NCERT Exemplar Problems Class 9 Science – Sound

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
Answer:
(a) The sound which is produced due to a mixture of several frequencies is called a note and it is pleasant to listen too. An octave consists of eight different note, ranging from 256Hz to 512 Hz.

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
Answer:
(c) In the second case, sound will be louder but pitch will be lower, because pitch of sound directly depends on frequency.

Question 3:
In SONAR, we use
(a) ultrasonic waves (b) infrasonic waves
(c) radio waves (d) audible sound waves
Answer:
(a) SONAR is a device that uses ultrasonic waves to measure the distance, direction and speed of under water objects.

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
Answer:
(c) Sound travels in air if disturbance moves. As during the propagation of sound waves, particles only vibrates in its own position and disturbances created by the vibration of particle moves from one place to other.

Question 5:
When we change feeble sound to loud sound we increase its
(a) frequency (b) amplitude
(c) velocity (d) wavelength
Answer:
(b) The loudness or softness of a sound is determined basically by its amplitude. So for a loud sound it must have higher amplitude.

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

Answer:
(b) In this curve, half the wavelength is BD. Because it is half the length of one complete 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
Answer:
(b) Earthquakes produces low frequency (i.e., 5 Hz) infrasound before the main shock waves begin which possibly alert the animals and they get disturbed.

Question 8:
Infrasound can be heard by
(a) dog (b) bat
(c) rhinocerose (d) human beings
Answer:
(c) Infrasound are the waves whose frequency is less than 20 Hz. Rhinoceroses communicate using infrasound of frequency as low as 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
Answer:
(c) Sitarist is adjusting frequency of the sitar string with the frequency of other musical instruments. Because if it is not done so, the sound will be unpleasent to listen.

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.

Answer:

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.

Answer:
Graph (i) represents the male voice. Since the pitch and frequency of male voice is lower than the pitch of female voice and vibration of graph (ii) Represents higher frequency and higher pitch.

Question 12:
A girl is sitting in the middle of a park of dimension 12 m x 12 iti . 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.
Answer:
No, it is not possible for the girl to hear the echo of this sound, because the distance between girl and obstacle (building) is only 6m approx but echo is heard only if the minimum distance between the observer at the source of sound and the obstacle is 11.3 m.

Question 13:
Why do we hear the sound produced by the humming bees while the sound of vibrations of pendulum is not heard?
Answer:
The frequency of vibrations of pendulum is below 20 Hz (infrasound). We cannot hear infrasound but humming produce audible sound which can be heard by human beings.

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

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).
Answer:
Given, time ft) = 10 s and speed (v) = 340 m/s
We know that, distance = speed x time = 340 x 10 = 3400 m

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

Answer:
We know that, in laws of reflection, the angle of incidence (x) is always equal to the angle of reflection (x).
Since, AOB is a straight line.

∠AOB = 180° 50° + x + x + 50° = 180°
(∴sum of all angles lies on the same side of aline 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?
Answer:
The ceiling of concert halls, conference halls and cinema halls are curved so that sound after reflection reaches all corners of the hall uniformly.

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.
Answer:

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.
Answer:
The speed of sound is defined as the distance which a point on a wave, such as a compression or a rarefaction, travels per unit time.


Thus, frequency of sound is 400 Hz.

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.
Answer:
We have a curve showing density or pressure variations with respect to distance for a disturbance produced by sound.

denoted by X.
Time taken by the waves to complete one full cycle, so that its particles are in same phase is called time period. It is denoted by T.