5.3 Sound Waves
What are Sound Waves?
Sound waves are longitudinal waves caused by vibrations. They show areas of compression (high pressure) and rarefaction (low pressure).
Sound waves are longitudinal waves caused by vibrations. They show areas of compression (high pressure) and rarefaction (low pressure).
⚡ Key Properties of Sound:
Compressions and Rarefactions:
• Sound is a mechanical wave
• It needs a medium to travel (air, water, solid)
• It cannot travel through a vacuum
• Speed depends on the medium
• It needs a medium to travel (air, water, solid)
• It cannot travel through a vacuum
• Speed depends on the medium
Compression
(high pressure) Rarefaction
(low pressure) Compression
(high pressure) Rarefaction
(low pressure)
(high pressure) Rarefaction
(low pressure) Compression
(high pressure) Rarefaction
(low pressure)
Solids
~5,000 m/s
Fastest - particles closest
Liquids
~1,500 m/s
Medium speed
Gases (Air)
~340 m/s
Slowest - particles furthest
💡 Why Does Sound Travel Faster in Solids?
Sound travels by particles passing vibrations to neighbouring particles.
• In solids: particles are close together → vibrations pass quickly
• In gases: particles are far apart → vibrations take longer to pass
This is why you can hear a train coming by putting your ear to the track.
• In solids: particles are close together → vibrations pass quickly
• In gases: particles are far apart → vibrations take longer to pass
This is why you can hear a train coming by putting your ear to the track.
🎯 Sound Properties Quiz
What Determines How a Sound... Sounds?
Low frequency = Low pitch
Think: whistle (high) vs bass drum (low)
Low amplitude = Quiet sound
Think: shouting vs whispering
Frequency → Pitch
High frequency = High pitchLow frequency = Low pitch
Think: whistle (high) vs bass drum (low)
Amplitude → Loudness
High amplitude = Loud soundLow amplitude = Quiet sound
Think: shouting vs whispering
🎵 Interactive Wave Demo:
Low pitch
High pitch
Quiet
Loud
Human Hearing Range:
Humans can hear sounds with frequencies between:
Note: As we age, our upper limit decreases. Teenagers can often hear frequencies that adults cannot.
Lower Limit
20 Hz
Very low bass sounds
Upper Limit
20,000 Hz
Very high pitched sounds
Note: As we age, our upper limit decreases. Teenagers can often hear frequencies that adults cannot.
🎯 Pitch and Loudness Quiz
Beyond Human Hearing
Uses:
• Medical imaging (baby scans)
• Finding cracks in metal/pipes
• Cleaning delicate equipment
• Sonar (submarines, fishing)
Sources:
• Earthquakes
• Volcanic eruptions
• Elephant communication
• Whale songs
Infrasound
<20 Hz
<20 Hz
Human Hearing
20 Hz - 20,000 Hz
20 Hz - 20,000 Hz
Ultrasound
>20,000 Hz
>20,000 Hz
Ultrasound
Frequency above 20,000 HzUses:
• Medical imaging (baby scans)
• Finding cracks in metal/pipes
• Cleaning delicate equipment
• Sonar (submarines, fishing)
Infrasound
Frequency below 20 HzSources:
• Earthquakes
• Volcanic eruptions
• Elephant communication
• Whale songs
How Ultrasound Imaging Works:
Step 1: Ultrasound waves are sent into the body
Step 2: Waves reflect off boundaries between different tissues
(e.g., between fluid and bone)
Step 3: Reflected waves are detected and timed
Step 4: A computer builds an image from the reflections
Why ultrasound?
• Non-ionising (safe, unlike X-rays)
• Can image soft tissue
• Real-time images possible
Step 2: Waves reflect off boundaries between different tissues
(e.g., between fluid and bone)
Step 3: Reflected waves are detected and timed
Step 4: A computer builds an image from the reflections
Why ultrasound?
• Non-ionising (safe, unlike X-rays)
• Can image soft tissue
• Real-time images possible
💡 Animals and Sound:
| Animal | Hearing Range | Special Ability |
|---|---|---|
| Dogs | 40 - 60,000 Hz | Can hear dog whistles we can't |
| Bats | 1,000 - 100,000 Hz | Echolocation using ultrasound |
| Dolphins | 75 - 150,000 Hz | Underwater echolocation |
| Elephants | 14 - 12,000 Hz | Communicate using infrasound |
Example: Ultrasound Distance Calculation
An ultrasound pulse is sent into the body. It reflects off a structure and returns after 0.00004 seconds. If the speed of ultrasound in tissue is 1,500 m/s, how deep is the structure?
Step 1: Calculate total distance travelled
$\text{Distance} = \text{speed} \times \text{time}$
$\text{Distance} = 1500 \times 0.00004 = 0.06$ m
Step 2: Remember - the wave travels there AND back.
$\text{Depth} = 0.06 \div 2 = 0.03$ m = 3 cm
Step 1: Calculate total distance travelled
$\text{Distance} = \text{speed} \times \text{time}$
$\text{Distance} = 1500 \times 0.00004 = 0.06$ m
Step 2: Remember - the wave travels there AND back.
$\text{Depth} = 0.06 \div 2 = 0.03$ m = 3 cm
🧮 Ultrasound Distance Calculator:
Time (s):
Speed (m/s):
🎯 Ultrasound & Infrasound Quiz
Real Life Applications:
• Medical: Pre-natal scans, kidney stone treatment
• Industrial: Finding flaws in metal structures
• Navigation: Sonar for submarines and ships
• Nature: Bats use echolocation to hunt
• Cleaning: Ultrasonic cleaners for jewellery and glasses
• Medical: Pre-natal scans, kidney stone treatment
• Industrial: Finding flaws in metal structures
• Navigation: Sonar for submarines and ships
• Nature: Bats use echolocation to hunt
• Cleaning: Ultrasonic cleaners for jewellery and glasses