Important (Short) Questions - Sound, Class 8, Science PDF Download

1.How is sound produced ?
Ans: Sound is produced by a vibrating body e.g. when a tightly stretched bond is plucked, it vibrates and produced sound. When it stops vibrating , it does not produced any sound . 

2.   How is sound produced by humans? 

Ans: In humans, the sounds is produced by the voice box or the larynx. Put your fingers on the throat and find a hard bump that seem to move when you swallow. This part of the body is known as the voice box. This is at the upper end of the wind pipe. Two vocal cords are stretched across the voice box or larynx in such a way that it leaves a narrow slit between them for the passage of air. When the lungs force air through the suit the vocal cords vibrate, producing sound.

3.   why the voices of men, women and children are different.

Ans: The vocal cords in men are about 20 mm long. In women these are about 15 mm shorter. Children have very short vocal cords. This is the reason why the voices of men, women and children are different.

4.   What is oscillatory motion 

Ans: The to and fro motion of an object is known as vibration. This motion is also called inaudible sounds or infrasonic. . Such sound can not be detected by the human ear. The sound of frequencies higher than about  20,000 vibrations per second (20 kHz) are called the ultrasonic sound. For human ear, the range of audible frequencies is roughly from 20 to 20,000 Hz

8.   Why are sound waves called mechanical waves?

Ans:  Sound waves force the medium particles to vibrate. because of the interaction of the particles present in that medium

9.   What is SONAR?

Ans: SONAR stands for Sound Navigation and Ranging and it works on the principle of reflection of sound waves. The SONAR technique is used to determine the depth of the sea and to locate under water hills, valleys, submarines, icebergs sunken ships etc.

10.               What is the intensity of sound?

Ans: The amount of sound energy passing each second through unit area is called the intensity of sound.

11. Define :  (i) Reverberation of sound.  (ii)Echo (iii) Time period (iv)Wavelength.(v) wave velocity (vi) compression. (vi) rarefaction                                    Ans:  (1)The persistence of sound due to repeated reflection and its gradual fading away is called reverberation of sound.

 (2) Echo is a repetition of sound due to the reflection of original sound by a large and hard obstacle.

(3) Frequency is defined as the number of oscillations per second.  The time taken by the wave for one complete oscillation of the density or pressure of the medium is called the time period, T.

(4) The distance between two consecutive compressions or two consecutive rarefactions is called the wavelength.

(5)The distance traveled by a wave in one second is called wave velocity. It depends upon the nature of the medium through which it passes. 

(6) The part or region of a longitudinal wave in which the density of the particles of the medium is higher than the normal density is known as compression. 

(7) The part or region of a longitudinal wave in which the density of the particles of the medium is lesser than the normal density is called a rarefaction.