AP 9th Class Physics 11th Lesson Questions and Answers Sound

AP State Board new syllabus AP Board Solutions Class 9 Physics 11th Lesson Sound Questions and Answers.

AP 9th Class Physical Science 11th Lesson Questions and Answers Sound

9th Class Physics 11th Lesson Sound Questions and Answers (Exercise)

Question 1.
What is sound and how is it produced?
Answer:
Sound is a form of energy which produces the sensation of hearing in our ears. Sound is produced by forcing an object to vibrate. In other words, sound is produced by vibrating object.

Question 2.
Describe with the help of a diagram, how compressions and rarefactions are produced in air near a source of sound.
Answer:
1. A vibrating membrane moved back and forth to produce sound.
2. When the membrane pushed forward it pushes the particles of air in front of it.
3. Due to this layers of air get compressed increasing the density of air.
4. This compressed air pushes the layer of air next to it.
5. Creating a compression pulse.

6. After pushing the compression pulse forward the air particles comes to normal.
7. When the membrane pushed back.
8. It drags the layer of air particles towards it.
9. As result density of air reduces.
10. Creating a rarefaction pulse.
11. As the membrane moves to & fro rapidly compression and rarefaction pulse are produced continuously one after the other.
12. When these pulses (energy) reaches the ears the eardrum vibrates.
13. And thus we hear the Sound.

Question 3.
Why is sound wave called a longitudinal wave ?
Answer:
The particles of a medium move along the direction of sound wave that why it is a longitudinal wave.

Question 4.
Which characteristic of the sound helps you to identify your friend by his voice while sitting with others in a dark room ?
Answer:
The characteristic of the sound that helps us to identify our friends voice is – “timbre”. Timbre – quality of voice or music.

Question 5.
Flash and thunder are produced simultaneously. But thunder is heard a few sec­onds after the flash is seen, why ?
Answer:
Lightning travels at 3 x 10⁸ ms^-1 where as the thunder travels in air at 344 ms¹, so thunder is heard slower than lightning.

Question 6.
A person has a hearing range from 20 Hz to 20 kHz. What are the typical wave­lengths of sound waves in air corresponding to these two frequencies ? Take the speed of sound in air as 344 ms^-1
Answer:
i) Sound Frequency o = 20 Hz; Speed of sound v = 344 ms^-1

ii) Frequency υ = 20000 Hz ; Speed of sound v = 344 ms^-1

Question 7.
Two children are at opposite ends of an aluminium rod. One strikes the end of the rod with a stone. Find the ratio of times taken by the sound wave in air and in aluminium to reach the second child.
Answer:
Speed of sound in air (s) = 346 ms^-1
Distance transversed = s x t₁ ⇒ d = 346 t₁ …………… (1)

Speed of sound in aluminium = 6420 ms^-1
Distance transversed = s x t₂ = 6420 t₂     …………… (2)

Distance transversed by soundwave is same.

Question 8.
The frequency of a source of sound is 100 Hz. How many times does it vibrate in a minute ?Answer:
Source of sound frequency = 100 Hz
No. of vibrations in is = 100
No. of vibrations in 1 min = 100 x 60 = 6000
It will vibrate 6000 times in 1 min.

Question 9.
Does sound follow the same laws of reflection as light does ? Explain.
Answer:
Yes, sound and light follows the same laws of reflection.

The direction in which sound is incident and is reflected makes equal angles.
∠i = ∠r

Question 10.
When a sound is reflected from a distant object, an echo is produced. Let the distance between the reflecting surface and the source of sound production remains the same. Do you hear echo sound on a hotter day?
Answer:
If the temperature increases speed of sound increases, this reduces the time duration of sound and its reflection, so no echo is heard.

Question 11.
Give two practical applications of reflection of sound waves.
Answer:
Applications of reflection of Sound Waves :

• Reflection of sound waves are used in sonar.
• It is also use to heartbeat in stethoscope.
• Megaphones, loudspeakers, horns, musical instruments all are use this property.

Question 12.
A stone is dropped from the top of a tower 500 m high into a pond of water at the base of the tower. When is the splash heard at the top?Given, g =10 ms^-2 and speed of sound=340 ms^-1
Answer:
Decent of stone :
Initial velocity u =0 ms^-1; Displacement s=500 m

Acceleration due to gravity g=10 ms^-2

Question 13.
A sound wave travels at a speed of 339 ms^-1. If its wavelength is 1.5 cm. What is the frequency of the wave? Will it be audible?
Answer:
Sound wave : Speed ms^-1
Wavelengty λ = 1.5 cm = \(\frac{1.5}{100}\) =0.015 m
[∵ 1m =100 cm; 1cm= \(\frac{1}{100}\) m

Question 14.
What is reverberation ? How can it be reduced ?
Answer:
The persistence of sound due to repeated reflection and its gradual fading away is called reverberation of sound.
1. Excessive reverberation is highly undesirable to reduce it.
2. Use sound absorbing material like

• compressed fibre board,
• rough plaster (or) draperies,
• cushions that absorb sound.

Question 15.
What is loudness of sound ? What factors does it depend on ?
Answer:
Loudness of sound depends on the amplitude (energy) of the sound wave.
It depends on the following factors:

• Loudness α (Amplitude)²
• Loudness depends on distance,
• It also depends on area of vibrating body.

Question 16.
How is ultrasound used for cleaning ?
Answer:

• The object are put in a tank called ultrasonic vibrator.
• When it is filled with water.
•  It creates vibrations and removes dust particles which are impossible to remove normally. Ex : Greasy machine parts, gold jewellery are cleaned by using this.

Question 17.
Explain how defects in a metal block can be detected using ultrasound.
Answer:
Ultrasonic waves are allowed to pass through the metal block.

• Detectors are used to detect transmitted waves.
• If there is a defect the ultrasound waves get reflected back.
• Indicating the presence of flaw (or) defect.

9th Class PS 11th Lesson Questions and Answers (InText)

Question 1.
How does the sound produced by a vibrating object in a medium reach your ear?
Answer:

• A vibrating membrane moved back and forth to produce sound.
• When the membrane pushed forward it pushes the particles of air in front of it.
• Due to this layers of air get compressed increasing the density of air.
• This compressed air pushes the layer of air next to it.
• Creating a compression pulse.
• After pushing the compression pulse forward the air particles comes to normal.
• When the membrane pushed back.
• It drags the layer of air particles towards it.
• As result density of air reduces.
• Creating a rarefaction pulse.
• As the membrane moves to & fro rapidly compression and rarefaction pulse are produced continuously one after the other.
• When these pulses (energy) reaches the ears the eardrum vibrates.
• And thus we hear the Sound.

Question 2.
Explain how sound is produced by your school bell.
Answer:

• The school bell produces sound through a process known as vibration.
• When the bell is struck, it starts to vibrate, causing the air molecules around it to vibrate as well.
• This creates a pressure wave that travels through the air as a sound wave, eventually reaching our ears and allowing us to hear the sound of the bell.

Question 3.
Why are sound waves called mechanical waves?
Answer:
Sound waves require a material medium to travel that’s why they are called mechanical waves.

Question 4.
Suppose you and your friend are on the moon. Will you be able to hear any sound produced by your friend ?
Answer:
No, we will not be able to hear any sound on moon, as moon doesn’t have any atmosphere.

Question 5.
Which wave property determines (a) loudness, (b) pitch ?
Answer:
The amplitude of the wave determines the loudness.
Loudness α (Amplitude)²

Question 6.
Guess which sound has a higher pitch guitar or car horn ?
Answer:
Pitch describes how the brain interprets the frequency of Sound. So, the guitar has a higher pitch.

Question 7.
What are wave length, frequency, time period and amplitude of a sound wave ?
Answer:
i) Wavelength: The distance between two consecutive crest (or) troughs is called the Wavelength.

It is denoted with Greek letter λ (lambda).

ii) Frequency: The number of oscillations of the density of the medium at a place per unit time is called the frequency of the sound wave. It is denoted with Greek letter v (nu) \(=\frac{1}{\mathrm{~T}}\)

iii) Time Period : The time taken to complete one oscillation of the density of the
medium is called the time period of the sound wave.

iv) Amplitude: The magnitude of the maximum disturbance in the medium on either side of the mean value is called the Amplitude. It is represented by letter (A).

(or) It is the \(\boldsymbol{\leftarrow}\) distance of crest or trough.

Question 8.
How are the wavelength and frequency of a sound wave related to its speed ?
Answer:
Speed of Sound = Frequency × Wavelength
v = λν

Question 9.
Calculate the wavelength of a sound wave whose frequency is 220 Hz and speed is 440 m/s in a given medium.
Answer:
Sound Wave : Frequency ν=220 Hz; Speed v=440 ms^-1
v = λν
Wave Length =

∴ Wavelength of a sound wave λ = 2m

Question 10.
A person is listening to a tone of 500 Hz sitting at a distance of 450 m from the source of the sound. What is the time interval between successive compressions from the source?
Answer:
Tone: Frequencey v=500 Hz
Time interval between successive compression of the source T= \(\frac{1}{v}=\frac{1}{500}\)= 0.002sec

Question 11.
Distinguish between loudness and intensity of sound.
Answer:

Loudness	Intensity
1) Loudness is determined by Amplitude.	1) Intensity of sound depends on energy.
2) It is energy per unit area of wave.	2) It is also energy per unit area of wave.
3) It is dependent on the interpretation of the ear.	3) It is independent on the interpretation of the ear.

Question 12.
In which of the three media, air, water or iron, does sound travel the fastest at a particular temperature ?
Answer:
Sound travels fastest in solids, so sound travels fast in iron than water and air.

Question 13.
An echo is heard in 3 s. What is the distance of the reflecting surface from the source, given that the speed of sound is 342 ms^-1?
Answer:
Speed of sound = 342 ms^-1;
Time taken to hear echo = 3 sec
Distance of the reflecting source

Question 14.
Why are the ceilings of concert hails curved ?
Answer:
The ceilings of the concert halls are curved in order make the sound to reach all cor­ners after reflection, so that it is audible to every person.

Question 15.
What is the audible range of the average human ear ?
Answer:
20 Hz to 20000 Hz (or) 20 Hz to 20 kHz is the audible range.

Question 16.
What is the range of frequencies associated with (a) Infrasound ? (b) Ultrasound?
Answer:
a) Infrasound : Sound wave frequencies between 1 Hz to 20 Hz.
b) Ultrasound : Sound wave of the frequency above 20 kHz.

Examples

Question 1.
A sound wave has a frequency of 2 kHz and wavelength 35 cm. How long will it take to travel 1.5 km?
Answer:
Given, Frequency, o = 2 kHz = 2000 Hz ; Wavelength, X= 35 cm = 0.35 m
We know that speed, v of the wave = wavelength x frequency
The time taken by the wave to travel a distance, d of 1.5

Thus sound will take 2.1 s to travel a distance of 1.5 km.

Question 2.
A person clapped his hands near a cliff and heard the echo after 2s. What is the dis­tance of the cliff from the person if the speed of the sound, v is taken as 346 ms^-1 ?
Answer:
Given, Speed of sound, v = 346 ms^-1
Time taken for hearing the echo, t = 2s
Distance travelled by the sound = v x t = 346 ms^-1 x 2 s = 692 m
In 2 s sound has to travel twice the distance between the cliff and the person.
Hence, the distance between the cliff and the person = 692 m/2 = 346 m.

9th Class Physical Science Chapter 11 Questions and Answers (Lab Activities)

(Page No. 82) Activity -11.1

Question 1.
1. Take a tuning fork and set it vibrating by striking its prong on a rubber pad. Bring it near your ear.
2. Do you hear any sound ?
3. Touch one of the prongs of the vibrating tuning fork with your finger and share your experience with your friends.
4. Now, suspend a table tennis ball or a small plastic ball by a thread from a support [Take a big needle and a thread, put a knot at one end of the thread and then with the help of the needle pass the thread through the ball].Touch the ball gently with the prong of a vibrating tuning fork.
5. Observe what happens and discuss with your friends.

Vibrating tuning fork just touching the suspended table tennis ball
Answer:
When the ball is touched by a vibrating prong the tennis ball also starts vibrating.

(Page No. 82) Activity 11.2

Question 2.
1) Fill water in a beaker or a glass up to the brim. Gently touch the water surface with one of the prongs of the vibrating tuning fork, as shown in figure (a).
2) Next dip the prongs of the vibrating tuning fork in water, as shown in figure (b).
3) Observe what happens in both the cases?
4) Discuss with your friends why this happens.

Answer:
In case -1 vibrating prong brought near the surface of water the water particles on the surface starts vibrating (riples gets produced).
In case – II vibrating prong produces waves in water when it immersed in glass of water.

(Page No. 84) Activity 1.3

Question 3.
Make a list of different types of musical instruments and discuss with your friends which part of the instrument vibrates to produce sound.
Answer:
String Instruments → Veena, Violin, Guitar, Sitar, Ek-tara.
Membrane Instruments → Mridangam, Tabla, Dholak, Dappu. ’
Aerophones → Flute, Harmonica.

(Page No. 86) Activity 11.4

Question 4.
1) Take a slinky. Ask your friend to hold one end. You hold the other end. Now, stretch the slinky as shown in Fig (a). Then give it a sharp push towards your friend.
2) What do you notice ? If you move your hand pushing and pulling the slinky alternatively, what will you observe?
3) If you mark a dot on the slinky, you will observe that the dot on the slinky will move back and forth parallel to the direction of the propagation of the disturbance.

Answer:
1) The region where turns of the coil come closer called Compression (C).
2) The region where turns move further called rarefactions (R).
So, waves produced by a slinky are longitudinal waves.

(Page No. 94) Activity 11.5

Question 5.
1) Take two identical pipes, as shown in below figure. You can make the pipes using chart paper. The length of the pipes should be sufficiently long as shown.
2) Arrange them on a table near a wall.
3) Keep a clock near the open end of one of the pipes and try to hear the sound of the clock through the other pipe.
4) Adjust the position of the pipes so that you can best hear the sound of the clock.
5) Now, measure the angles of incidence and reflection and see the relationship between the angles.
6) Lift the pipe on the right vertically to a small height and observe what happens.
Answer:

Sound follows laws of reflection ∠i=∠r
When we lift the pipe on the right vertically to a small height the sound can’t be head at other end. This is due to ∠i ≠∠r