SSC and CBSE Std 9 Chapter on "Sound"

1. Standard 9
Chapter 15
The Music of Sound
GURUDATTA WAGH

2. Std 9 Chapter 15 The Music of Sound
15.1 Nature of sound
Sound is a form of energy which produces
sensation of hearing in our ears
15.2 Production of sound
Sound is produced by striking, plucking, blowing,
scratching, rubbing, flapping, shaking
Vibrations in the object are responsible to
produce sound

3. 15.3 Propagation of sound
Wave It is produced at the point of disturbance.
Particles of the medium do not change their
positions. They vibrate about the mean position
when the disturbance reaches that point. Wave is a
disturbance that moves through a medium when
particles of medium set neighbouring particles into
motion, i.e. the wave motion is a mode of
transmission of energy through a medium.
Vibration To and
fro motion of an
object

4. Propagation Sound also gets propagated in the
form of waves. Sound is produced when an object
is disturbed and starts vibrating.
Sound gets
transmitted
through
matter or
substance
called
medium,
e.g. solid,
liquid, gas.

5. Sound cannot travel through vacuum

6. Particles do not travel from the vibrating object to
the listener. A particle of medium in contact with a
vibrating object is first displaced from its
equilibrium position. Then it exerts a force on the
adjacent particle.
The adjacent particle gets displaced from its
position at rest. The first particle comes back to its
original position after displacing the adjacent
particle.
The process goes on repeating from source to
listener and sound reaches the listener in the form
of waves.

8. 15.4 Sound waves are longitudinal waves
Transverse wave Wave produced in a string.
Particles oscillate up and down about their mean
positions. They do not oscillate along the line of
wave propagation but perpendicular to direction of
propagation.

9. Longitudinal waves In a slinky the propagation of
disturbance is in the form of longitudinal waves.
Individual particles of the medium move in a
direction parallel to direction of propagation of
disturbance. Particles do not move from one place
to other, they just oscillate back and forth about the
mean position, e.g. sound waves.
Slinky

10. 15.5 Characteristics of sound waves
When sound waves travel through a medium, there
is change in density and pressure of the medium.

11. Compressions Regions where particles are
crowded together; density and pressure is high
Rarefactions Regions where particles are spread
apart; density and pressure is low

12. Wavelength Distance between two consecutive
compressions or rarefactions.
SI unit metre, Greek letter lambda ƛ

13. Frequency Change of density from maximum to
minimum to maximum value, makes one oscillation
Frequency of wave Number of oscillations/ unit
time, SI unit hertz Hz, Greek letter nu ʋ
Oscillation The term
'vibration' is precisely
used to describe
mechanical oscillation
but used as a
synonym of
'oscillation' too, e.g. a
swinging pendulum
and alternating
current.

14. Heinrich Hertz
First to broadcast
and receive radio
waves; produced
electromagnetic
waves, sent them
through space and
measured their
velocity and
wavelength

15. Time period of wave/ Time period of sound wave
T
Time taken by two consecutive compressions and
rarefactions to cross a fixed point, i.e.
Time taken for one complete oscillation in the
density of medium
SI unit - second
Relation between frequency and period = 1/Tʋ

16. Pitch is the frequency of vibrations of a sound
source. For faster vibrations frequency is higher
and hence pitch is higher.
High pitch sound corresponds to more number of
compressions and rarefactions passing through a
fixed point per unit time.
Pitch We can distinguish a
male voice from a female
voice due to different
frequencies produced by
them. Pitch high = shrill
sound; pitch low = soft sound

17. Amplitude of the wave (A) Magnitude of maximum
displacement of particle in the medium on either
side of the mean value
‘A’ changes with density or pressure; depends upon
the force with which an object is made to vibrate
When sound waves move away from a source its
amplitude and loudness decreases
Amplitude
Determines
the loudness
or softness of
sound

18. Timbre Enables us to distinguish one sound from
another having the same pitch and loudness. Sound
which is more pleasant is said to be of rich quality.
Tone A sound of single frequency
TONE

19. Note Sound produced due to a mixture of different
frequencies. Pleasant to listen
Intensity Amount of sound energy passing per
second through unit area
Intensity

20. Loudness Measure of response of the ear to sounds

21. 15.6 Speed of sound
Distance which a point on a wave, such as
compression or rarefaction travels per unit time is
the speed of sound
Speed = Distance/ Time
If we consider the distance travelled by the sound
wave in one time period, then
Speed = wavelength/ Time period
V = / Tƛ
V = because 1/ T=ʋƛ ʋ
Speed = frequency x wavelength

22. Under the same physical conditions, for all the
frequencies in a given medium, the speed of sound
remains almost the same.
The physical conditions of the given medium are
related to the state of medium and temperature.
The speed of sound decreases from solid to
gaseous medium.
If we increase the temperature of any medium, the
speed of sound also increases.

24. 15.7 Range of hearing in humans
Ultrasound/ Ultrasonic Bats can produce and hear
sounds of frequency higher than 20000Hz which
humans cannot hear, e.g. dolphins, rats. Moths of
certain families can hear the ultrasound produced by
bats and they can protect themselves from capture
by bats
Infrasound/ Infrasonic Human ear cannot hear
sound of frequency less than 20 Hz, e.g. sound
produced before an earthquake by vibrations of the
earth surface

25. Audible range of sound human beings 20–20000 Hz
Children < 5 years, dogs can hear up to 25000 Hz
As people grow older, ears become less sensitive to
higher frequencies
A decibel dB(A) is a unit of measurement that
indicates how loud a sound is. Humans can hear
sounds between 0 and 140 decibels. 0 decibel does
not mean that there is no sound, merely that we
cannot hear it. 0 decibel is the so-called hearing
threshold for the human ear.

26. Decibel levels

27. Ultrasound
Able to travel
along well
defined paths
even in
presence of
obstacles
Uses Ship to ship communication. Welding plastic surfaces. Kill
bacteria in liquids like milk. Echocardiography. Scan and obtain
images of internal organs of the human body. Clean parts located in
places which are difficult to reach. Detecting cracks and flaws in
metal blocks

30. 15.8 Reflection of
sound
Similar to light waves,
sound waves get
reflected at the
surface of solids or
liquids.
Follow the same laws
of reflection.Light waves
Laws of reflection

31. Only requirement for reflection of sound is an
obstacle of large size having a rough or polished
surface.
Direction in which sound is incident and reflected
makes equal angles with the normal to the
reflecting surface at the point of incidence and
these are in the same plane.

32. 15.8.1 Echo Repetition of sound due to reflection of
original sound from any surface
The minimum distance of the reflecting surface
from the source of sound must be 17.2 m for
related temperature 22 °C. At 22 °C the speed of
sound is 344 m/ s in air.
Sensation of sound persists in our brain for 0.1 s.
Total distance covered by sound from the point of
generation to the reflecting surface and back
should be at lease 34.3 m, since
Distance = velocity x time = 344 x 0.1 = 34.4 m

33. For hearing different echoes, the minimum
distance between the source of sound and obstacle
must be half the distance, i.e. 17.2 m.
Distances are different for different temperatures
Due to successive or multiple reflection, echoes
may be heard a number of times. Gol Gumbaz,
Bijapur, Karnataka. Reflection of sound –
measuring the depth of oceans.

34. 15.9 Reverberation is the collection of reflected
sounds from the surfaces in an enclosure like an
auditorium.
Persistence of sound The time interval between the
successive arrival of the same sound signal goes on
decreasing. The intensity of sound gradually
decreases. These reflected sounds interfere and
cause a jumbling and prolongation of sound in the
room. Due to this pitch of the music will not be
heard distinctly.

35. Some theatres, auditoriums are considered acoustically
poor because of reverberation.

36. Reducing reverberation Roofs and walls of
auditoriums are generally covered with sound-
absorbent materials like compressed fibre-boards,
rough plaster or draperies (curtains). The material
for the seat is selected such that it is a good
absorber of sound waves.

37. 15.10 SONAR
Sound Navigation And Ranging (Acronym an
abbreviation formed from the initial letters of
other words and pronounced as a word, e.g.
NASA )
Uses To measure the distance, direction and speed
of under water objects using ultrasonic waves;
determine the depth of the sea; locate underwater
hills, valleys, submarines, sunken ships, etc.

38. SONAR = transmitter + detector
•Installed in a ship or a boat
•Transmitter produces and transmits ultrasonic waves
•Waves travel through water

39. • Waves get reflected after striking the object on the sea
bed
• Reflected waves are sensed by the detector
• Detector converts the ultrasonic waves into electrical
signals which are interpreted
• Time interval between transmission and reception of
the ultrasound is recorded
• By knowing the speed of sound in water and considering
the above time interval, the distance of the object that
reflected the sound wave can be calculated

40. 15.11 The human ear
Converts pressure variations with audible
frequencies into electrical signals that travel to the
brain via the auditory nerve

41. 3 major parts 1) external, 2) middle, 3) inner
External (Pinna)
Collects the sound which is carried by auditory canal to the
ear drum.
The ear drum is a thin membrane which is set into
vibrations due to the sound reaching it.
When a compression of medium reaches the ear drum, the
pressure outside the membrane increases and forces the
ear drum inwards.
The ear drum moves outwards when a rarefaction reaches
it and the ear drum vibrates.
The vibrations are amplified and carried to the inner part of
the ear.
Auditory nerve connects the part of inner ear to the brain.

42. Inner ear
Spiral chamber like a snail shell called cochlea is
present.
Cochlea is responsible for recognizing the qualities
of sound, i.e. loudness and pitch.
Vibration received from the ear drum are converted
into nerve impulses or electrical signals, which are
sent to the brain through the auditory nerve.
The brain interprets them as sound.
The amplitude of vibration of membrane gives the
brain a measure of loudness of sound being heard.

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