# Sound

> Physics · CIE IGCSE 0625
> Source: https://www.owlsprep.com/study/cie-0625-u3-sound/

This guide covers all Core and Extended Sound subtopic content for CIE IGCSE Physics 0625, including wave properties, speed calculations, pitch/loudness links, and Extended ultrasound applications.

**Prerequisites:** [General wave properties (v=fλ, wave types)](https://www.owlsprep.com/study/cie-0625-u3-general-wave-properties/); [Wave speed calculation methods](https://www.owlsprep.com/study/cie-0625-u3-wave-speed/)

## Learning objectives

- Describe how sound is produced by vibrations and transmitted as a longitudinal wave
- Recall that sound cannot travel through a vacuum, and that the speed of sound in air is about 330-350 m/s
- Calculate speed of sound using $v=f\lambda$ and echo measurement methods
- Explain links between frequency/pitch and amplitude/loudness
- (Extended) Recall that, in general, sound travels faster in solids than in liquids, and faster in liquids than in gases
- (Extended) Define ultrasound and recall its practical applications

## 1. Core: Nature, Production and Transmission of Sound

**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.

> **Vacuum Test Proof**
>
> Sound cannot travel through a vacuum, as there are no particles to transmit vibrations. This is proven by the bell jar experiment, where a ringing bell cannot be heard when air is pumped out of the sealed jar.

**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. Core: Sound Properties and Calculations

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

$$v = f \lambda$$

**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: $\lambda = v/f$
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: $speed = distance / time$
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. Extended Only: Ultrasound and Applications

**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. Extended Only: Speed of Sound in Different Media

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.

> **Speed order**
>
> Solids > liquids > gases. For comparison, sound travels at about 330-350 m/s in air, roughly 1500 m/s in water, and around 5000 m/s in steel.

## Common pitfalls

- **Wrong:** (Extended) Stating sound travels fastest in gases
  - Why it fails: Sound travels via particle interactions; particles in solids are closest together so vibrations pass much faster than in gases
  - Correct: Recall the speed order (Extended): solids > liquids > gases. At Core level, only the speed in air (~330-350 m/s) is required
- **Wrong:** Forgetting to double distance in echo calculations
  - Why it fails: Sound travels to the reflecting surface and back to the observer, so total distance is twice the one-way distance to the surface
  - Correct: Always divide total distance traveled by 2 to get the one-way distance to the surface
- **Wrong:** Mixing up links between pitch/amplitude and loudness/frequency
  - Why it fails: Pitch is determined by frequency, not amplitude; loudness is determined by amplitude, not frequency
  - Correct: Memorise the fixed links: frequency = pitch, amplitude = loudness
- **Wrong:** Stating sound can travel through a vacuum
  - Why it fails: Sound is a mechanical wave that requires particles to transmit vibrations; vacuums have no particles
  - Correct: Explicitly state sound cannot travel through a vacuum when answering questions about sound in space
- **Wrong:** (Extended) Defining ultrasound as sound below 20 Hz
  - Why it fails: Sound below 20 Hz is infrasound; ultrasound is above the upper limit of human hearing
  - Correct: Recall human hearing range is 20 Hz to 20 kHz, so ultrasound is >20 kHz

## 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 | $v=f\lambda$, 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 |

## 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).

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