Sound

What you'll learn

Sound forms a core component of the CIE IGCSE Physics waves specification, typically accounting for 8-12 marks across Paper 2 and Paper 4. This topic covers the production and transmission of sound waves, their properties including frequency and amplitude, how sound travels through different media, and practical applications such as ultrasound and echo calculations.

Key terms and definitions

Longitudinal wave — a wave in which particles vibrate parallel to the direction of energy transfer; sound waves are longitudinal waves consisting of compressions and rarefactions.

Frequency — the number of complete waves passing a point per second, measured in hertz (Hz); determines the pitch of a sound.

Amplitude — the maximum displacement of particles from their rest position; determines the loudness or volume of a sound.

Echo — a reflected sound wave that arrives at the listener with sufficient time delay (typically more than 0.1 seconds) to be heard as a distinct sound.

Ultrasound — sound waves with frequencies above 20,000 Hz (20 kHz), above the upper limit of human hearing.

Compression — a region in a longitudinal wave where particles are closer together than normal, corresponding to higher pressure.

Rarefaction — a region in a longitudinal wave where particles are further apart than normal, corresponding to lower pressure.

Speed of sound — the velocity at which sound energy travels through a medium; approximately 330 m/s in air at room temperature, faster in liquids and solids.

Core concepts

Nature of sound waves

Sound requires a medium to travel and cannot propagate through a vacuum. This distinguishes sound from electromagnetic waves, which can travel through empty space. The classic bell jar experiment demonstrates this principle: when a ringing electric bell is placed inside a jar and the air is gradually removed using a vacuum pump, the sound becomes progressively quieter until it cannot be heard at all, even though the bell continues to vibrate.

Sound waves are longitudinal waves produced by vibrating objects. When an object vibrates, it causes the surrounding particles (air molecules, water molecules, or particles in a solid) to oscillate back and forth parallel to the direction the wave travels. This creates alternating regions of:

• Compressions where particles bunch together (high pressure)
• Rarefactions where particles spread apart (low pressure)

These pressure variations travel through the medium as a wave, carrying energy from the source to the detector (such as the human ear or a microphone).

Speed of sound in different media

The speed of sound varies significantly depending on the medium through which it travels. Sound travels fastest through solids, slower through liquids, and slowest through gases. This occurs because particles in solids are more tightly packed and can transmit vibrations more efficiently.

Typical speeds at room temperature:

• Air: approximately 330 m/s (often taken as 340 m/s in calculations)
• Water: approximately 1500 m/s
• Steel: approximately 6000 m/s
• Concrete: approximately 5000 m/s

Temperature affects the speed of sound in gases. As temperature increases, air molecules move faster and collide more frequently, allowing sound to propagate more quickly. For air, the speed increases by approximately 0.6 m/s for each degree Celsius rise in temperature.

The speed of sound can be calculated using the relationship between distance, time and speed. For measuring the speed of sound in air, students should know two experimental methods frequently tested in CIE IGCSE papers.

Frequency, wavelength and pitch

Frequency determines the pitch of a sound. High-frequency sounds have high pitch (such as a whistle or a bird chirping), while low-frequency sounds have low pitch (such as thunder or a bass drum). The human hearing range extends from approximately 20 Hz to 20,000 Hz (20 kHz), though this range decreases with age, particularly at the upper limit.

The relationship between speed, frequency and wavelength applies to sound waves:

speed = frequency × wavelength

or v = f λ

Since the speed of sound in air is approximately constant (at a given temperature), higher frequency sounds have shorter wavelengths, while lower frequency sounds have longer wavelengths.

Infrasound refers to sound with frequencies below 20 Hz, below the human hearing range. Elephants use infrasound to communicate over long distances. Earthquakes and volcanic eruptions also produce infrasound.

Ultrasound refers to sound with frequencies above 20 kHz, above the human hearing range. Bats, dolphins and some insects can detect ultrasound.

Amplitude, loudness and energy

The amplitude of a sound wave determines its loudness or volume. Greater amplitude means the particles vibrate with larger displacement from their rest position, corresponding to greater pressure variations and more energy transmitted. Loud sounds have large amplitudes; quiet sounds have small amplitudes.

On an oscilloscope display:

• The horizontal axis represents time
• The vertical axis shows voltage (proportional to pressure variation)
• Amplitude is measured from the centre line to the peak (or trough)
• Wavelength can be determined by measuring the distance for one complete wave pattern

Doubling the amplitude does not double the loudness perceived by the human ear, as loudness perception is logarithmic. However, increasing amplitude does increase the energy transmitted by the wave, which is proportional to the square of the amplitude.

Reflection of sound and echoes

Sound waves reflect from hard, smooth surfaces following the same law as light reflection: the angle of incidence equals the angle of reflection. This principle explains why empty rooms sound different from furnished ones—soft furnishings absorb sound rather than reflecting it.

An echo occurs when reflected sound arrives at the listener's ear with sufficient time delay to be distinguished from the original sound. The human brain requires approximately 0.1 seconds between sounds to perceive them as separate. Since sound travels at approximately 330 m/s in air, echoes become noticeable when the reflecting surface is at least:

distance = speed × time = 330 × 0.1 = 33 m away

However, for the sound to travel to the surface and back, the minimum distance to the reflecting surface is actually 16.5 m.

Echo calculations form a common CIE IGCSE exam question type. The total distance travelled by the sound equals twice the distance to the reflecting surface (there and back). Using:

distance = speed × time

students can calculate unknown distances or time delays.

Applications of ultrasound

Ultrasound has numerous practical applications that appear regularly in CIE IGCSE examinations:

Medical imaging: Ultrasound scans use high-frequency sound waves (typically 1-15 MHz) to create images of internal body structures. A transducer sends pulses of ultrasound into the body and detects reflections from boundaries between different tissues (such as organ surfaces or a developing fetus). The time delay between sending and receiving reflections allows the device to calculate depths and construct an image. Ultrasound is preferred for pre-natal scanning because, unlike X-rays, it does not damage living cells.

Sonar (Sound Navigation and Ranging): Ships use sonar to detect underwater objects and measure sea depth. A pulse of ultrasound is transmitted downward; the time taken for the echo to return allows calculation of depth using:

depth = (speed × time) ÷ 2

The division by 2 accounts for the sound travelling down and back up.

Industrial quality control: Ultrasound can detect cracks and flaws inside metal structures without damaging them. The ultrasound reflects from the crack, producing an echo that reveals the defect's location.

Cleaning: Ultrasonic cleaners use high-frequency vibrations to remove dirt from delicate objects like jewellery and dental equipment. The rapid pressure changes create tiny bubbles that dislodge contaminants.

The Doppler effect (Extended tier only)

The Doppler effect describes the change in observed frequency (and therefore pitch) when there is relative motion between a sound source and an observer. This concept appears in Extended papers but not Core tier.

When a sound source moves toward an observer:

• The compressions are pushed closer together
• The wavelength decreases
• The observed frequency increases
• The pitch sounds higher

When a sound source moves away from an observer:

• The compressions are spread further apart
• The wavelength increases
• The observed frequency decreases
• The pitch sounds lower

A common example is an ambulance siren: as it approaches, the pitch sounds high; as it passes and moves away, the pitch suddenly drops to a lower value. The actual frequency emitted by the siren remains constant—only the observed frequency changes due to relative motion.

The Doppler effect also applies when the observer moves while the source remains stationary, though the mathematical treatment is slightly different. For CIE IGCSE purposes, students need to explain the effect qualitatively and predict whether pitch increases or decreases in described scenarios.

Worked examples

Example 1: Echo calculation

Question: A student stands 82.5 m from a large building and claps once. She hears the echo 0.50 seconds after the clap. Calculate the speed of sound in air. [3 marks]

Solution:

The sound travels to the building and back, so:

• Total distance = 2 × 82.5 m = 165 m [1]

Using speed = distance ÷ time:

• speed = 165 ÷ 0.50 [1]
• speed = 330 m/s [1]

Key points: Remember the sound travels twice the distance to the reflecting surface. Show your working clearly for method marks.

Example 2: Wavelength calculation

Question: A sound wave in air has a frequency of 256 Hz. The speed of sound in air is 340 m/s. Calculate the wavelength of this sound. [3 marks]

Solution:

Using the wave equation: speed = frequency × wavelength

Rearranging: wavelength = speed ÷ frequency [1]

wavelength = 340 ÷ 256 [1]

wavelength = 1.33 m (or 1.3 m) [1]

Key points: Always rearrange the equation before substituting values. Include the unit in your answer.

Example 3: Ultrasound sonar

Question: A ship uses sonar to measure the depth of the sea. An ultrasound pulse is sent out and the echo is detected 0.24 seconds later. The speed of sound in seawater is 1500 m/s.

(a) Calculate the depth of the sea beneath the ship. [3 marks] (b) Explain why ultrasound is used rather than sound waves within the human hearing range. [2 marks]

Solution:

(a) Distance travelled by sound = speed × time Distance = 1500 × 0.24 = 360 m [1]

This is the total distance (down and back)
Depth = 360 ÷ 2 [1]
Depth = 180 m [1]

(b) Ultrasound has a higher frequency [1], which provides better resolution/more accurate measurements and can detect smaller objects. Ultrasound also travels further through water without spreading out as much [1].

Common mistakes and how to avoid them

• Mistake: Stating that sound is a transverse wave. Correction: Sound is a longitudinal wave where particle vibration is parallel to the direction of energy transfer. Water waves and electromagnetic waves are transverse; sound waves are always longitudinal regardless of the medium.

• Mistake: Forgetting to halve the distance in echo and sonar calculations. Correction: When sound reflects from a surface, it travels to the surface and back, covering twice the distance to the object. Always divide the total distance by 2 to find the distance to the reflecting surface.

• Mistake: Confusing frequency with amplitude when describing loudness and pitch. Correction: Frequency determines pitch (high frequency = high pitch); amplitude determines loudness (large amplitude = loud sound). These are independent properties.

• Mistake: Believing sound travels faster through gases than solids. Correction: Sound travels fastest through solids (approximately 6000 m/s in steel), slower through liquids (approximately 1500 m/s in water), and slowest through gases (approximately 330 m/s in air) because particles are more tightly packed in solids.

• Mistake: Writing that sound cannot travel through liquids or solids, only air. Correction: Sound can travel through any material medium (solid, liquid or gas) but cannot travel through a vacuum. Sound actually travels faster through liquids and solids than through air.

• Mistake: Stating humans can hear ultrasound or that ultrasound has low frequency. Correction: Ultrasound has frequencies above 20 kHz, which is above the upper limit of human hearing. The prefix "ultra" means beyond or above. Humans cannot detect ultrasound; specialised equipment or animals like bats and dolphins can.

Exam technique for Sound

• Define and explain questions: When asked to define terms like echo, ultrasound or longitudinal wave, provide the complete definition including numerical values where relevant (such as "above 20,000 Hz" for ultrasound). Command words like "explain" require you to give reasons, not just descriptions.

• Calculation questions: Always show your working for method marks—even if your final answer is incorrect, you can score marks for correct method. State the equation you're using, rearrange if necessary, substitute values with units, and give your answer with the appropriate unit. For echo/sonar calculations, explicitly state "distance = 2 × depth" to show you understand the sound travels both ways.

• Describe practical procedures: Questions about measuring the speed of sound or demonstrating that sound cannot travel through a vacuum require step-by-step descriptions. Include the measurements taken, how calculations are performed, and any safety considerations or sources of error.

• Application questions: Be prepared to apply sound principles to unfamiliar contexts. CIE papers often present scenarios involving animal communication, building acoustics, or industrial applications. Read the context carefully, identify which sound principle applies (reflection, frequency, speed in different media), and apply your knowledge to the specific situation described.

Quick revision summary

Sound waves are longitudinal waves requiring a medium to travel; they cannot propagate through a vacuum. Sound consists of compressions and rarefactions travelling through materials at speeds of approximately 330 m/s in air, 1500 m/s in water, and 6000 m/s in steel. Frequency (measured in Hz) determines pitch, while amplitude determines loudness. The wave equation v = f λ links speed, frequency and wavelength. Echoes result from sound reflection and enable depth measurements using distance = speed × time (remembering to halve the total distance). Ultrasound (above 20 kHz) has medical, industrial and navigational applications. The Doppler effect describes pitch changes when sources and observers move relative to each other.