top of page

Unlock the Mysteries of Sound with Future Classes' Science Course for CBSE Class 9

Work and Energy is an important concept in CBSE Class 9 Science. Learning about this topic is essential for understanding the world around us. To help students understand this topic, notes and other resources are available for download on our website. These notes provide an in-depth explanation of concepts such as kinetic energy, potential energy, and the law of conservation of energy. We also provide example questions and solutions to help students practice and apply their knowledge. Download our CBSE Class 9 Science notes today and start exploring the fascinating world of work and energy

Our website provides comprehensive PDF downloads for CBSE Class 9 Science Chapter Sound. These PDFs are regularly updated and provide clear, comprehensive information on all topics related to the chapter. They are easy to download and can be printed for use as handy reference material. With our PDFs, students can gain a better understanding of the concepts and be better prepared for their exams.

CHAPTER 11 SOUND

Sound

  • The sensation felt by our ears is called sound.

  • Sound is a form of energy which makes us hear.

  • Law of conservation of energy is also applicable to sound.

  • Sound travels in form of wave.

Production of Sound

Sound is produced when object vibrates.

  • Example: Sound of our voice is produced by vibration of two vocal cords in our throat

  • Sound of a drum or table is produced by vibration of its membrane when struck

Sound can be produced by following methods:

  1. By vibrating string (sitar)

  2. By vibrating air (flute)

  3. By vibrating membrane (table, drum)

  4. By vibrating plates (bicycle bell)

  5. By friction in objects

  6. By scratching or scrubbing the objects etc.

Propagation of Sound

  • The substance through which sound travels is called a medium.

  • The medium may be solid, liquid or gas.

  • When an object vibrates, then the air particles around it also start vibrating in exactly the same way and displaced from their stable position.

  • These vibrating air particles exert a force on nearby air particles so they are also displaced from their rest position and start to vibrate.

  • This process is continued in the medium till sound reaches our ears.

  • The disturbance produced by sound travels through the medium (not the particles of the medium).

  • Wave is a disturbance which travels through a medium and carries energy. So sound travels in wave form known as mechanical waves.

Longitudinal wave

A wave in which the particles of the medium vibrate back and forth in the same direction in which the wave is moving, is called a longitudinal wave.

  • The direction of vibrations of the particles is parallel to the direction of wave.

Transverse wave

When one end of a slinky is moved up and down rapidly whose other end is fixed, it produces transverse wave.

  • Thus, in transverse wave particles of the medium vibrate up and down at right angles to the direction of wave.

Characteristics of Sound Wave

The characteristics of sound waves are: wavelength, frequency, amplitude, time period and velocity.

  • When a wave travel in air the density and pressure of air changes from their mean position.

  • Compression is shown by crest while rarefaction is shown by trough.

  • Compression is the region of maximum density or pressure.

  • Rarefaction is the region of minimum density or pressure.

Wavelength

In sound waves the combined length of a compression and an adjacent rarefaction is called its wavelength. It is denoted by the Greek letter lamda λ. Its SI unit is meter.

Frequency

Number of complete waves produced in one second or number of vibrations per second is called frequency.

  • The SI unit of frequency is hertz (Hz). The symbol of frequency is v (nu).

  • 1 Hertz: One Hz is equal to 1 vibration per second.

  • Bigger unit of frequency is kilohertz kHz = 1000 Hz.

Time Period

Time taken to complete one vibration is called time period.

  • SI unit of time period is second (s). Time period is denoted by T.

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

Amplitude

The maximum displacement of the particle of the medium from their original undisturbed position is called amplitude of the wave. Amplitude is denoted by A and its SI unit is meter (m).

Velocity

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

Its SI unit is meter per second (m/s).

Speed of Sound in Various Mediums

  1. Speed of sound depends on the nature of material through which it travels. It is slowest in gases, faster in liquids and fastest in solids.

  2. Speed of sound increases with the rise in temperature.

  3. Speed of sound increases as humidity of air increases.

  4. Speed of light is faster than speed of sound.

  5. ​In air, speed of sound is 344 m/s at 22 C.

Reflection of Sound

Sound also bounces back when it falls on a hard surface. It is called reflection of sound.

Law of reflection of sound

  1. The incident sound wave, the reflected sound wave and normal at the point of incidence lie in the same plane.

  2. Angle of reflection of sound is always equal to the angle of incidence of sound.

Echo

The repetition of sound caused by the reflection of sound waves is called an echo.

  1. We can hear echo when there is a time gap of 0.1 second in original sound and echo (reflected sound).

  2. Echo is produced when sound reflected from a hard surface.

Q. To calculate the minimum distance to hear an echo:

Reverberation

  1. The persistence of sound in a big hall due to repeated reflection of sound from the walls, ceiling and floor of the hall is called reverberation.

  2. If it is too long, sound becomes blurred, distorted and confusing.

Methods to reduce reverberation in big halls or auditoriums

  1. Panels made of felt or compressed fiber board are put on walls and ceiling to absorb sound.

  2. Heavy curtains are put on doors and windows.

  3. Carpets are put on the floor.

  4. Seats are made of material having sound absorbing properties.

Applications of Reflection of Sound

  1. Megaphone, loudspeakers, bulb horns and trumpets, shehnai etc. are designed to send sound in a particular direction without spreading all around. All these instruments have funnel tube which reflects sound waves repeatedly towards audience. In this amplitude of sound waves adds up to increase loudness of sound.

  2. Stethoscope: It is a medical instrument used for listening the sounds produced in human body mainly in heart and lungs. The sound of the heartbeats reaches the doctor’s ears by the multiple reflection of the sound waves in the rubber tube of stethoscope.

  3. Sound Board: In big halls or auditoriums sound is absorbed by walls, ceiling, seats etc. So, a curved board (sound board) is placed behind the speakers so that his speech can be heard easily by audiences. The soundboard works on the multiple reflection of sound.

  4. The ceiling of concert halls is made curved, so that sound after reflection from ceiling, reaches all the parts of the hall.

Range of Hearing

  1. Range of hearing in human is 20 Hz to 20000 Hz.

    1. Children younger than 5 years and dogs can hear up to 25 KHz.

  2. The sounds of frequencies lower than 20 Hz are known as ‘infrasonic sounds.

    1. A vibrating simple pendulum produces infrasonic sounds.

    2. Rhinoceroses communicate each other using frequencies as low as 5 Hz.

    3. Elephants and whales produce infrasonic waves.

    4. Earthquakes produces infrasonic waves (before shock waves) which some animals can hear and get disturbed.

  3. The sounds of frequencies higher than 20 KHz are known as ‘ultrasonic waves.

    1. Dogs, dolphins, bats and rats can hear ultrasonic sounds.

    2. Bats and rats can produce ultrasonic sounds.

Applications of Ultrasound

  1. It is used to detect cracks in metal blocks in industries without damaging them.

  2. It is used in industries to clean ‘hard to reach’ parts of objects such as spiral tubes, odd shaped machines etc.

  3. It is used to investigate the internal organs of human body such as liver, gall bladder, kidneys, uterus and heart.

  4. Echocardiography: These waves are used to reflect the action of heart and its images are formed. This technique is called echocardiography.

  5. Ultrasonography: The technique of obtaining pictures of internal organs of the body by using echoes of ultrasound waves is called ultrasonography.

  6. Ultrasound is used to split tiny stones in kidneys into fine grains.

PDF DOWNLOAD FREE 

Sing Up/Login Required

You have to sign up to download the PDF. You will get the PDF of the full chapter.

Click Here

Direct Download

To download the PDF you just have to click on the button. You will get the PDF of HALF chapter.

Click Here
bottom of page