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Short Notes - Sound Notes | Study Science Class 9 - Class 9

Document Description: Short Notes - Sound for Class 9 2022 is part of Science Class 9 preparation. The notes and questions for Short Notes - Sound have been prepared according to the Class 9 exam syllabus. Information about Short Notes - Sound covers topics like What is Sound?, Sound is Produced by a Vibrating Body, Sound needs a Medium for Propagation, Properties of Sound Waves, Reverberation, Audible and Inaudible Sounds and Short Notes - Sound Example, for Class 9 2022 Exam. Find important definitions, questions, notes, meanings, examples, exercises and tests below for Short Notes - Sound.

Introduction of Short Notes - Sound in English is available as part of our Science Class 9 for Class 9 & Short Notes - Sound in Hindi for Science Class 9 course. Download more important topics related with notes, lectures and mock test series for Class 9 Exam by signing up for free. Class 9: Short Notes - Sound Notes | Study Science Class 9 - Class 9
Table of contents
What is Sound?
Sound is Produced by a Vibrating Body
Sound needs a Medium for Propagation
Properties of Sound Waves
Reverberation
Audible and Inaudible Sounds
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What is Sound?

Sound is a form of energy that produces a sensation of hearing in our ears.  

  • Propagation of Sound: Sound is produced by vibrating objects.
  • Medium: The matter or substance through which sound is transmitted is called a medium. It can be solid, liquid or gas. Air is the most common medium for sound propagation.
  • Wave: A wave is a disturbance that moves through a medium when the particles of the medium set neighbouring particles into motion. They in turn produce similar motion in others. The particles of the medium do not move forward themselves, but the disturbance is carried forward. This is what happens during the propagation of sound in a medium, hence sound can be visualised as a wave. Sound waves are characterised by the motion of particles in the medium and are called mechanical waves.

Sound is Produced by a Vibrating Body

  • When a vibrating object moves forward, it pushes and compresses the air in front of it creating a region of high pressure. This region is called a compression (C), as shown in the following figure. This compression starts to move away from the vibrating object. When the vibrating object moves backwards, it creates a region of low pressure called rarefaction (R).

Short Notes - Sound Notes | Study Science Class 9 - Class 9

  • As the object moves back and forth rapidly, a series of compressions and rarefactions are created in the air. These make the sound wave that propagates through the medium. Compression is the region of high pressure and rarefaction is the region of low pressure.

Sound needs a Medium for Propagation

Experiment to show Sound needs a medium:
Activity: 

  • Take an electric bell and airtight jar.
  • Fit the bell inside the jar and attach it to the battery.
  • With the air still in jarring the bell.
  • Now take out the air by vaccumization.
  • Ring the bell again.
    Bell Experiment
    Bell Experiment

Observation: The sound of the bell can be heard when air is inside the jar. When air is taken out then a sound cannot be heard.
Conclusion: This shows that the presence of a medium is necessary for the propagation of sound waves.

Question for Short Notes - Sound
Try yourself:Sound travels in air if
View Solution


Properties of Sound Waves

Short Notes - Sound Notes | Study Science Class 9 - Class 9

  • Wavelength: Distance between two consecutive peaks (crest) or and troughs is called wavelength. Wavelength is represented by λ (lambda) and its SI unit is metres.
  • Frequency: The number of oscillations of sound waves per unit time is called its frequency. A number of peaks or troughs per unit of time will give frequency. It is represented by ν (nu) and its SI unit is Hertz (Hz).
  • Time Period: The time taken to complete one oscillation is called its time period. Its unit is second and is represented by T.
  • Relation of frequency and time period:

Short Notes - Sound Notes | Study Science Class 9 - Class 9

  • Amplitude: The magnitude of the maximum disturbance in the medium on either side of the mean value is called the amplitude of the wave. It is usually represented by the letter A. Its unit is metre.
  • Softness or Loudness of Sound: If the amplitude is smaller then the sound will be softer and if it is larger then the sound will be louder. Higher amplitude helps the sound wave is traveling faster.
  •  Speed of sound: It is the distance which compression or a rarefaction travels per unit of time.
    Speed of sound = Short Notes - Sound Notes | Study Science Class 9 - Class 9
    Short Notes - Sound Notes | Study Science Class 9 - Class 9
    So, Speed = Wavelength × Frequency The speed of the sound remains almost the same for all frequencies in a given medium under the same physical condition.
    Speed of Sound in Different Media at 25º C.Short Notes - Sound Notes | Study Science Class 9 - Class 9
  • Reflection of Sound: Sound reflects off a surface in the same way as light reflects and follows the same rules of reflection. The incident sound and the reflected sound make equal angles with the normal and all three are in the same plane.
  • Echo: In bigger monuments and in large empty halls you can hear reflections of your sound after you speak something. This effect is known as echo. The sensation of sound persists in our brain for 0.1 seconds, so to hear an echo the time difference between the original sound and reflected sound should be more than that. As you know the speed of sound in air = 344 m/s And 344 × 0.1 = 34.4 metres So, the minimum distance required to hear an echo from a reflecting wall or surface should behalf of 34.4, which means it should be 17.2 metres.

Question for Short Notes - Sound
Try yourself:When we change feeble sound to loud sound we increase its
View Solution


Reverberation

  • Repeated reflections of sound result in the persistence of sound and is called reverberation.
  • Use of Reverberation of Sound: Following instruments use this property of sound:
    1. Megaphones, Shehanais, Trumpets
    2. Stethoscope
    3. Curved dome of concert halls

Audible and Inaudible Sounds

  • Range of Hearing in humans: From 20 Hz to 20000 Hz Sounds below 20 Hz are called infrasonic.
  • Sounds above 20000 Hz are called ultrasonic.
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