Sound — Waves, Pitch, Volume
In 2020, Australian researchers at CSIRO used 20–200 Hz low-frequency sound pulses to track blue whales across 1,500 km of ocean — proving that sound carries energy far further than light in water.
Printable Worksheets
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Q1 · If someone plays a very high note on a flute and a very low note on a tuba, what is physically different about the sound waves?
Q2 · Put your fingers on your throat and hum. What do you feel? What does this tell you about how sound is made?
● Know
- Sound travels as a longitudinal wave through a medium
- Pitch depends on frequency; volume depends on amplitude
- Human hearing range is 20 Hz – 20,000 Hz
● Understand
- Why sound cannot travel through a vacuum (needs particles)
- Why sound travels faster in solids than in gases
- How echo and sonar use the speed of sound
● Can do
- Identify compressions and rarefactions in a sound wave diagram
- Calculate distance using echo timing (basic)
- Give Australian examples of infrasound and ultrasound applications
- Longitudinal wave
- Compression
- Rarefaction
- Frequency
- Amplitude
- Number of vibrations per second — controls pitch
- Wave where particles move in the same direction as the wave
- Size of vibration — controls loudness
- High-pressure region where particles are squashed together
- Low-pressure region where particles are spread apart
When you clap, you compress the air in front of your hands. That compression ripples outward in all directions — a wave of pressure changes. That's sound. No air, no sound — which is why space is completely silent.
Sound is a longitudinal wave: the particles of the medium vibrate in the same direction the wave travels (push-pull, not side-to-side). This creates alternating zones of:
- Compressions — high-pressure regions where particles are pushed close together
- Rarefactions — low-pressure regions where particles are spread apart
Key rule: sound needs a medium (solid, liquid or gas — any material with particles). In the vacuum of outer space there are no particles to vibrate, so sound cannot exist. The wrench dropped outside the ISS? You'd see it clang against the hull but hear nothing.
Sound travels fastest in solids, slower in liquids, slowest in gases. This is because particles in solids are closer together and pass the vibration on more quickly.
| Medium | Speed of sound (approx.) |
|---|---|
| Air (20°C) | ~340 m/s |
| Water | ~1500 m/s |
| Steel | ~5000 m/s |
This is why you can hear a train coming by putting your ear to the rail long before you hear it through the air.
Pitch is controlled by frequency — how many complete vibrations occur each second, measured in hertz (Hz).
- High frequency → high pitch — bird call, flute, piccolo (~2000–8000 Hz)
- Low frequency → low pitch — thunder, tuba, bass guitar (~50–200 Hz)
Volume (loudness) is controlled by amplitude — how far particles move from their rest position.
- Large amplitude → louder (more energy, more pressure change)
- Small amplitude → softer (less energy)
Changing pitch does NOT change volume, and vice versa — they are independent properties. A whispered high note has high frequency and low amplitude.
Speed of sound in air is approximately 340 m/s and increases with temperature (warm air lets particles vibrate more freely). This is why thunder from lightning 1 km away arrives about 3 seconds after the flash.
Echo: sound bounces off a hard surface and returns to the source. The time delay between sending and receiving tells you the distance:
Distance = (speed × time) ÷ 2
(Divide by 2 because the sound must travel there and back.)
SONAR (Sound Navigation and Ranging): sends an ultrasound pulse underwater and measures echo return time → maps the ocean floor or locates fish and submarines. Dolphins and whales use a biological version of sonar (echolocation). SONAR is used in Great Barrier Reef surveys to map reef structures.
Human hearing range: 20 Hz to 20,000 Hz (20 kHz).
- Infrasound (<20 Hz) — below our range. Elephants and whales use it for long-distance communication. Also produced by earthquakes — useful for early warning. Cannot be heard but can be felt.
- Ultrasound (>20 kHz) — above our range. Dog whistles (~25 kHz); bat echolocation (~50–100 kHz); medical scans (obstetric ultrasound, physiotherapy); cattle pregnancy testing in rural Australia.
Wrong: "Louder sounds travel faster." Speed of sound depends on the medium and temperature — not on amplitude or pitch. A whisper and a shout travel at exactly the same speed through the same air.
Right: Loudness (amplitude) has no effect on the speed of sound. Only the medium and temperature affect speed.
Wrong: "Sound is a transverse wave like light." Light is transverse (vibrates perpendicular to wave travel). Sound is longitudinal — particles push and pull in the same direction as the wave moves.
Right: Sound = longitudinal wave. Light = transverse wave. They are completely different types.
Wrong: "High pitch and loud volume are the same thing." A piccolo plays high-pitched softly, a tuba plays low-pitched loudly. Pitch = frequency, Volume = amplitude — independent properties.
Right: Pitch and volume are independent: a sound can be high-pitched and quiet, or low-pitched and loud.
You're watching a thunderstorm from a safe distance. The lightning flash and the thunder are produced at the SAME moment. The flash reaches you almost instantly, but the thunder arrives 3 seconds later. Predict: what does this tell you about how sound and light travel differently? How far away is the lightning?
How close was your prediction?
The hook at the start of this lesson asked: if an astronaut dropped a wrench outside the International Space Station, could the crewmate inside hear it clang? Now you know — absolutely not! Sound cannot travel through the vacuum of space.
Explain why the crewmate would hear nothing, then describe what is physically different between a high flute note and a low tuba note. Use the words frequency, pitch and amplitude at least once each.
Q1. Explain why the sound of a guitar string changes when you tighten it (increase tension). Use the terms frequency, pitch and amplitude in your answer. (3 marks)
Q2. A ship uses SONAR and detects an echo from the ocean floor after 2 seconds. If sound travels at 1500 m/s in water, how deep is the ocean at that point? Show your working. (3 marks)
Q3. Describe how bats use echolocation to navigate and hunt insects at night. Include the relevant wave properties in your answer. (4 marks)
Answers
▾MCQ 1
B — Sound is a longitudinal wave: particles vibrate in the same direction as the wave travels, through alternating compressions and rarefactions. It is not electromagnetic and always needs a medium.
MCQ 2
C — Volume (loudness) is controlled by amplitude — the size of the vibration. A louder sound has larger amplitude pressure changes. Frequency controls pitch, not loudness.
MCQ 3
C — Pitch is determined by frequency (vibrations per second, in Hz). High frequency = high pitch; low frequency = low pitch. Amplitude controls loudness only.
MCQ 4
C — Sound is a mechanical wave and needs particles to vibrate. Space is a near-perfect vacuum with no particles, so no sound wave can exist there.
MCQ 5
D — Sound travels fastest through solids (~5000 m/s in steel) because particles are tightly packed and transfer vibrations quickly. A vacuum (B) cannot carry sound at all.
Short Answer 1
Model answer: Tightening the guitar string increases the tension, causing it to vibrate more rapidly. This increases the frequency of the sound wave. Higher frequency means higher pitch, so the guitar produces a higher-pitched note. The amplitude (and therefore volume) is not necessarily changed by tightening — it depends on how hard you pluck the string.
Short Answer 2
Model answer: Total distance = speed × time = 1500 m/s × 2 s = 3000 m. This is the distance there and back. Depth = 3000 ÷ 2 = 1500 m.
Short Answer 3
Model answer: Bats emit ultrasound pulses (frequency >20,000 Hz — above human hearing). These longitudinal waves travel through the air, reflect (echo) off objects including insects, and return to the bat's ears. The bat's brain processes the time delay between sending and receiving the echo to calculate the distance and location of the insect. The amplitude of the returned echo also gives information about the size of the object. This system is so precise that bats can navigate and catch flying insects in complete darkness, using only sound.