Class 9 Science Solutions English Medium

Reflection of sound: 

Factors which influences the speed of sound: 

(i) Temperature: The speed of sound changes with temperature. 

For example, the speed of sound in air is 331 m s^–1 at 0 ^०C and 344 m s^–1 at 22 ^०C. The speeds of sound at a particular temperature.

(ii) Particles of Medium : The speed of sound decreases when we go from solid to gaseous state. So in different medium the speed of sound is different. 

Speed of sound and speed of light: The speed of sound is less than the speed of light. For example, The sound of a thunder is heard
a little later than the flash of light is seen. Because the speed of light is very faster than speed of sound. So light comes to us soon while sound travels very late to us. 

Sound Boom : When the speed of any object exceeds the speed of sound it is said to be travelling at supersonic speed. Bullets, jet
aircrafts etc. When a sound, producing source moves with a speed higher than that of sound, it produces shock waves in air. These shock waves carry a large amount of energy. The air pressure variation
associated with this type of shock waves produces a very sharp and loud sound called the “sonic boom”.

Reflection of Sound: 

The reflection of sound is same phenominom like reflection of sound. It also follows the same law of reflection. 

Law of reflection of sound: 

(i) The directions in which the sound is incident and is reflected make equal angles.

(iii) The angle of incident, angle of reflection and the normal to the reflecting surface are in the same plane.  

Essential factor for reflection of sound: 

(i) It need a large size of barrier/obstacle of polished or rough surafce. 

Echo Sound : When any sound refelects by any obstacle, we hear the same sound again a little later. This sound which we hear is called an echo sound. 

Condition to hear the echo sound: 

1. Sensation of sound : The sensation of sound persists in our brain for about 0.1 s.

2. Condition to hear echo sound: To hear a distinct echo the time interval between the original sound and the reflected one must be at least 0.1s.

3. Distance of obstacle : For hearing distinct echoes, the minimum distance of the obstacle from the source of sound must be half of this distance, that is, 17.2 m.

Common Use of the reflection of sound : 

(i) By using the multiple reflection of sound megaphones or loudspeaker, horns and some musical instruments are made. 

(ii) Stethoscope: Stethoscope is a medical instrument which is used to hear various internal sounds of human heart and lungs. It also helps to identify the varietes of different organs sound. 

Method of stethoscope:

In stethoscopes the sound of the patient’s heartbeat reaches the doctor’s ears by multiple reflection of sound.

(iii) The ceiling of concert halls are design in such way that sound after reflection reaches to all corners of the hall.  

Reverberation : The repeated reflection of sound that results in the persistance of sound is called reverberation. 

Methods to reduce reverberation: 

(i) The curtains are suspended in the buildings to reduce it so that the sound can be absorbed.

(ii) By increasing the presence of listeners in the rooms or auditoriums, the sound is absorbed by it as well.

(iii) To reduce it, compressed fiber board, rough plaster etc. are applied.

(iv) The right choice of seat's materials in the seats also act as sound absorbers.