This is a ppt useful for class 9 physics learners.

1. SOUND
Jaya: Arvind! We hear many different kinds of sounds all day.
Arvind: Yes. Jaya. We can hear human voice- talking, laughing, crying, singing,
traffic noise, sounds made by animals, and also the musical sound.

2. Jaya: So, Arvind how sound is produced?
Arvind: Jaya, sounds are produced due to the vibrations of a body or an
object.
Jaya: Arvind, should we discuss it in detail with our teacher?
Arvind: Sure.

3. After completing the lesson, we will be able to:
Explain propagation of sound.
Explain transmission of sound in a medium.
Discuss speed of sound in solid, liquid and gases.
List out the characteristics of a sound.
Discuss and describe Doppler Effect in sound.
Illustrate Reflection of Sound.

4. Teacher (30 yr old): Good morning children.
Students (collectively): Good morning teacher.
Jaya: Mam! What is sound and how is it produced?
Teacher: It is simply the vibrations of air molecules. Here we perform some
activities to understand this.

5. As you speak, place a finger lightly on your throat. You will easily feel the
movement in your throat.
Now place your hand over the speaker of a transistor radio. Your hand will
feel some vibrations.

6. Thus, vibration of a source is necessary for sound to be produced.
When the strings of a sitar are plucked, we hear musical notes.
When the stretched membrane of tabla or drum is struck, it vibrates and we
hear its characteristic sound.

7. In case of a bell, a loud sound is produced due to its outer layer. When the
bob at the center strikes the inner layer, it starts vibrating and hence
producing a sound.
Teacher, how is sound produced in a flute?
Good query Arvind. When we blow air into one end of a flute, air molecules
in the flute vibrates and sound is produced.