Noadswood Science, 2011

To understand how amplitude, wavelength and frequency affect sound waves

Friday, April 21, 2023

Sound Waves

Sound waves are caused by vibrating objects – they are longitudinal waves travelling at various speeds in differing substances (generally quicker as the density of the material increases)

Sound cannot travel in a vacuum (as they are not particles)

Sound waves also pass into frequencies that we cannot hear (ultrasound) used for industrial cleaning, pre-natal scanning, SONAR etc…

Waves

Copy the following CRO traces of the sounds…

Increase the amplitude to sound 1, keeping everything else the same

Increase the pitch or frequency (decrease the wavelength) to sound 2, keeping everything else the same

Sound 1

Sound 2

Waves

Sound 1 – amplitude increased

Waves

Sound 2 – pitch (frequency) increased (wavelength shortened)

Wave Speed

Waves are vibrations that transfer energy from place to place without matter being transferred

Waves travel at different speeds (light waves travel about a million times faster than sound waves)

The speed of a wave - its wave speed - is related to its frequency and wavelength…

Wave speed (v) = frequency (f) × wavelength (λ)

Wave speed (m/s) = frequency (Hz) × wavelength (m)

Frequency(Hz)

Wave speed(m/s)

Wavelength

(m)

A wave with a frequency of 100Hz and a wavelength of 2 m travels at what speed?

Wave speed of 100 Hz × 2 m = 200 m/s

Frequency(Hz)

Wave speed(m/s)

Wavelength

(m)

A wave with a frequency of 100 Hz and a wavelength of 2 m travels at what speed?

Wave speed of 100 Hz × 2 m

Wave speed = 200 m/s

f

v

λ

1. Match the correct units: -

Wave speed (______) = Frequency (______) x Wavelength (______)

2. What is the wave speed when the frequency is 25 Hz and the wavelength is 4 m?

3. What is the correct equation for frequency?

f

v

λ

1. Match the correct units: -

Wave speed (______) = Frequency (______) x Wavelength (______)

Wave speed (m/s) = Frequency (Hz) x Wavelength (m)

2. What is the wave speed when the frequency is 25 Hz and the wavelength is 4 m?

100 m/s

3. What is the correct equation for frequency?

frequency = wave speed ÷ wavelength

f

v

λ

4. What is the frequency when the wave speed is 330 m/s and the wavelength is 3.3 m?

5. What is the equation for wavelength?

6. What is the wavelength of a radio station signal when the wave speed is 300’000’000 m/s and the frequency is 100’000’000 Hz

f

v

λ

4. What is the frequency when the wave speed is 330 m/s and the wavelength is 3.3 m?

100 Hz

5. What is the equation for wavelength?

wavelength = wave speed ÷ frequency

6. What is the wavelength of a radio station signal when the wave speed is 300’000’000 m/s and the frequency is 100’000’000 Hz

3 m

f

v

λ

Wave Types

All waves carry energy without transferring matter

Waves can be transverse or longitudinal

Transverse waves have sideways vibrations, whilst longitudinal waves have vibrations along the same line…

Vibrations from side to side

Wave travelling this way

Vibrations in the same direction

Wave travelling this way

Transverse waves

Longitudinal waves

Rarefactions & Compressions In longitudinal waves where the vibration occurs on the same

line (such as sound waves and shock waves) rarefactions and compressions occur

Rarefactions is the name given to the region where the wave is pulled apart

Compression is the name given to the region where the wave is pushed together

The wavelength can be measured as the distance between the centre of two compressions…

CompressionCompression

RarefactionRarefaction Wavelength

Pitch Variations

What happens to the noise of a vehicle when it drives past you (e.g. an emergency vehicle with the siren on)?

Doppler Effect

When an ambulance or police car goes past, its siren is high-pitched as it comes towards you, then becomes low-pitched as it goes away – this effect, where there is a change in frequency and wavelength, is called the Doppler effect

The Doppler effect happens with any wave source that moves relative to an observer – there is a change in the observed wavelength and frequency

What are echoes?

Sound can reflect from the surface of an object - this is called an echo

Which surfaces reflect sound better?

Hard surfaces reflect sound better than soft surfaces – which is one reason why classrooms without carpets or curtains can be noisy places

Why do you hear echoes in caves?

Hard surfaces reflect sound better than soft surfaces

Sound is reflected well by large solid objects, so in caves the sound is reflected well from the solid cave wall

Why do echoes usually sound quieter?

As the sound wave travels some energy is lost, so you usually hear your echo with less amplitude (volume)

You can work out how far away something is using the reflection of waves

If you stood at the entrance of a cave and shouted, how could you work out how far away the back of the cave was?

Distance = Speed x Time

If the echo took 20 seconds to be heard after you shouted, how far back is the cave?

Distance = 330 m/s x 20 seconds

Distance = 6600 m

However, the distance is from your mouth, to the back of the cave, and back again! So we need to halve this result – distance to back of cave is therefore 3300m

Sonar stands for sound navigation and ranging – a technique used to measure how far away something is – how does it do this?

The boat sends out a sound wave which hits the sea bed

The sound wave reflects off the surface, back to the receiver – the time it takes can be used to calculate the distance

Sound travels at 1480 m/s through water - if the sound took 0.3 seconds to get back to the boat, what is the distance?

Sound travels at 1480m/s through water

If the sound took 0.3 seconds to get back to the boat, what is the distance?

Distance = Speed x Time

Distance = 1480 m/s x 0.3 seconds

Distance = 444 m

However, this is the sound travelling there and back, so we need to ÷ 2

Distance = 222 m

Sonar

Sonar stands for sound navigation and ranging – a technique used to measure how far away something is