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Lakhmir Singh Solutions for Class 9th Physics Chapter 5: Sound

This chapter includes a wide account of sound waves, their production, and their propagation. Sound energy is the form of energy because of which we can hear. It generally travels in the form of longitudinal waves where the particles of the medium vibrate in a uni-direction where the waves are moving. Other examples of longitudinal waves include the water waves formed on the water surface in a pond or the waves that travel along with a spring when it is pushed at one end. Following are some important characteristics of sound waves:
   1. Wavelength
   2. Amplitude
   3. Time-period
   4. Frequency
   5. Velocity of wave

The chapter further includes understanding the relationship between velocity, frequency, and wavelength of a wave. For example, the vibration of two vocal cords in our throat or the vibration of a drum membrane produces a sound that propagates through the layers of air and reaches our ear. Sound waves are mechanical waves because they require a material medium for their propagation. Sound waves cannot travel through the vacuum or free space. Hence the astronauts communicate via wireless sets using radio waves on the surface of the moon.

The speed of sound and factors affecting it is another important topic of this chapter. The speed of sound depends on the air's material medium, temperature, and humidity. Bouncing back of sound waves on striking a hard surface is called reflection of sound, and two laws of reflection are obeyed during the reflection of sound. Megaphones, stethoscopes, and soundboards are some devices which work on the principle of reflection of sound waves. A simply reflected sound is called an echo. When sound waves encounter multiple reflections, the phenomenon is called reverberation. The frequency range of hearing in humans is 20-20,000 Hz.

  • Infrasonic is < 20Hz
  • Ultrasonic is > 20,000 Hz
  • Supersonic is the speed of an object which is > Speed of sound in air

The chapter explores various applications of ultrasound in different fields and the concept of SONAR. The characteristics of sound include loudness, pitch, and quality. Construction of the human ear and its working are the concluding topics of the chapter.


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