9th standard syllabus notes and 30 questions on the content attached

1. Sound-2

2. Reflection of Sound
▪ Echo: When we clap near a mountain the same
sound is heard again a little later.This is known as a
echo.
▪ Minimum distance of the obstacle from the
source=1/2(17.2)
SOUND 2

3. Reverberation
▪ A sound created in a big hall will persist by
repeated reflection from the walls until it is
reduced to a value where it is no longer audible.
▪ The repeated reflection that results in this
persistence of sound is called reverberation.
SOUND 3

4. Range of Hearing
SOUND 4
▪ RANGE OF HEARING: 20 Hz to 20000 Hz
Above 20,000Hz
Ultrasound
Below 20Hz
Infrasound

5. Applications of ultrasound
▪ Detect cracks and flaws in metal blocks.
▪ Echocardiography
▪ Ultrasound scanner
▪ Objects to be cleaned are placed in a cleaning
solution and ultrasonic waves causes dust, grease
and dirt to get detached and drop out.
SOUND 5

6. SONAR
 Sound Navigation And
Ranging
 Uses ultrasonic waves to
measure the distance,
direction and speed of
underwater objects
SOUND 6

7. Working of SONAR
Transmitter
• Produces and transmits
ultrasonic waves.
• Waves travel through
water.
• After striking the
object on the seabed,
get reflected back.
Detector
• Reflected waves reach
the detector.
• They are sensed by the
detector.
• Converts the ultrasonic
waves into electrical
signals which are
appropriately
interpreted
SOUND 7

8. Calculating distance
▪ The time interval between transmission and
reception of ultrasound signal be t
▪ The speed of sound through seawater be v.
▪ The total distance, 2d travelled by the ultrasound
is then, 2d = v × t.
SOUND 8

9. Human Ear
▪ Involves three major parts:
▪ Outer ear
▪ Middle ear
▪ Inner ear
SOUND 9

10. SOUND 10
Eardrum moves outward when a rarefaction reaches it
Forces the eardrum inward
Pressure on the outside of the membrane increases
Compression of the medium reaches the eardrum.
Reaches the thin membrane called the ear drum
The collected sound passes through the auditory canal.

11. SOUND 11
Brain interprets them as sound
Electrical signals are sent to the brain via the auditory nerve
Pressure variations turned into electrical signals by cochlea
Middle ear transmits the amplified waves to the inner ear
Vibrations are amplified several times by three bones
The eardrum vibrates.

12. SOUND 12
1. Explain reflection of sound and mention its laws
A. Sound gets reflected at the surface of a solid or liquid and follows
the same laws of reflection.The directions in which the sound is
incident and is reflected make equal angles with the normal to the
reflecting surface, and the three are in the same plane.
2. A person clapped his hands near a cliff and heard the echo after 10
s.What is the distance of the cliff from the person if the speed of the
sound, v is taken as 346 m s–1?
A: Speed of sound, v = 346 m s–1,
Time taken for hearing the echo, t = 5 s
Distance travelled by the sound
= v × t = 346 m s–1 × 10 s = 3460 m
In 5 s sound has to travel twice the distance between the cliff and the
person. Hence, the distance between the cliff and the person
= 3460 m /2 = 1730 m.

13. SOUND 13
3. Define Echo
A. If we shout or clap near a suitable reflecting object such as a tall
building or a mountain, we will hear the same sound again a little
later.This sound which we hear is called an echo.
4.What is reverberation
A. A sound created in a big hall will persist by repeated reflection
from the walls until it is reduced to a value where it is no longer
audible.The repeated reflection that results in this persistence of
sound is called reverberation.
5.Why do you hear the thunder roll ?
A. The rolling of thunder is due to the successive reflections of the
sound from a number of reflecting surfaces, such as the clouds
and the land.

14. SOUND 14
6.What can be done to reduce reverberation
A. To reduce reverberation, the roof and walls of the auditorium are
generally covered with sound-absorbent materials like
compressed fibreboard, rough plaster or draperies.The seat
materials are also selected on the basis of their sound absorbing
properties.
7. How does a doctor hear the heartbeat of a patient using
stethoscope? Show a diagram explaining this
A. In stethoscopes the sound of the patient’s heartbeat reaches the
doctor’s ears by multiple reflection of sound.

15. SOUND 15
8.What is the range of hearing in human beings?
A. The audible range of sound for human beings extends from
about 20 Hz to 20000 Hz.
9.What are infra and ultra sound?
A. Sounds of frequencies below 20 Hz are called infrasonic sound
or infrasound. Frequencies higher than 20 kHz are called
ultrasonic sound or ultrasound.
10. Give one instance on what will happen if we hear infrasound.
A. If we could hear infrasound we would hear the vibrations of a
pendulum just as we hear the vibrations of the wings of a bee.

16. SOUND 16
11. Explain the working of a Hearing aid
A. A hearing aid is an electronic, battery operated device.The hearing
aid receives sound through a microphone.The microphone
converts the sound waves to electrical signals.These electrical
signals are amplified by an amplifier.The amplified electrical
signals are given to a speaker of the hearing aid.The speaker
converts the amplified electrical signal to sound and sends to the
ear for clear hearing.
12. How does animals recognize any natural disaster like Earthquake
A. Earthquakes produce low-frequency infrasound before the main
shock waves begin which possibly alert the animals. Whales and
elephants produce sound in the infrasound range. So, animals get
disturbed before earthquakes.

17. SOUND 17
13.What is Echocardiography?What is its principal?
A. Ultrasonic waves are made to reflect from various parts of the
heart and form the image of the heart.This technique is called
‘echocardiography’. It is based on ultrasound
14.What is a ultrasound scanner? Mention its uses
A. Ultrasound scanner is an instrument which uses ultrasonic waves
for getting images of internal organs of the human body. A doctor
may image the patient’s organs such as the liver, gall bladder,
uterus, kidney, etc. It helps the doctor to detect abnormalities,
such as stones in the gall bladder and kidney or tumours in
different organs.

18. SOUND 18
15. Calculate the minimum distance of the obstacle from the source
of sound.
A. To hear a distinct echo the time interval between the original
sound and the reflected one must be at least 0.1s. If we take the
speed of sound to be 344 m/s at a given temperature, say at 22 ºC
in air, the sound must go to the obstacle and reach back the ear of
the listener on reflection after 0.1s. Hence, the total distance
covered by the sound from the point of generation to the
reflecting surface and back should be at least (344 m/s) × 0.1 s =
34.4 m.
16.What is a SONAR?
A. The acronym SONAR stands for Sound Navigation And Ranging.
Sonar is a device that uses ultrasonic waves to measure the
distance, direction and speed of underwater objects.

19. SOUND 19
17. Mention the two important parts of a SONAR
A. Sonar consists of a transmitter and a detector and is installed in a
boat or a ship.
18. Explain the working of a SONAR
A. The transmitter produces and transmits ultrasonic waves.These
waves travel through water and after striking the object on the
seabed, get reflected back and are sensed by the detector.The
detector converts the ultrasonic waves into electrical signals
which are appropriately interpreted.
19.What are the uses of SONAR?
A. Uses of Echo-ranging:The sonar technique is used to determine
the depth of the sea and to locate underwater hills, valleys,
submarine, icebergs, sunken ship etc.

20. SOUND 20
20. A ship sends out ultrasound that returns from the seabed and is
detected after 5 s. If the speed of ultrasound through seawater is
1531 m/s, what is the distance of the seabed from the ship?
Speed of ultrasound in sea water, v = 1531m/s
Distance travelled by the ultrasound = 2 × depth of the sea = 2d
where d is the depth of the sea.
2d = speed of sound × time = 1531 m/s × 5 s = 7655 m
d = 7655 m/2 = 3827.5 m.
Thus, the distance of the seabed from
the ship is 3827.5 m or 3.83 km.
21. How does bats detect their prey?
A. Bats search out prey and fly in dark night by emitting and
detecting reflections of ultrasonic waves.The high-pitched ultrasonic
squeaks of the bat are reflected from the obstacles or prey and
returned to bat’s ear .The nature of reflections tells the bat where
the obstacle or prey is and what it is like.

21. SOUND 21
22.What are the three bones in the middle ear?
A. The hammer, anvil and stirrup.
23. How ultrasound is used in cleaning?
A. Ultrasound is generally used to clean parts located in hard-to-
reach places, for example, spiral tube, odd shaped parts,
electronic components etc. Objects to be cleaned are placed in a
cleaning solution and ultrasonic waves are sent into the
solution. Due to the high frequency, the particles of dust, grease
and dirt get detached and drop out.The objects thus get
thoroughly cleaned.
24. How is ultrasound used in treating kidney stone?
A. Ultrasound may be employed to break small ‘stones’ formed in
the kidneys into fine grains.These grains later get flushed out
with urine.

22. SOUND 22
25. Give two practical application of reflection of sound waves
A. Megaphones which send waves in a particular direction is based on
reflection of sound.
B. In stethoscope the heartbeat of patient is heard using the
phenomenon of multiple reflection.
26.The distance between a reflecting source and the source of sound
remains same. But will you hear a echo on a hot day?
A. As the temperature increases, speed of sound increases.This in turn
will increase the minimum distance required for hearing a echo. No
sound is heard because the distance between reflecting and
producing source does not increase.
27. Explain why echo cannot be heard in a small room?
A. For hearing an echo a minimum distance of 17m should be present
between the source of sound and the reflecting source. Since this
condition is not satisfied in a small room echo is not heard.

23. SOUND 23
28.Why we do not hear sound from oscillations of pendulum.
A. The vibrations produced by a oscillating pendulum does not fall
in the human range of hearing (20Hz-20,000Hz). Hence sound is
not heard.
29. Say whether these fall in range of hearing, ultrasound or
infrasound: 50Hz, 50,000Hz, 10Hz
A. 50Hz- Range of hearing
B. 50,000Hz- Ultrasound
C. 10Hz- Infrasound
30.Why are ceilings of concert halls curved?
A. Generally the ceilings of concert halls, conference halls and
cinema halls are curved so that sound after reflection reaches all
corners of the hall. Curved soundboard may be placed behind
the stage so that the sound, after reflecting from the sound
board, spreads evenly across the width of the hall.