Concepts

Class 9
Chapter 12 Class 9 - Sound

## What is Reflection of Sound

When sound strikes a hard surface, it bounces back

This is called Reflection of Sound

## Does Reflection of Sound happens in all types of matter?

No

We know that there are 3 forms of matter-Solids,Liquids and Gases

Reflection of Sound happens mainly when sound strikes solid or liquid (not gases)

This is because sound reflects back only when it strikes a hard surface

Note:- Sound waves can be reflected back by both polished and rough surfaces.

## How is Reflection of Light Similar to Reflection of Sound?

Laws of Reflection says that

• Incident Ray,Reflected Ray and Normal Ray all lie on same place
• Angle of Incidence is equal to Angle of Reflection

Both these rules are equally applicable in case of Reflection of Sound

Hence, we can say that

• Incident Sound Wave, Reflected Sound Wave and Normal all lie on same plane
• Angle of Incidence of Sound wave is equal to Angle of Reflection of Sound wave

## Activity to Show Reflection of Sound

Make 2 pipes of Chart Paper

Arrange them on Table near wall

Keep a clock near one of pipes

Try to hear from other pipe

Adjust position of second pipe till sound is largest

We draw a line perpendicular to point where 2 pipes meet

This is normal line

We Measure the angle at this place

At this position ,we observe that

Angle of Incidence of Sound is equal to Angle of Reflection of Sound

Also, Incident Sound Wave, Reflected Sound Wave and Normal Sound Wave all lie on same plane (which is the table in this case)

Actual figure looks like

## Practical Use of Multiple Reflection of Sound

### Ceilings of Cinema halls are Curved

Concert halls are very big, so the sound might not reach every corner of the hall.

So, ceilings of concert halls are made curved so that after reflecting, it reaches all parts of the hall.

### Stethoscope

It is used by doctors to listen to sounds produced within the body, mainly in heart or lungs.

Doctor is able to hear sound due to multiple reflection of sound in stethoscope

### Bulb Horns

When bulb of horn is pressed,

It uses principle of reflection to reflect sound in forward direction

### Musical Instruments

When a person blows trumpet or shehnai

It send sound in particular direction without spreading it in all directions

Hence It also uses principle of reflection to reflect sound in forward direction

## Questions

Question 1 Page 169 -  Why are the ceilings of concert halls curved?

NCERT Question 10 - Does sound follow the same laws of reflection as light does? Explain.

NCERT Question 12 Give two practical applications of reflection of sound waves.