Factors affecting diffraction  
o
The wavelength (the shorter wavelength, the greater the of angle of diffraction  
)
ퟏ
o
The size of the opening gap (the size of gap  
∝
)
풅풊풇풇풓풂풄풕풊풐풏 풐풇 풘풂풗풆  
Applications of diffraction of Waves  
▪
It is used in determining the crystal structure of materials  
▪
It is used in measuring the coefficient of thermal expansion, crystalline size and  
thick of thickness of thin films  
▪
It is used in determining the types and phases present in a specimen where the  
spacing of obstacles (atoms) is between 1 and 3nm  
Sound Waves  
•
•
Sound wave is a longitudinal wave that produced by vibrating object.  
For example, turning fork  
Propagation of Sound Wave  
•
Sound travels by vibration of particles to transfer energy to the next particles  
until the sound reaches another point  
Qn:  
Why solid materials transfer sound faster than liquid/gas  
Answer  
:
The molecules/particles of solid materials are packed together  
Sources of Sound Wave  
•
Almost everything ranging from people  
,
animals  
,
plants and machines  
Factors affecting speed of sound in air  
❖
Temperature  
V
∝
T, e.g, Vda = V a + 0.6 Tc (Vda = velocity of sound in dry air  
)
❖
Direction of Wind  
o
In the same direction : V  
∝
W
o
1
In opposite direction  
:
푉 ∝  
푊
❖
❖
Humidity  
ퟏ
푽 ∝  
푽 ∝  
푯
Density of Air  
ퟏ
흆
Audibility Range  
•
NB:  
o
Audibility range is the range of frequency detected by human ear  
The human ear is most sensitive to sound with a frequency around 3000Hz  
Sound below 20Hz is called infrasonic Sound  
o
o
o
Sound above 20000Hz is called ultrasonic sound  
Dogs, cat, bat and dolphins detect ultrasonic sound  
o
Bats can fly in the dark without hitting anything because they are guided by  
ultrasonic waves produced by them  
o
Human ear can distinguish two simultaneous sound if their frequencies differ  
by at least 7Hz  
The Human Ear  
•
Human ear is the human organ responsible for conversion of sound energy to  
mechanical energy to nerve impulse that transfers to the brain for  
interpretation. It can distinguish frequency, amplitude and direction.  
Parts of human Ear  
It consists of three basic parts include  
➢
➢
➢
outer ear  
Middle ear  
Inner ear  
Mechanism of Hearing  
•
Earflap collect sound waves, which pass through the ear canal to hit drum  
which results vibrations of interconnected bones where vibrate cochlea fluid  
through (oval window) results vibrations of hair cells which transform  
mechanical energy to electrical impulses, which transmitted to the brain where  
they are decoded and interpreted as sound  
N.B  
Hearing  
:
Is the process by which the ear transforms sound vibrations in the  
external environment into nerve impulses that are conveyed to the brain  
where they are interpreted as sounds  
Echo  
•
•
Echo is a reflected sound which is heard distinctly from the original sound  
Since sound waves go and bounce back its distance becomes 2d.  
ퟐ풅  
•
Hence speed (v) of sound associated with echo is calculated by, v =  
풕
NB:  
Always echo reaches the ear more than 0.1s  
ퟐ풅  
From  
:
v =  
풕
Then  
:
2d = v x t = 0.1 x 340,  
d = 17m  
•
Therefore: An obstacle must be at least 17 m away for a distinct echo to be heard  
The accoustics of buildings  
•
•
Is the science of controlling the quality and amount of sound inside a building  
It is used to allow for pleasant sound in a concert hall and to reduce echoes  
and noise within an office building  
•
•
When a hall has many people, most of the sound (including echoes) is absorbed by  
clothes and skins of the audience, thus echoes do not occur (noise reduced)  
It also concerns suppressing sound coming from outside the building such as apartments  
Reverberation  
•
Are the multiple reflections of sound produced when it travels in an enclosed room  
Reverberation time  
•
Is the time required for the sound to decay in a closed space or cavity  
Examples  
1. An echo sounder produces a pulse and an echo is received from the sea – bed  
after 0.4 seconds. If the speed of sound in water is 1500 m/s, calculate the  
depth of the sea –bed  
Soln:  
Given: v = 1500 m/s, total time = 0.4 s  
2푠  
From: 푣 =  
→ 풔 = 풗풕  
=
ퟏퟓퟎퟎ 풙 ퟎ.ퟒ = ퟑퟎퟎ 풎  
푡
ퟐ
ퟐ
2. In determining the depth of an ocean an echo sounder produces ultrasonic  
sound. Give reasons why ultra sound is preferred for this purpose  
ANS: (a) Ultra sound penetrates deepest  
(b) Ultra sound is reflected easily by tiny grains of sand  
Class Activity  
1. A gun was fired and the echo from a cliff was heard 8s later. How far was the gun  
from the cliff? (ANS: d= 1400m)  
2. (a) Define an echo  
(b) Name any two factors that affect the speed of sound in air  
(c)Explain briefly why sound produced in hall with many people is heard more  
clearly than when the hall has few people?  
(d) A person standing 99m from the foot of mountains claps his hands and hears  
an echo 0.6 second later. Calculate the speed of the sound in the air  
(AN: V = 330m/s)  
ANS (c) When a hall has many people, most of the sound (including echoes) is absorbed  
by clothes and skins of the audience, thus echoes do not occur  
3.  
I
t is possible to hear sound round obstacles but not possible to see light. Give reason  
4. Two sets of transverse waves arrive at the same time. Under what conditions do they:  
(i)  
(ii)  
Cancel out (ANS: trough from one arrive at the same time as crest from the other)  
Produce a larger wave ANS: A crest from one arrive at the same time as a crest  
from the other (constructive interference))  
(
5. A fathometer produces sound in a ship and receives two echoes where there is a  
raised sea bed one after 2.5 seconds and the other after 3.0 seconds. Find the  
height of the raised sea bed (Take V in water = 1460 m/s) (ANS: h1-h2 = 365 m  
)
6.  
A
girl standing 200 m from the foot of a high wall claps her hands and the echo  
reaches her 1.16 seconds later. Calculate the velocity of sound in air using this  
observation (ANS: V = 344.8 m/s)  
7. From the figure below, give reason why, an observer moving along the line AB  
hears loud sound at same point and soft sound at other points. How is the sound  
along the line OC? (when he moves along line OC hears loud sound only)  
8. A person stands 100 m from the foot of a tall building claps his hands and hears  
an echo 0.588 seconds later .Calculate the velocity of sound in air (ANS: 340 m/s)  
9. The velocity of sound in air is 330 m/s, Find the wavelength in water of sound  
wave of frequency 660 Hz if the velocity of sound in water is 1. 32 km/s  
10. A source of sound produces waves of wavelength 0.8 m in air. The same source  
of sound produces waves of wavelength 4.0 m in air. If the velocity of sound in air  
is 332 ms-1 , find the velocity of sound in water (ANS: v = 332 m/s)  
11. How far does sound travel in air when a turning fork of frequency 250 Hz  
completes 50 vibrations? The speed of sound in air is 340 m/s. (ANS: 흀 = ퟏ. ퟑퟔ 풎)  
12. A bat emits ultrasonic sound of frequency 100 kHz in air. If this sound meets a  
water surface, what is the wavelength of (i) the reflected sound  
(ii) the transmitted  
sound? If the speed of sound in air = 340m m/s and in water = 1486 m/s (ANS:  
(i) 흀 = ퟑ. ퟒ x 10-3 m (ii) 흀 = 1.486 x 10-3 m)  
13. If the frequency of radio waves is 600 kHz, find the wavelength of the waves. The  
speed of radio waves = 3 x 108 m/s. (ANS: 흀 = ퟓퟎퟎ 풎)  
14. A stone is dropped into a well 19.6 m deep and the impact of sound is heard after  
2.056 seconds. Find the velocity of sound in air (ANS: v = 350 m/s)  
15. A personal with deep voice singing a note of frequency 200Hz is producing sound  
waves whose velocity is 330m/s. find the sound's wave length. (ANS: λ = 1.65m)  
16. A hospital uses an ultrasonic scanner to locate tumours in a tissue. What is the  
wavelength of sound in a tissue in which the speed of sound is 1.7 kms-1? The  
operating frequency of the scanner is 4.2 MHz. (ANS: 흀 = ퟎ. ퟒퟎퟓ 풎)  
17. Calculate the velocity of the wave whose wavelength is 1. 7 x10-2m and frequency  
2x1014Hz  
(
ANS: 3.4 1x1012m/s)  
18. Find the wavelength of sound wave whose frequency is 550Hz and speed is  
330m/s  
(ANS: The wavelength is 0. 6m) NB: The higher the frequency of  
a wave, the shorter the wavelength and the lower is the frequency on the wave,  
the longer is the wavelength.  
19. The radio waves have a velocity of about 3.0 x108m/s and the wavelength of  
1500m. Calculate the frequency of these waves?  
(
ANS: f = 2. 0 x 105 Hz)  
20. Consider the figure below illustrates part of a wave traveling across the water at a  
particular place, Calculate;  
(a) The frequency of the wave  
(ANS: f = 10 Hz)  
(b) The wavelength of the wave (ANS: = 0.1 m)  
(c) The amplitude of the wave  
(d) The velocity  
(ANS: A = 0.2 m)  
(ANS: v = 2m/s)  
21. The wavelength of signals from a radio transmitter is 1500m and the frequency is  
the 200 KHz. To what speed does the radio wave travel?.What is the wavelength  
of a transmitter operating at 1000 KHz? (ANS: V = 3 x 108 m/s,  
흀
= 3. 0 x 102m  
)
22. A certain wave has a periodic time of 0.04 second and travels at 30 x 107 m/s Find  
its wavelength.  
(ANS:  
= 1.2 x 107 m)  
23. A signal is sent to the seabed from the bottom of a ship. The signal comes back in  
one – fifth of a second .How deep is the water?  
Uses of Echo  
❖
❖
❖
❖
❖
❖
Used to find depth of ocean (lakes)  
Looking at babies in the womb ( pre – natal scanning  
To detect the submarines  
To detect large groups of fish  
To detect the wrecked ships  
)
by Ultrasound  
To detect the dangerous rocks  
Musical sounds and noise  
•
All sounds which produce sensation of hearing may be roughly dived into two  
classes namely (i) musical sounds (ii) noise  
(a)  
Musical sound  
•
Is a pleasant, continuous and uniform sound produced by regular and periodic  
vibrations  
OR  
•
•
Is a sound produced by continuous and regular vibrations  
Example sound produced by turning fork, flute and piano  
(b)  
Noise  
•
Is an unpleasant, discontinuous and non–uniform sound produced by irregular  
succession of disturbances  
OR  
•
•
Is the random and non-structured sound musical scale that not appealing to the  
human ear  
Example is sound produced by a falling brick, the clapping of two wooden blocks  
NB:  
•
Tone (Musical note) is a sound of regular frequency  
Properties of Musical Sounds  
•
These are loudness, pitch and timbre (quality)  
Loudness  
•
•
Loudness is the intensity of the sound as perceived by the human ear.  
The large the amplitude, the louder the sound