of  Sound Waves

  • Wavelength, frequency, amplitude, time period, and velocity are the characteristics of sound waves.
  • The density and pressure of the air move from their mean location when a wave travels across it.
  • Whereas the trough indicates rarefaction, the crest indicates compression
  •  The area with the highest density or pressure is called compression.
  •  The area of lowest pressure or density is known as rarefaction.

Wavelength

  • Definition: The distance between two consecutive compressions or rarefactions in a sound wave.
  • Measurement: It is measured in meters (m).
  • It is denoted by the Greek letter lamda (λ​) 

Frequency

  • Definition: The number of complete oscillations or vibrations that occur in one second.
  • Unit: It is measured in Hertz (Hz), where 1 Hz equals one vibration per second.

Time Period

  • Definition: A wave's time period (T) is the amount of time it takes for a single wave cycle to pass a specific spot. It is the amount of time that passes between two successive points that are in phase, such as the compressions in a sound wave.

  • Measurement: The time period is measured in seconds (s).

  • The frequency of a wave is the reciprocal of the time period.

  • v=1/T

Amplitude

Definition: The greatest displacement of a wave's points from its rest position (equilibrium) is known as its amplitude. Stated differently, it gauges the wave's oscillation's distance from its centre value. It has to do with changes in the medium's pressure for a sound wave.

Measurement: Usually, amplitude is expressed in meters (m).
 On the other hand, pressure (Pascals) can also be used to represent sound.

Velocity

• The distance travelled by a wave in one second is called velocity of the wave.

• Its SI unit is metre per second (ms-1).
Velocity = Distance travelled/Time taken
⇒ v = λ/T
(λ is the wavelength of the waves travelled in one time time period T)
v = λv (1/T = v)

Sonic Boom

When an object travels faster than sound (supersonic speed), such as a bullet or a jet plane, a sonic boom happens. Shock waves are high-energy sound waves produced while the object moves. The loud, piercing sound produced by these shock waves is referred to as a "sonic boom." These shock waves can have enough energy to break glass and even cause structural damage.

Reflection of Sound
Similar to how a rubber ball bounces off a wall, sound waves can bounce off solid or liquid surfaces. The same rules that govern light reflection also apply to this reflection:
  • The angle of incidence (incoming sound) equals the angle of reflection (reflected sound).
  • Both angles are measured from the normal (a line perpendicular to the surface).
  • The incident sound, reflected sound, and the normal all lie in the same plane.

Echo

  • Definition: An echo is the repetition of sound caused by the reflection of sound waves from a surface.
  • Conditions for Hearing an Echo:
    • The distance between the reflecting surface and the sound source must be at least 17.2 meters.
    • For the echo to be clearly audible, there must be a minimum of 0.1 seconds between the initial sound and the reflected sound.
  • Examples: When you yell in a big, deserted hall or next to a mountain, you can hear an echo.

Minimum distance to hear an echo

Speed = Distance/Time
Here, Speed of sound in air = 344 ms-1 at 22ºC
Time = 0.1 second
344 = Distance/0.1 sec
⇒ Distance = 344 × 0.1 = 34.4 m
So, distance between reflecting surface and audience = 34.4/2 = 17.2 (at 22ºC).

Reverberation
  • The persistence of sound in a specific area after the initial source of sound has ceased is known as reverberation.
  • It happens when sound waves bounce off objects (such as floors, walls, and ceilings) and produce a number of overlapping, blending echoes.

Methods to reduce reverberation in big halls or auditoriums
  1. Sound-Absorbing Materials: Install panels on the walls and ceilings using materials such as compressed fibreboard.
  2. Carpets: Cover floors with carpets to absorb sound.
  3. Heavy Curtains: Use heavy curtains on doors and windows.
  4. Upholstered Furniture: Use seats and other furniture that absorb sound

Applications of Reflection of Sound

(i) Horns, shehanais, trumpets, and megaphones, often known as loudhailers, are all made to emit sound in a specific direction without dispersing it in all directions. These instruments move the majority of the sound waves from the source in a forward direction towards the audience by reflecting sound successively through a tube and a conical aperture.

(ii) A medical device called a stethoscope is used to listen to noises made inside the body, primarily in the heart or lungs. with several sound reflections, the doctor can hear the patient's heartbeat with stethoscopes.

(iii) To improve sound quality and clarity, auditoriums use soundboards, which are big, flat surfaces that reflect sound waves towards the audience.

Range of Hearing
Hearing Aid
  • Microphone: Captures sound waves and converts them into electrical signals.
  • Amplifier: Increases the strength of these electrical signals.
  • Speaker: Converts the amplified electrical signals back into sound waves and directs them into the ear.

Application of Ultrasound

  1. Cleaning High-frequency sound waves in a solution are used in ultrasonic cleaning to get rid of oil and grime from difficult-to-reach areas like tubes and electronic components. 

  2. Flaw Detection: Ultrasound can detect cracks and flaws in metal structures. Waves passed through the metal reflect back if defects are present, ensuring structural integrity.

  3. Medical Imaging:

    • Echocardiography: Uses ultrasound to create images of the heart.
    • Ultrasonography: In addition to imaging internal organs (such as the liver and kidneys) to detect anomalies, an ultrasound scanner is used throughout pregnancy to assess foetal growth and malformations. 

  4. Kidney Stone Treatment: Kidney stones can be broken up by ultrasound into tiny fragments that are then eliminated through the urine.


Sonar

The technology known as SONAR (Sound Navigation and Ranging) uses sound waves to find and identify objects underwater.
  • Full Form: Sound Navigation and Ranging.
  • Working Principle: Sound waves are reflected, which is how SONAR operates. It measures the distance, direction, and speed of underwater objects using ultrasonic waves, which are high-frequency sound waves.
  • Components:
    • Transmitter: Produces and transmits ultrasonic waves.
    • Receiver/Detector: Detects the reflected waves (echoes) from objects
  • Process:
  1. Ultrasonic waves are sent into the water by the transmitter.
  2. When these waves strike an object, they bounce back after moving through the water.
  3. After detecting the reflected waves, the receiver calculates how long it takes for the echo to return.
  4. The speed of sound in water and the time it takes for the echo to return are used to determine the object's distance.

Applications:

  • Navigation: Used for obstacle avoidance and navigation in ships and submarines.
  • Depth Measurement: Determines the ocean's or sea's depth.
  • Fishing: Helps in locating schools of fish.
  • Underwater Exploration: Used to find sunken ships and map the ocean floor.

Structure of Human Ear

  1. Outer Ear:

    • Pinna (Auricle): The visible part of the ear that collects sound waves.
    • Ear Canal: A tube that carries sound waves to the eardrum.
    • Eardrum (Tympanic Membrane): A thin membrane that vibrates when sound waves hit it.

  2. Middle Ear:

    • Ossicles: Three tiny bones (malleus, incus, and stapes) that amplify the vibrations from the eardrum.
    • Eustachian Tube: Connects the middle ear to the throat and helps equalize pressure.

  3. Inner Ear:

    • Cochlea: A spiral-shaped organ that converts vibrations into electrical signals.
    • Semicircular Canals: Help maintain balance.
    • Auditory Nerve: Carries electrical signals from the cochlea to the brain.