NCERT Exemplar Solutions Class 9 Science Sound

Multiple Choice Questions (MCQs)...........................

Question 1:  Note is a sound

(a) of mixture of several frequencies                     

(b) of mixture of two frequencies only

(c) of a single frequency                                            

(d) always unpleasant to listen

Solution 1:   (a) of mixture of several frequencies.

A note is a sound that is made by the combination of several frequencies and is nice to listen to. Eight separate notes, ranging from 256Hz to 512Hz, make up an octave.

Question 2:  A key of a mechanical piano struck gently and then struck again but much harder this time. In the second case
(a) sound will be louder but pitch will not be different
(b) sound will be louder and pitch will also be higher
(c) sound will be louder but pitch will be lower
(d) both loudness and pitch will remain unaffected

Solution 2:   (c) sound will be louder but pitch will be lower.

The sound will be louder in the second case, but the pitch will be lower, since pitch is directly proportional to frequency.

Question 3:  In SONAR, we use

(a) ultrasonic waves                                   

(b) infrasonic waves

(c) radio waves                                            

(d) audible sound waves

Solution 3:   (a) ultrasonic waves. 

SONAR is a system that measures distance, direction, and speed of underwater objects using ultrasonic waves.

Question 4:  Sound travels in air, if
(a) particles of medium travel from one place to another
(b) there is no moisture in the atmosphere
(c) disturbance moves
(d) both particles as well as disturbance travel from one place to another

Solution 4:   (c) disturbance moves. 

If a noise passes, sound travels through the air. Particles only vibrate in their own direction during the propagation of sound waves, and disruptions caused by particle vibrations travel from one location to another.

Question 5:  When we change feeble sound, to loud sound we increase its

(a) frequency                                               

(b) amplitude

(c) velocity                                                   

(d) wavelength

Solution 5:   (b) amplitude. 

The intensity of a sound is what determines its loudness or softness. As a result, a loud sound needs larger amplitude.

Question 6:  In the given curve, half the wavelength is

(a) AB
(b) BD
(c) DE
(d) AE

Solution 6:   (b) BD. 

Half of the wavelength in this curve is BD. It's because it's half the duration of a full cycle.

Question 7:  Earthquake produces which kind of sound before the main shock wave begins

(a) ultrasound                                           

(b) infrasound

(c) audible sound                                      

(d) None of these

Solution 7:   (b) infrasound. 

Before the main shock waves begin, earthquakes emit low frequency (i.e., 5 Hz) infrasound, which can alarm animals and cause them to become disturbed.

Question 8: Infrasound can be heard by

(a) dog                                                         

(b) bat

(c) rhinoceros                                            

(d) human beings

Solution 8:  (c) rhinoceros. 

Waves with a frequency less than 20 Hz are referred to as infrasound. Rhinoceroses interact by emitting infrasound with a frequency of 5 Hz.

Question 9:   Before playing the orchestra in a musical concert, a sitarist tries to adjust the tension and pluck the string suitably. By doing so, he is adjusting
(a) intensity of sound only
(b) amplitude of sound only
(c) frequency of the sitar string with the frequency of other musical instruments
(d) loudness of sounds

Solution 9:   (c) frequency of the sitar string with the frequency of other musical instruments. 

The frequency of the sitar string is adjusted in relation to the frequency of other musical instruments by the sitarist. For otherwise, the sound would be unpleasant to listen to.

Short Answer Type Questions.......................

Question 10:  The given graph shows the displacement versus time relation for a disturbance travelling with velocity of 1500 ms’¹. Calculate the wavelength of the disturbance.

Solution 10:

Question 11:  Which of the two graphs (i) and (ii) representing the human voice is likely to be the male voice? Give reason for your answer.

Solution 11:  The male voice is represented by graph (i) Since the male voice has a lower pitch and volume than the female voice, the vibration of graph (ii) represents a higher frequency and pitch.

Question 12:  A girl is sitting in the middle of a park of dimension 12 m x 12 m. On the left side of it there is a building adjoining the park and on right side of the park, there is a road adjoining the park. A sound is produced on the road by a cracker. Is it possible for the girl to hear the echo of this sound? Explain your answer.

Solution 12:   No, the girl will not hear the echo of this sound because the distance between her and the obstacle (building) is only about 6 meters, but echo is only heard if the distance between the observer at the source of sound and the obstacle is at least 11.3 meters.

Question 13:  Why do we hear the sound produced by the humming bees while the sound of vibrations of pendulum is not heard?

Solution 13:   The pendulum's vibration frequency is less than 20 Hz (infrasound). We can't detect infrasound, but humming produces audible vibration that humans can hear.

Question 14:  If any explosion takes place at the bottom of a lake, what type of shock waves in water will take place?

Solution 14:  If there is an explosion at the bottom of a lake. Shock waves of the infrasound kind will occur in the water.

Question 15:  Sound produced by a thunderstorm is heard 10 s after the lightning is seen. Calculate the approximate distance of the thunder cloud (Given speed of sound = 340 ms^-1). 

Solution 15:  

Given, time = 10s and speed (v) = 340 m/s

We know that, distance = speed x time = 340 x 10 = 3400 m

                              = 3400/1000 km = 3.4 km 

Question 16:  For hearing the loudest ticking sound heard by the ear, find the angle x in the given figure.

Solution 16:  We know that the angle of incidence (x) is always equal to the angle of reflection in reflection rules (x)

AOB is a straight line, so
∠AOB = 180° 50° + x + x + 50° = 180°
(∴sum of all angles lies on the same side of a line is 180°)
2x + 100° = 180°
2x = 180° – 100°
2x = 80°
x=80°/2
x = 40°
Hence, the value of x is 40°.

Question 17:  Why is the ceiling and wall behind the stage of good conference halls or concert halls made curved?

Solution 17:   Concert halls, meeting halls, and cinema halls have curved ceilings so that sound hits all corners of the hall equally after reflection.

Long Answer Type Questions..........................

Question 18:  Represent graphically by two separate diagrams in each case.
(i)   Two sound waves having the same amplitude but different frequencies.
(ii)  Two sound waves having the same frequency but different amplitudes.
(iii) Two sound waves having different amplitudes and also different wavelengths.

Solution 18:

Question 19:  Establish the relationship between speed of sound, its wavelength and frequency. If velocity of sound in air is 340 ms^-1. Calculate -
(i) wavelength when frequency is 256 Hz.
(ii) frequency when wavelength is 0.85 m.

The speed of sound is defined as the distance travelled per unit time by a point on a wave, such as a compression or rarefaction.

Question 20:  Draw a curve showing density or pressure variations with respect to distance for a disturbance produced by sound. Mark the position of compression and rarefaction on this curve. Also define wavelengths and time period using this curve.

Solution 20:   For a sound-induced disturbance, we have a curve depicting density or pressure changes as a function of distance.

The time it takes for waves to complete one full cycle, with all of their particles in the same phase, is referred to as the time period. T is the symbol for it.