Grade 9
  1. Mechanics  1.1. Motion  1.1.1. Types of motion  1.1.2. Distance and displacement  1.1.3. Speed and Velocity  1.1.4. Acceleration  1.1.5. Graphical representation of motion  1.1.6. Equations of motion  1.1.7. Uniform and non-uniform motion  1.1.8. Free fall and acceleration due to gravity  1.1.9. Relative speed  1.1.10. Circular motion  1.2. Laws of force and motion  1.2.1. The concept of force  1.2.2. newton's first law of motion  1.2.3. Newton's second law of motion  1.2.4. Newton's third law of motion  1.2.5. Inertia and types of inertia  1.2.6. Motion  1.2.7. Conservation of momentum  1.2.8. Application of Newton's laws in daily life  1.2.9. Types of Forces (Contact and Non-Contact)  1.2.10. Friction and its effects  1.3. Gravitational force  1.3.1. Newton's law of universal gravitation  1.3.2. Importance of Gravitational Force  1.3.3. Acceleration due to gravity  1.3.4. Free fall and weightlessness  1.3.5. Mass and weight  1.3.6. Variation of g with height and depth  1.3.7. Kepler's laws of planetary motion  1.3.8. Satellites and their orbits  1.4. Work, Energy and Power  1.4.1. Concept of work  1.4.2. Positive and negative actions  1.4.3. Work done by constant and variable force  1.4.4. Energy and its forms  1.4.5. Kinetic energy and potential energy  1.4.6. Law of conservation of energy  1.4.8. Commercial unit of energy  1.4.9. Work–energy theorem  1.5. Simple machines  1.5.1. Types of simple machines  1.5.2. Mechanical advantage  1.5.3. Machine efficiency  1.5.4. Levers and their types  1.5.5. Pulleys and Inclined Planes  1.5.6. Gears and their uses
  2. Properties of matter  2.1. States of matter  2.1.1. Solids, liquids and gases  2.1.2. Properties of different states of matter  2.1.3. Change in state of matter  2.1.4. Plasma and Bose–Einstein condensate  2.2. Density and pressure  2.2.1. Concept of density and relative density  2.2.2. Pressure in solids, liquids and gases  2.2.3. Pascal's law and its applications  2.2.4. Atmospheric pressure and its variations  2.2.5. Surface tension and capillarity  2.3. Buoyancy and Archimedes' principle  2.3.1. Buoyant force  2.3.2. Archimedes principle  2.3.3. Applications of Archimedes' Principle  2.3.4. floating and sinking objects  2.3.5. Theory of Flotation and Archimedes' Principle
  3. Heat and Thermodynamics  3.1. Temperature and heat  3.1.1. Concept of heat and temperature  3.1.2. Temperature measurement  3.1.3. Temperature scales  3.2. Heat transfer  3.2.1. Conduction of heat  3.2.2. Convection of heat  3.2.3. heat radiation  3.2.4. Applications of heat transfer  3.2.5. Thermal conductivity  3.3. Thermal expansion  3.3.1. Expansion of solids, liquids and gases  3.3.2. Abnormal expansion of water  3.3.3. Applications of Thermal Expansion  3.4. Specific heat capacity and latent heat  3.4.1. Concept of specific heat capacity  3.4.2. Measurement and applications of specific heat  3.4.3. Latent heat of fusion and vaporization  3.4.4. Heat Engines and Efficiency
  4. Waves and sound  4.1. Waves and their types  4.1.1. Mechanical and electromagnetic waves  4.1.2. Longitudinal and Transverse Waves  4.1.3. Properties of Waves  4.1.4. Wavelength, frequency and speed of the wave  4.1.5. Superimposition of waves  4.2. Sound waves  4.2.1. Nature and transmission of sound  4.2.2. Speed of sound in different mediums  4.2.3. Reflection of sound and echo  4.2.4. Sonar and ultrasound  4.2.5. Resonance and pulsation  4.3. Sound characteristics  4.3.1. Intensity, loudness and quality of sound  4.3.2. Doppler effect in sound
  5. Lighting and Optics  5.1. Reflection of light  5.1.1. Laws of reflection  5.1.2. Reflection from a plane mirror  5.1.3. Reflection from a spherical mirror  5.1.4. Image formation by concave and convex mirrors  5.1.5. Applications of Reflection  5.2. Refraction of light  5.2.1. Laws of refraction  5.2.2. Refractive index  5.2.3. Refraction through a glass slab  5.2.4. Refraction in water and air  5.2.5. Lenses and their types  5.2.6. Image formation by convex and concave lenses  5.2.7. Total internal reflection and its applications  5.3. Dispersion and scattering of light  5.3.1. Spectrum of white light  5.3.2. Prisms and Dispersion  5.3.3. Tyndall effect and blue sky
  6. Electricity and Magnetism  6.1. Electric charge and static electricity  6.1.1. The concept of electric charge  6.1.2. Charging by friction, contact and induction  6.1.3. Electroscope and its working  6.2. Current Electricity  6.2.1. The concept of electric current  6.2.2. Ohm's Law and Resistance  6.2.3. Factors affecting resistance  6.2.4. Series and Parallel Circuits  6.2.5. Heating effect of electric current  6.2.6. Electric power and energy  6.3. Magnetism  6.3.1. Natural and artificial magnets  6.3.2. Properties of magnets  6.3.3. Earth's magnetism  6.3.4. Magnetic field and field lines  6.3.5. Electromagnets and their applications
  7. Modern Physics  7.1. structure of the atom  7.1.1. Atomic Structure and Subatomic Particles  7.1.2. Electrons, Protons, and Neutrons  7.1.3. Rutherford and Bohr model  7.2. Radioactivity  7.2.1. Types of radioactive emissions  7.2.2. Half-life and radioactive decay  7.2.3. Uses and Hazards of Radioactivity
  8. Space science and astronomy  8.1. The universe and its components  8.1.1. Stars and galaxies  8.1.2. Planets and Solar System  8.2. Satellites  8.2.1. Natural and artificial satellites  8.2.2. Use of satellites

Grade 9 → Waves and sound → Sound waves ↓


Nature and transmission of sound


Sound is an important part of our daily lives. We use sound to communicate with each other, listen to music and even get information about our surroundings through its various signals. But what exactly is sound? How does it travel from one place to another? In this lesson, we will explore the nature of sound and how it is transmitted through various mediums.

What is sound?

Sound is a type of energy that is produced when an object vibrates. When an object vibrates, it creates waves that travel through a medium such as air, water, or solids. These waves are called sound waves, and they carry sound from the source to our ears.

Imagine you are at a concert and the guitarist plays the strings. The strings begin to vibrate. These vibrations cause the air molecules around the strings to vibrate as well. These vibrating molecules collide with their neighbouring molecules, and the vibrations propagate outward in the form of waves.

Nature of sound waves

Sound waves are longitudinal waves, which means the vibrations occur in the same direction as the wave is traveling. To understand this better, let's compare sound waves to water waves.

When a stone is dropped into a pond, it creates waves that move across the surface of the water. These are transverse waves, where the motion of the water is perpendicular (at right angles) to the direction of the wave. However, sound waves don't work exactly that way.

In sound waves, the motion is parallel to the direction of the wave. You can imagine this like a Slinky toy. If you stretch a Slinky out on a table and push one end, you will see the coils move back and forth along the length of the Slinky, creating regions where the coils are close to each other and regions where they are far from each other.

In the above diagram, sound waves are shown as circles moving along a line. The blue circles represent regions where the molecules are closer together, called compression. The spaces between them where the molecules are more spread out are called rarefactions.

Properties of sound waves

Sound waves have several important properties that affect their hearing:

  • Wavelength: It is the distance between two successive compressions or rarefactions. In a slinky, it would be the distance between the coils in the compressed state.
  • Frequency: This is the number of waves passing a given point in one second. It is measured in Hertz (Hz). High frequency means high pitched sound, such as a whistle. Low frequency means low pitched sound, such as a drum.
  • Amplitude: It is the maximum extent of vibration or oscillation, measured from the equilibrium position. Practically, amplitude is related to how loud the sound is. Higher amplitude means louder sound.
  • Speed: This refers to the speed of the wave through the medium. Sound travels at about 343 meters per second in air at room temperature.

The properties of sound waves can be expressed in formulas. The formula for the speed of sound is given as:

v = f × λ

Where:

  • v is the speed of sound,
  • f is the frequency, and
  • λ (lambda) is the wavelength.

Transmission of sound

Sound needs a medium to travel, and it can propagate through gases, liquids, and solids. However, it cannot travel through a vacuum because there are no particles to carry the vibrations.

Let us analyse the propagation of sound through different mediums:

Sound in the air

Air is the usual medium through which we experience sound. As sound waves travel, they move air molecules, creating compressions and rarefactions. You can look at the molecules as a series of crowded and spaced segments, transferring energy through collisions between molecules.

Sound in the water

Sound travels faster in water than in air. This is because water molecules are more closely spaced than air molecules. If you have ever talked underwater or heard the movement of water, you know that sound is transmitted more easily.

Sound in solids

Sound travels even faster in solids because of the compactness of the particles. Consider how easily sound can travel through the walls of a building – or how the vibrations of a train can be felt through tracks.

In general, sound travels fastest in solids, slowest in liquids, and slowest in gases. The speed of sound in different media can be summarized as follows:

Speed in solids > Speed in liquids > Speed in gases

Examples of sound transmission

To understand the transmission of sound in a practical context, let us consider some examples:

Sound on a stringed instrument

When you pluck a guitar string, the vibrations of the strings create sound waves in the air. These vibrations resonate with our hearing ability, producing music or sound.

The two gray lines show the string vibrating back and forth, producing sound wave patterns on both sides. The solid black line shows its resting state.

Sound in a hollow tube

If you speak into one end of a hollow tube, the sound waves you create travel through the tube, causing the particles inside it to vibrate. The sound level increases as it travels through the tube because fewer waves are lost to the outside environment.

Reflection, refraction and diffraction of sound

Apart from passing through different mediums, sound waves can also reflect, refract and diffract under different circumstances.

Reflection

When sound waves hit a hard surface, they can bounce back. This is the principle behind echo. You can experience it in a large empty hall or valley, where your voice comes back just moments after you speak.

Refraction

Refraction occurs when sound waves travel from one medium to another, changing their speed and direction. This can happen on a hot day when the air near the ground is warmer than the air above, causing the sound waves to bend and travel farther.

Diffraction

Diffraction involves sound waves bending around obstacles or through holes. This is why you can hear someone talking even if you're around a corner.

Applications and implications of sound transmission

Understanding the nature and propagation of sound has many applications. Here are some major areas:

  • Communication: From simple speaking and listening to advanced devices such as telephones, radios, and alarms, sound is essential for transmitting information.
  • Medical: Techniques such as ultrasound use sound waves to create pictures of the inside of the body. It is a powerful tool in prenatal scanning and diagnosing health conditions.
  • Music and entertainment: Instruments and sound systems rely on sound physics to create and enhance music.
  • Environment and navigation: Animals such as bats and dolphins use echolocation to find their way. Ships use sonar to detect other objects underwater.

Conclusion

Sound is a fascinating type of energy that relies on wave vibrations to travel through various mediums. By understanding the fundamental properties and behaviors of sound, we gain insight into both natural and technological phenomena. Whether in communication, medicine or entertainment, the role of sound and its waves is integral to our experience of the world.


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Nature and transmission of sound

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