Science 2nd prep 2nd term unit 2 lesson 2

You may notice: the domes of worship places are built in concave shapes! Your ear pinna is concave and the ears of some animals as

gazelle are big! Have you ever asked yourself how doctors know the sex of

embryo before birth?!

** all these are applications on the property of the reflection of sound waves.

When sound waves strike a reflecting surface (wall) , they return back in the same medium, in the form of reflected waves. ** this phenomenon is known as (( Sound reflection )).

Sound reflection: It is the return of sound waves in the same direction due to hitting a reflecting surface.

- the reflection of sound waves is governed by two laws known as " two laws of sound reflections"

Activity >>> to verify the laws of sound reflection.

Ms. Marim 1

Reflection of sound waves

Laws of reflection of sound

Science 2nd prep 2nd term unit 2 lesson 2

Materials and tools: - wooden plate - foam (cork) plate - a table - two cardboard tubes, one of them contains a

cellular phone.

Steps: Do the steps as shown in figure:

Observation: The angle of incidence = the angle of reflection.

** move the tube B upwards, and change the inclination of the wooden plate away from the plane of the two tubes. Do you hear sound?!

Observation: The sound can not be heard.

Conclusion: The reflection of sound waves is governed by two law known as: the two laws of sound reflection"

First lawThe angle of incidence = the angle of reflection.

Second law: The incident sound ray , the reflected sound ray and the perpendicular line from the point of incidence on the reflecting surface, all lie on the same plane, perpendicular to the reflecting surface.

G.R:When sound ray is incident perpendicular to a reflecting surface. It reflects on itself? Because the angle of incidence = the angle of reflection = zero.

Example:

Ms. Marim 2

Science 2nd prep 2nd term unit 2 lesson 2 Calculate the angle of reflection if the angle between the incident sound ray and the reflected sound ray is 30ºAnswer: Angle of reflection = 30 / 2 = 15 º

G.R : A piece of moquette is put under the washing machine? To absorb the noise produced due to vibration instead of its reflection from the glistening surfaces of walls.

Enrichment information: Ceilings and walls of broadcast studios are covered with rubber of felt to absorb sound and prevent its reflection.

Sound ray : It is the direction of the line of propagation of sound wave .

Angle of incidence of sound ray :It is the angle between the incident sound ray and the line perpendicular to the reflecting surface at the point of incidence.

Angle of reflection of sound ray.It is the angle between the reflected sound ray and the line perpendicular to the reflecting surface at the point of incidence.

If you make a sound in a large empty room, you will hear a repetition of your sound !

If you visit a large empty place containing hard flat surfaces like Karnak temple in Luxor and produce a sound, you will

hear a repetition of your sound .

This phenomenon is known as ((Echo)) ,

Ms. Marim 3

Concepts related to sound reflection

Echo

Science 2nd prep 2nd term unit 2 lesson 2 Echo :

It is a repetition of sound produced due to its reflection.

The conditions necessary for hearing the echo : 1 – presence of wide and big reflecting

surface such as walls, mountains or water bodies .

2 – the distance between the sound source and the reflecting surface must not less than 17 meters, where the time period between hearing the original sound and its echo should not less than 1/10 of a second.

Therefore : the human ear can not distinguish between two successive sounds if the period between them is less than 0.1 second .

Notice that : If the velocity of sound through air is 340 m/sec, so the distance traveled by the sound and its echo in 0.1 sec, = velocity v time = 340 v 0.1 = 34 metres.

So, 17 metres from the sound source to the reflecting surface and 17 meters from the reflecting surface to the ear (( echo )).

G.R:: Echo cannot be heard if the distance between the sound source and the reflecting surface is less than 17 meters ?

Because the time between hearing the main sound and its echo will be less than 1/10 of a second ,so human ear can not distinguish between the two successive

sounds .

Enrichment information : When the distance between the source of sound and the reflecting surface is multiplies of 17 meters , the echo is heard in the form of the last two or three phrases of the whole produced sound.

1 – Determination of sound speed (velocity) through air .

Ms. Marim 4

Real life application of echo (sound reflection)

Science 2nd prep 2nd term unit 2 lesson 2

2 – Concentration of sound .

3 – Determination of sea depth and fish shoals .

4 – Detecting industrials defects .

5 – Medical diagnosis .

6 – Locating the position of surfaces and barriers .

We follow the following steps : 1 – stand away from a big screen and produce a

sound by using two pieces of wood .2 – record the time (seconds) between the moments

of producing sound and hearing the echo .3 – measure the distance between the sound source

and the reflecting surface ( meter) .4 – repeat step 2 several times and find the average

value of time (sec).

So, you can calculate the velocity of sound through air using the relation : Velocity of sound (V) = twice the distance between source of sound and the reflecting surface (2d) / the average time of echo in seconds (t).

V = 2d (meters) / t (sec) = ……… m/sec.

Problems : A gril stood at the shore of an island and produced a sound, its echo was heard after 3 seconds, Calculate the distance between the shore and the cliff given that the velocity of sound through air in

this region is 330 m/s .

Solution :

As V = 2d / t Then d = t V / 2 = 3 ×330 / 2 = 495 meters.

Ms. Marim 5

1 - Determination of sound speed through air.

2 – Concentration of sound.

Science 2nd prep 2nd term unit 2 lesson 2

-when sound waves fall on a concave surface , the reflected sound waves will be collected at a point known as (( the focal centre (focus) of this surface. ))

-thus, it is possible to determine the positions and dimensions of concave surfaces then direct the reflected sound waves on definite direction to make

sound mare clear and to increase its intensity .

** This phenomenon is called "concentration of sound "

Concentration of sound : It is the conversion (collection) of sound at a point due to its reflection on a concave surface.

This phenomenon is applied by sound engineers when building worship places and big conference halls .

These places are supplied by concave walls and ceilings to concentrate the reflected sound and make the sound more clear and more intense .

Note : In the past , Moslem scientists used the property of sound reflection to concentrate and enforce the voice of Imam to all parts of the mosque.

G.R: The voice of Imam can be heard clearly in all parts of large mosques without using microphones ?

Because the surface of large mosques are concave which concentrate the reflected sound waves and make the sound clearer and more intense .

G.R : Fennec fox has large ability of hearing ?Because it has large and concave ear pinna that concentrate the reflected sound and make it more clear and more intense.

**Fennec fox: its hearing is sensitive enough to hear prey moving underground.

Ultrasonic waves of frequencies higher than 20000 Hz are used for :

-estimating the depth of seas

Ms. Marim 6

3 – Determination of sea depth and fish shoals

Science 2nd prep 2nd term unit 2 lesson 2

-exploring the fish gathering (shoals) in seas .

How to determine the sea depth !!? a. two devices are fixed at the bottom of the ship .

1 – sonar device which receives the reflected waves .2 – a hydrophone set which receives the reflected waves from seabed (fish

shoals).

b. measure the time period (t) between sending and receiving the reflected waves (echo time) in seconds .

c. if the velocity of ultrasonic waves through water is (V) m/sec., so the depth of sea or loction of fish shoals can be determined by the relation :

depth (d) = velocity of ultrasonic waves (V) × echo time (t) / 2

Then d (m) = t V / 2

Problem : A sailor produced a sound in the sea, he heard its echo after 0.6 second, if the velocity of sound through water is 1435 m/sec, calculate the depth of sea.

Solution : d = tV / 2 = 0.6 × 1435 / 2 = 430.5 m .

-the ultrasonic waves are used to discover the presence of cracks in metallic pipes as those used in

conveying natural gas or parts of nuclear reactors .

-these cracks, holes , air bubbles and welding defects can be detected by passing ultrasonic waves through

the materials to be examined .

-defects in manufactures can be detected through the variation in the intensity (amplitude of vibration) of the

reflected waves .

Ms. Marim 7

4 – Detecting industrial defects:

Science 2nd prep 2nd term unit 2 lesson 2

G.R :: The ultrasonic waves can be used in detecting the industrial defects ?Because the waves reflected from the areas which contain air bubbles (defects in welding) have a different intensity than those reflected from well welded areas.

-Ultrasonic waves are used nowadays in many medical investigations and laboratory tests such as detecting the sex of embryo and his health

state .

-This process depends on the ability of different parts of body to reflect ultrasonic waves.

* Bats can use the phenomenon of sound reflection (echo) in their daily life to :

1 – Locate the position of their preys .2 – Fly in the dark without colliding with anything .

This is achieved by transmitting ultrasonic waves of frequencies (50: 100 KHz) and receiving them after reflection.

Similarly : dolphins can avoid obstacles on their way under water .

G.R: Bats can fly in the dark without colliding with anything ?

Because they produce ultrasonic waves that reflect on the surfaces and barriers then receive them back and locate their positions.

Ms. Marim 8

5 – Medical diagnosis

6 – Locating the position of surfaces and barriers

Science 2nd prep 2nd term unit 2 lesson 2

Questions on lesson : Complete the following statements :

1 …………………………… - is the return of sound waves in the same direction due to hitting a ……………………… surface.

2 – the first law of sound reflection states that ..…………………………

3 – when sound waves strike a hard flat surface, they.……………………………

4 – the second law of sound reflection states that..……………………………………………

5 – the angle between the incident sound ray and the perpendicular to the reflecting surface from the point of incidence is called.…………………………………

6 – for an echo to be heard, the distance between the ear and the sound source must be = ………………………….. meters.

7 ……………………………… - and …………………………….. are from the real life applications fro echo.

8 ………………………… - of sound is the conversion of sound at a point due to its ………………………… on a concave surface .

9 – the depth of sea and the fish shoals in the sea can be determined by ……………………. Waves whose frequency more than …………………………. Hz .

10 – the depth of sea can be determined from the relation : depth (d) = ………… /2 ×.…………

11 – sonar set is used to produce …………………….. waves, while hydrophone set is used to …………………………… the ………………………… waves.

12 – welding defects in the manufactures can be detected through the variation in the …………………………….. of the ……………………… waves .

13 – some animals like ………………….. use echo to fly in dark without colliding with any surface because they produce …………………… waves whose frequencies more

than .……………………

14 – ultrasonic waves are used in many …………………. Application such as determination the ………………………… of the embryo and its..……………………………

Choose the correct answer : 1 – if the angle of reflection of a sound ray is 30 , so the angle of incidence equals

.…………a. 30 b. 90 c. 80 d. 180

2 – the angle of incidence of sound rays …………………… the angle of its reflection .a. more than b. less than c. equal to d. no correct answer .

Ms. Marim 9

Science 2nd prep 2nd term unit 2 lesson 2

3 – the echo can be heard if the time between producing the sound and hearing the echo is not less than ………………… second .

a. 0.01 b. 0.1 c. 1 d. 1.5

4 – when the sound travels 300 m in one second , so the minimum distance for the echo to happen is .…………………………

a. 5m b. 17 m c. 30 m d. no correct answer .

5 – the repetition of hearing the sound as a result of its reflection is called ..………………a. sound reflection b. sound refraction c.echo d. no correct answer.

6 – all of the following are applications of sound reflection except …………………………

a. estimating the depth of seabeds .b. examining the defects in welding of metals .

c. concentrating the sound d. determining the sound pitch .

7 ..………………………… - surface are used to concentrate sound waves .a. flat b. convex c. concave d. smooth .

8 – fennic fox has large ability of hearing because ..………………………………… a. it has a concave ear pinna only. b. its ear pinna is convex only .c. it has a large ear pinna only. d. a and c together.

9 -a person was shouting in front of a mountain at a distance 990 meters, the time between producing the sound and hearing its echo is ………………… knowing that the velocity of sound is 330 m/sec.a. 6 sec b. 3 sec c. 1/3 sec d. 2/3 sec .

10 the depth of seas is estimated by using …………………………. Waves .a. ultrasonic b. infrasonic c. sonic d. no correct answer .

11 – the variation in the intensity of the reflected waves at definite part of an alloy indicates on ……………………………………………

a. good welding b. presence of air bubbles .c.the alloy is pure. D. no correct answer .

12 – the bats depend on the phenomenon of sound ………………………… in hunting their preys.a. refraction b. reflection c. rarefaction d. no correct answer.

Give reasons : 1 – a piece of rubber is put under the washing machine ?

………………………………………………………………………………………………………………………….…………

Ms. Marim 10

Science 2nd prep 2nd term unit 2 lesson 2 2 – echo is used to determine the depth of the seas ?

……………………………………………………………………………………………………………………………………

3 – in mosques, the ceilings are built I the shape of domes ?………………………………………………………………………………………………………………………….………

4 – Bats can fly in the dark without colliding with anything ?……………………………………………………………………………………………………………………………………

5 – the echo doesn't occur in the classroom ?…………………………………………………………………………………………………………………………………

Ms. Marim 11