Sound

Q: Is sound a transverse or longitudinal wave?

Sound is a longitudinal wave, meaning particles vibrate parallel to the direction of wave travel. It is made up of alternating high-pressure compressions and low-pressure rarefactions in the medium it travels through.

Q: How do I calculate distance to a surface using an echo?

An echo travels to the surface and back to the observer, so divide the total distance traveled by 2 to get the one-way distance: , where is speed of sound and is total time between emitting the sound and hearing the echo.

Physics· 3.4· 25 min read

1. 1. Core: Nature, Production and Transmission of Sound★★☆☆☆⏱ 8 min

📘 Definition

Sound wave

A longitudinal mechanical wave produced by vibrating objects, which requires a solid, liquid or gas medium to travel.

Sound is generated when an object vibrates, causing adjacent particles in the surrounding medium to oscillate parallel to the direction of wave travel. These oscillations create alternating regions of high pressure (compressions) and low pressure (rarefactions) that move through the medium.

📐 Worked Example

Explain why an astronaut standing close to a falling rock on the Moon cannot hear the impact, even if they are not wearing a helmet.

1
The Moon has no atmosphere, so the space between the rock and the astronaut is a vacuum.
2
Sound is a mechanical wave that requires particles in a medium to transmit vibrations.
3
There are no particles in the vacuum to carry the sound of the impact to the astronaut's ear.

Exam tip:

Always explicitly state that sound requires a medium to travel when answering questions about sound in space/vacuum; this is a mandatory marking point.

2. 2. Core: Sound Properties and Calculations★★★☆☆⏱ 10 min

Two subjective sound properties are directly linked to measurable wave properties, and the speed of sound follows the standard wave speed equation:

v = f \lambda

📘 Definition

Pitch and Loudness

Pitch is proportional to wave frequency (higher frequency = higher pitch). Loudness is proportional to wave amplitude (higher amplitude = louder sound). The normal human hearing range is 20 Hz to 20,000 Hz.

The speed of sound depends on the medium it travels through. At Core level you need to recall that the speed of sound in air is approximately 330-350 m/s at room temperature. (How the speed compares across solids, liquids and gases is Extended content, covered in the next section.)

📐 Worked Example

A 440 Hz sound wave (middle A) travels through air at 330 m/s. Calculate its wavelength, and state if it is high or low pitch.

1
Rearrange the wave speed equation to solve for wavelength: $λ = v / f$
2
$\lambda = \frac{330}{440} = 0.75 m$
3
440 Hz falls in the middle of the 20 Hz to 20,000 Hz human hearing range, so it is a medium pitch sound.

📐 Worked Example

A student claps and hears an echo from a 80 m away wall 0.5 s later. Calculate the speed of sound from this data.

1
The sound travels to the wall and back, so total distance traveled = $2 \times 80 = 160$ m
2
Use the speed equation: $s p ee d = d i s t an ce / t im e$
3
$v = \frac{160}{0.5} = 320 m/s$

Exam tip:

Always double the one-way distance when solving echo calculation problems; forgetting this step is the most common error in sound questions.

3. 3. Extended Only: Ultrasound and Applications★★★★☆Extended only⏱ 7 min

📘 Definition

Ultrasound

Sound waves with a frequency higher than the upper limit of human hearing (above 20,000 Hz).

Ultrasound is widely used in industrial and medical settings because it can penetrate most materials and reflects off boundaries between different media without causing damage:

Medical scanning: Used to create images of internal organs and fetuses, with no radiation risk from X-rays
Industrial flaw detection: Used to find hidden cracks or defects in metal structures and pipes
Sonar: Used on ships to measure water depth and detect underwater objects like fish or shipwrecks

📐 Worked Example

A fishing boat uses sonar to detect fish. An ultrasound pulse returns 0.02 s after it is emitted. If the speed of sound in water is 1500 m/s, calculate the depth of the fish.

1
Total distance traveled by the ultrasound pulse = $v \times t = 1500 \times 0.02 = 30$ m
2
The pulse travels to the fish and back, so depth is half the total distance
3
$Depth = \frac{30}{2} = 15 m$

Exam tip:

You must be able to name at least two practical applications of ultrasound to get full marks on Extended questions for this topic.

4. 4. Extended Only: Speed of Sound in Different Media★★★☆☆Extended only⏱ 4 min

In general, sound travels faster in solids than in liquids, and faster in liquids than in gases. This is because the particles in a solid are held close together with strong bonds, so a vibration is passed on to neighbouring particles very quickly. In a gas the particles are far apart, so the vibrations are transmitted much more slowly.

5. Common Pitfalls

Wrong move:

(Extended) Stating sound travels fastest in gases

Why:

Sound travels via particle interactions; particles in solids are closest together so vibrations pass much faster than in gases

Correct move:

Recall the speed order (Extended): solids > liquids > gases. At Core level, only the speed in air (~330-350 m/s) is required

Wrong move:

Forgetting to double distance in echo calculations

Why:

Sound travels to the reflecting surface and back to the observer, so total distance is twice the one-way distance to the surface

Correct move:

Always divide total distance traveled by 2 to get the one-way distance to the surface

Wrong move:

Mixing up links between pitch/amplitude and loudness/frequency

Why:

Pitch is determined by frequency, not amplitude; loudness is determined by amplitude, not frequency

Correct move:

Memorise the fixed links: frequency = pitch, amplitude = loudness

Wrong move:

Stating sound can travel through a vacuum

Why:

Sound is a mechanical wave that requires particles to transmit vibrations; vacuums have no particles

Correct move:

Explicitly state sound cannot travel through a vacuum when answering questions about sound in space

Wrong move:

(Extended) Defining ultrasound as sound below 20 Hz

Why:

Sound below 20 Hz is infrasound; ultrasound is above the upper limit of human hearing

Correct move:

Recall human hearing range is 20 Hz to 20 kHz, so ultrasound is >20 kHz

6. Quick Reference Cheatsheet

Concept	Core Fact	Extended Fact
Nature of sound	Longitudinal mechanical wave, needs medium, cannot pass through vacuum	Same as Core + ultrasound is >20,000 Hz
Speed of sound	$v = f λ$ , speed of sound in air ~330-350 m/s	Speed order: solids > liquids > gases; use echo/sonar to find distance to surfaces
Properties	Frequency = pitch, Amplitude = loudness, hearing range 20 Hz to 20 kHz	Ultrasound uses: medical scanning, flaw detection, sonar

7. Frequently Asked

Is sound a transverse or longitudinal wave?

Sound is a longitudinal wave, meaning particles vibrate parallel to the direction of wave travel. It is made up of alternating high-pressure compressions and low-pressure rarefactions in the medium it travels through.

How do I calculate distance to a surface using an echo?

An echo travels to the surface and back to the observer, so divide the total distance traveled by 2 to get the one-way distance: $d = \frac{v \times t}{2}$ , where $v$ is speed of sound and $t$ is total time between emitting the sound and hearing the echo.

Going deeper

What's Next

Now you have mastered the Sound subtopic for CIE IGCSE Physics 0625, move on to other wave subtopics in Unit 3. Next, you will study electromagnetic waves, a high-frequency exam topic that covers light, radio waves, X-rays and more. Practice structured calculation questions from past papers to solidify your echo and wave speed problem-solving skills, and make sure you only revise Extended content if you are sitting the Supplement papers (2/4).