Sound

Learning Objectives

a) describe the production of sound by vibrating sources

b) describe the longitudinal nature of sound waves in terms of the processes of compression and rarefaction

What is sound?

Sound is a form of energy that is transferred from one point to another as a longitudinal wave.

How is sound produced?

Let’s take a look at how the following produce sounds!

Guitar: https://www.youtube.com/watch?v=wGKYYcDNIsM Drum: https://www.youtube.com/watch?v=osFBNLA7woY Tuning Fork: https://www.youtube.com/watch?v=VCERs0v1OoI Voice

Sound is produced by vibrating sources placed in a medium.

How does a sound wave travel?

Tuning Fork

If a vibrating tuning fork is dipped into a glass of water, the water in the glass will splash out.

This is because the water is displaced by the vibrating tuning fork.

The vibrating tuning fork displaces the molecules of the medium around it, which in turn displaces their neighbouring molecules.

How does a sound wave travel?

From Chp 11: General Wave Properties

Sound waves are longitudinal waves direction of vibration of the medium’s molecules is parallel to the direction the wave travels.

How does a sound wave travel?

All longitudinal waves travel as a series of compressions (C) and rarefactions (R).

Compressions are regions where the medium’s density is higher than the surrounding density.

Rarefactions are regions where the medium’s density is lower than the surrounding density.

How does a sound wave travel?

Layers of air are in undisturbed positions.

When the prongs push outwards, a region of compression is produced.

1

2

When the prongs move inwards, a region of rarefaction is produced.

3

The prongs continue to vibrate (move inward and outward) and a series of compressions and rarefactions is set up.

54 ,

How does a sound wave travel?

Summary

Sound waves require a medium to travel.

When sound waves travel, a series of compressions and rarefactions is set up in the medium.

Learning Objectives

c) explain that a medium is required in order to transmit sound waves and the speed of sound differs in air, liquids and solids

Can sound travel in vacuum?

A bell jar experiment

The electric bell is switched on and the air in the jar is slowly pumped out to create a vacuum.

What do you think will be observed as the air is pumped

out?https://

www.youtube.com/watch?v=hIOqX4uJtYY

Can sound travel in vacuum?

A bell jar experiment

• When the electric bell is on and the vacuum pump is off, you will hear the bell.

• When all the air is pumped out, the bell jar becomes a vacuum.

• The sound of the bell becomes softer and softer until you can no longer hear it.

Medium of Transmission

Sound cannot travel in vacuum.

Sound needs a medium to travel from one point to another.

Travels at different speed in different media.

Medium Air Water Iron

Approximate speed of sound / m s-1

330 1500 5000

Speed of sound in gas < Speed of sound

in liquid < Speed of sound in solid

Medium of Transmission

Why does sound travel fastest in solid?

Recall: Kinetic Model of Matter

The closer the particles are packed, the faster the sound wave is able to be transferred.

Medium Air Water Iron

Particle arrangement

Very far apart Closely packed but further than

solid

Very closely packed

Approximate speed of sound / m s-1

330 1500 5000

How does a sound wave travel?

Summary

Sound waves require a medium to travel.

Sound travels fastest in solid, slowest in gas.

Learning Objectives

d) describe how the reflection of sound may produce an echo, and how this may be used for measuring distances

EchoesWhy do you hear echoes when you shout in an

empty room?

Forming Echoes

An echo is formed when a sound is reflected off a hard, flat surface such as a large wall.

An echo is the repetition of a sound due to the reflection of sound.

Using Echoes

Echoes can be used to find distances.

In the sea The ship sends out a pulse of

sound (a signal).

By noting the time taken for the sound to be reflected back to the ship, and with knowledge of the speed of sound in the sea, we can calculate the depth of the sea.

2 × depth of sea

time takenSpeed of sound in sea=

Using Echoes

Against a wall

A man stands at a distance, d, from a large brick wall Every time he claps, he hears an echo after time t s It is the time taken for the sound to travel from the man to the wall

and back, over a distance of 2d If the speed of sound travelling through air, v (340m/s), is known,

the distance of the reflecting surface from the source can be calculated using

2d

tSpeed of sound =

Using Echoes

Practice Qn 1

A boy standing 68 m from a high wall claps his hands and hears an echo 0.4 s later. Find the speed of the sound in air.

Ans:Total distance travelled by the sound= 2 × 68

Speed of sound = = = 340 m/s

Using Echoes

Practice Qn 2

A person standing at a distance, d, from a tall cliff claps his hands and hears an echo 0.6 s later. Assuming the speed of the sound in air is 330 m/s, calculate the distance between the person and the cliff.

Ans:Total distance travelled by the sound = 2d

Speed of sound = 330 = d = = 99 m

Learning Objectives

e) relate loudness of a sound wave to its amplitude and pitch to its frequency

Sound applet

Observe how the sound changes as the amplitude and frequency is being changed.

As the amplitude increases, the sound becomes louder.

As the frequency increases, the pitch of the sound becomes higher.

Loudness

Loudness is related to the amplitude of a sound wave.

The larger the amplitude, the louder the sound.

Recall that the amplitude of a wave is the maximum displacement of a point from its rest position.

Which sound is louder – Sound A or Sound B?

Sound A Sound B

Pitch

Pitch is related to the frequency of a sound wave.

The higher the frequency, the higher the pitch. Humans can only hear from 20 Hz to 20 000 Hz.

Recall that the frequency of a wave is the number of complete waves produced per second.

Which sound has the lower pitch – Sound A or Sound B?

Sound A Sound B

Questions?