S2/3 BGE Physics Course 1

Unit 1 - Waves and Radiation

2. Sound

Summary Notes

Topic 2 – Sound

Sound is a wave which transfers energy from one place

to another. It is most easily detected via our ears and

the hearing mechanism inside our head. This is a very

important sense that we often take for granted.

Sound is produced by the vibrations of particles through

solids, liquids and gases.

However sound cannot travel through a vacuum as

there are no particles to pass the vibrations along.

The vibrations are a result of some kind of movement.

e.g.

a hammer striking a bell.

a ruler held at the end of a desk being

twanged.

a drum stick being hit off the skin of a drum.

a guitar string being plucked.

Some opera singers have been known to break wine

glasses by singing. The sound energy that they produce

can make the glass vibrate so much that it shatters.

Speed of Sound in Air Experiment

The hammer hit a metal plate next to microphone 1

The sound reaches microphone 1 and the electronic

timer starts

When the sound reaches microphone 2 the

electronic timer stops

The electronic timer measures the time that it takes

the sound to travel from microphone 1 to

microphone 2

The distance between microphone 1 and

microphone 2 is measured by a ruler

Speed of sound = distance from mic1 to mic 2

time from mic1 to mic 2

To improve the accuracy of the experiment to find the

speed of sound in air, it is recommended that the time is

recorded five times.

Then the calculated experimental value can be

compared with the accepted value of 340ms-1

from the

data sheet.

Longitudinal Waves

Sound waves are longitudinal. This means that the

particles in a sound wave are vibrating parallel or anti-

parallel to the direction that the wave is travelling in.

This can be shown with a slinky.

The two diagrams above show the waveform in two

parts, compression and rarefaction.

Compression involves the waves being very close

together and rarefaction the waves are far apart. This

pattern continually repeats itself through a longitudinal

wave.

i.e. Compression, Rarefaction, Compression,

Rarefaction, Compression, Rarefaction etc etc

Frequency

Frequency is defined as the number of waves that pass

a point per second and it is measured in Hertz (Hz).

Frequency is the equivalent Physics term for the musical

term pitch.

When boys go through the journey of puberty their

voices often change from being high pitched to low

pitched, which they take into adulthood.

Frequency Calculations

f = frequency in Hertz (Hz).

N = Number of waves produced.

t = time in seconds.

Ex 1

If 30 waves pass a point in 6 seconds, then calculate the

frequency of the waves.

N = 30 waves f = N = 30 = 5Hz

t = 6 seconds t 6

f = ?

Ex 2

A wave generator produces 960 waves in 4 minutes.

Calculate the frequency of the generator.

N = 960 waves f = N = 960 = 4Hz

t = 4 minutes = 240 seconds t 240

f = ?

Ex 3

Calculate how many waves a 50Hz sources produces in 90

seconds.

N = ? N = f x t = 50 x 90 = 4500 waves

f = 50Hz

t = 90 seconds

Ex 4

Calculate how long it would take a 30Hz source to produce 240

waves.

N = 240 waves t = N = 240 = 8 seconds

f = 30Hz f 30

t = ?

Amplitude of a sound wave

The amplitude is a measure of the energy contained in a

wave. This can be displayed on an oscilloscope.

High amplitude waves are tall and low amplitude waves

are small.

Low amplitude and

Low frequency

Low amplitude and

High frequency

Frequency

High amplitude and Low

frequency

High amplitude and High

frequency

The range of human hearing

Humans can hear frequencies of sound in the range

between 20Hz and 20,000Hz.

In the experiment, the teacher gradually increases the

frequency of the sound emitted from the loudspeaker by

adjusting the signal generator. When the pupils start to

hear the sound, this is the lower end of the audible

frequency range.

This continues until the pupils can no longer hear the

sound. The last frequency of sound that the pupils can

hear is the higher end of the audible frequency range.

The older an adult is, the lower the frequency they can

hear at the higher end of the audible frequency range.

(Approximately 16,000Hz)

Mammals all have different audible frequency ranges.

A dog whistle for example has a frequency range

between 25,000Hz and 50,000Hz. Dogs can hear this

whistle while humans cannot.

Signal Generator

Loudspeaker

Ultrasound

Ultrasound involves frequencies of sound greater than

20,000Hz.

There are several applications in medicine and in

industry for ultrasound.

Ultrasound in Medicine

A very narrow beam is fired in short pulses through the

body of the mother. Echoes are produced as the waves

pass from one type of tissue to another, e.g. from

muscle to bone. When the echoes are received the spot

on the screen glows brightly. As the ultrasound probe

moves along the skin a series of bright spots are

produced and this eventually builds up into an image of

the unborn baby.

X-rays are not used to examine unborn babies, as they

can damage or kill living cells and tissue.

Ultrasound can also be applied to shattering kidney

stones so that they can pass out of the body, and they

can be used to check on foot and knee injuries.

Ultrasound in Industry

Ultrasound (sonar) is used by fishing boats to locate and

find the depth of the fish before putting out their nets.

Ultrasound is also used in non-destructive testing to

detect flaws in materials such as propellers on an

aircraft.

Sound Levels

Sound level is measured in decibels (dB) with a sound

level meter or indicator or a decibel meter.

These instruments are very important in the workplace

to protect the hearing of the workers, as they could be

subjected to loud sounds over a period of time. Health

and Safety regulations are very strict, as everyone has

the right to work in a safe environment.

A list of typical sound levels is shown below.

Sound Sources Sound Level (dB)

Threshold of Hearing 0

Leaves rustling in the distance 10

Quiet bedroom at night 30

Average Home 50

Conversation at 1m 60

Vacuum cleaner at 1m 70

Kerbside of a busy road 80

Disco at 1m from loudspeaker 100

Threshold of Pain 140

Ear Protectors

Ear protectors or ear defenders are used to protect your

hearing, not your ears.

Airport landing staff and pneumatic drill operators wear

ear protectors in their daily jobs. The ear protectors

absorb sound energy before it reaches the hearing

mechanism of the worker involved.

An activity that can

damage your hearing

An activity that can damage

your ears

Noise Cancellation Headphones

A microphone on the outside of the headphones picks

up background sounds, which turns them upside down

or inverts. This inverted signal is fed into the

loudspeaker in the headphones, which will cancel out

the background sound that the person would hear if they

were wearing ordinary headphones.

Sound waves produced from

the background sounds.

Inverted sound waves

produced by the headphones

A

B

Destructive Interference between

wave A and wave B to produce

wave C. This results in noise

cancellation.

C