Skip to content
Cambridge IGCSE0625

Sound

Physics 0625 Chapter Notes

What this chapter covers

Sound
ShareWhatsAppPost
Sound notes

Read online free. Download a watermarked copy with a free account.

Read the notes

The full Sound notes as text: skim, search, and jump between subtopics.

~8 min read

In this chapter

  1. 1Sound is a longitudinal wave from a vibrating source
  2. 2The audible range and ultrasound
  3. 3Loudness and pitch
  4. 4Sound needs a medium
  5. 5Measuring the speed of sound; echoes
  6. 6Compression and rarefaction
  7. 7Speed of sound in solids, liquids and gases
  8. 8Uses of ultrasound and sonar

1. Sound is a longitudinal wave from a vibrating source

Sound is produced by a vibrating source, such as the cone of a loudspeaker. The vibrating cone makes the air particles vibrate, and they transfer energy as sound.

Sound waves are longitudinal: the particles vibrate parallel to the direction in which the wave travels. The wave transfers energy without transferring matter.

Examiner insight

“A common error was to state that sound waves are transverse waves.” “Many candidates gave answers in terms of the loudspeaker receiving sound and did not refer to the loudspeakers being a vibrating source.”

Worked example 11 mark

Which row gives the correct description of a sound wave?

rowaction of wavetype of wave
Atransfers energy without transferring matterlongitudinal
Btransfers energy without transferring mattertransverse
Ctransfers matter without transferring energylongitudinal
Dtransfers matter without transferring energytransverse
  1. 1

    A: transfers energy without transferring matter, and longitudinal. [1]

2. The audible range and ultrasound

The approximate range of frequencies audible to humans is 20 Hz to 20 000 Hz.

Ultrasound is sound with a frequency higher than 20 kHz. It is not defined by what it is used for, or by what type of wave it is.

1 kHz = 1000 Hz

Examiner insight

“A common error was to give the upper limit of the range as 20 000 kHz.” “Stating that it is a sound outside the human hearing range was insufficient as that would also apply to sounds with a frequency below 20 Hz.”

Worked example 11 mark

Define ‘ultrasound’.

  1. 1

    (sound with frequency) above 20 kHz [1]

3. Loudness and pitch

A bigger amplitude makes a louder sound. A higher frequency makes a sound of higher pitch.

A sound can get louder while its pitch stays constant: the amplitude increases and the frequency stays the same.

Examiner insight

“Many candidates were unable to link pitch and frequency correctly.” “Most candidates gained credit for drawing a quieter sound, but many drew a sound with the same or even higher frequency.”

Worked example 11 mark

A quiet sound is produced by a loudspeaker. The pitch of the sound remains constant, but the loudness of the sound is increased. Which property of the sound wave is increased?

A amplitude · B frequency · C speed · D wavelength

  1. 1

    A: amplitude. Loudness depends on amplitude. Pitch depends on frequency, which stays constant. [1]

4. Sound needs a medium

Sound needs a medium to travel through. It cannot travel through a vacuum.

The speed of sound in air is approximately 330–350 m / s.

Examiner insight

“Some weaker candidates suggested that a vacuum might be the medium through which sound waves could travel.”

Worked example 11 mark

Which value is closest to the speed of sound in air?

A 30 m / s · B 300 m / s · C 300 000 m / s · D 300 000 000 m / s

  1. 1

    B: 300 m / s. The speed of sound in air is approximately 330–350 m / s. [1]

5. Measuring the speed of sound; echoes

An echo is the reflection of sound waves.

To find the speed of sound in air, measure a distance and a time. Measure the distance with a tape. Start a stopwatch when the source is seen to make a sound. Stop it when the sound is heard. Use a long distance, because reaction time is significant over a short one.

With an echo, the sound travels to the wall and back, so the distance to the wall is doubled.

v = s/t

v = 2d/t (echo from a wall at distance d)

Examiner insight

“Candidates should be aware that a stopwatch is started when a source is seen to make a sound, and stopped when the sound is heard.” “A metre rule or ruler is unsuitable for measuring a large distance.”

6. Compression and rarefaction

A sound wave in air is a series of compressions and rarefactions.

In a compression the air particles are closer together and the pressure is higher. In a rarefaction the particles are further apart and the pressure is lower.

Examiner insight

“Good answers referred to an area of high pressure or high density/the molecules being close together. Others mistakenly referred to the waves being closer together.”

Worked example 11 mark

In still air, the air pressure is p and the average distance between the air particles is d. A sound wave produces rarefactions as it travels through the air. In a rarefaction, the air pressure is P and the average distance between the air particles is D. Which statement is correct?

A P > p and D > d · B P > p and D < d · C P < p and D > d · D P < p and D < d

  1. 1

    C: P < p and D > d. In a rarefaction the pressure is lower and the particles are further apart. [1]

7. Speed of sound in solids, liquids and gases

In general, sound travels faster in solids than in liquids and faster in liquids than in gases.

So sound travels more slowly in air than in water, and more slowly in water than in a metal. You do not need to learn the speeds.

Examiner insight

“Many candidates believed that the speed of sound in gases is greater than in metals, leading to the incorrect option A.” “Candidates were not expected to know any of the relative velocities, but they should know that sound travels considerably faster in solids than in gases or liquids.”

8. Uses of ultrasound and sonar

Ultrasound is used in non-destructive testing of materials, in medical scanning of soft tissue and in sonar.

In sonar a pulse of ultrasound is sent out and reflects back. The depth or distance is calculated from the time and the wave speed. The pulse travels there and back, so the depth is half the total distance.

total distance = v × t

depth = (v × t)/2

Examiner insight

“A very common mistake was to ignore the factor of 2 that is needed because the time given in the question is for the journey from X to Y and back.” “A significant number omitted to divide by 2.”

Worked example 11 mark

Which statements about ultrasound are correct?

1 They diffract less than audible sound waves. · 2 They are used in the medical scanning of soft tissues. · 3 They travel at a smaller speed in air than audible sound waves.

A 1 and 2 · B 1 only · C 2 and 3 · D 2 only

  1. 1

    Statement 2 is correct: ultrasound is used in medical scanning of soft tissue.

  2. 2

    Statement 3 is wrong: ultrasound travels at the same speed in air as audible sound.

  3. 3

    A: 1 and 2. [1]

Recap

  • Sound is produced by a vibrating source and is a longitudinal wave.
  • Audible range: 20 Hz to 20 000 Hz. Ultrasound: frequency higher than 20 kHz.
  • Bigger amplitude = louder. Higher frequency = higher pitch.
  • Sound needs a medium. Speed in air is approximately 330–350 m / s.
  • Speed of sound: measure a distance and a time. An echo is a reflection, so double the distance.
  • Extended: a compression has particles closer together and higher pressure. A rarefaction has the opposite.
  • Extended: sound is generally fastest in solids, then liquids, then gases.
  • Extended: sonar depth = speed × time ÷ 2.

End-of-chapter exercise

Test yourself on the whole chapter. Work through these before moving on.

  1. State the range of frequencies audible to humans.1 mark
  2. A loudspeaker produces a sound wave in air. (a) State how the loudspeaker produces the sound. (b) A student says that the sound wave is transverse. Correct her and describe how the air particles move.3 marks
  3. A bell rings inside a sealed glass jar. The air is pumped out of the jar. State what happens to the sound heard outside the jar and explain why.2 marks
  4. A starting pistol is fired. A student 680 m away hears the bang 2.0 s after he sees the flash. Calculate the speed of sound in air.3 marks
  5. Sound X has twice the amplitude of sound Y and half the frequency of sound Y. State which sound is louder and which has the higher pitch.2 marks
  6. A student stands 85 m from a tall wall and claps once. She hears the echo 0.50 s later. Calculate the speed of sound.3 marks
  7. A ferret can hear frequencies from 16 Hz to 44 000 Hz. A whistle produces a sound of frequency 30 kHz. Explain why the ferret can hear the whistle but a human cannot.3 marks
  8. A loudspeaker produces a sound wave in air. (a) Describe a compression in terms of the spacing of the air particles and the air pressure. (b) The sound is made quieter. Its frequency stays the same. State what happens to the number of air particles in a region of compression and in a region of rarefaction.4 marks
  9. A worker hits a long steel rail once with a hammer. Two microphones are 1020 m from the hammer. One touches the rail and the other is in the air beside it. The microphones detect the sound 2.8 s apart. The speed of sound in air is 340 m / s. (a) State which microphone detects the sound first and give a reason. (b) Calculate the speed of sound in steel.5 marks
  10. An engineer uses ultrasound to test a metal bar of length 1.5 m for cracks without cutting it open. A transmitter at one end sends a pulse of ultrasound into the bar. A detector at the far end receives the pulse 0.25 ms later. A detector beside the transmitter receives an echo from a crack 0.18 ms after the pulse is sent. (a) Calculate the speed of ultrasound in the metal. (b) Calculate the distance of the crack from the transmitter. (c) Calculate the time taken for the echo from the far end of the bar to return to the transmitter.6 marks

Go deeper

Practise and revise with member-only material for this chapter.

Workbook

Exam-style practice questions with full worked solutions.

Unlock with a plan

Chapter at a Glance

A single-page summary of every key idea for fast revision.

Unlock with a plan

Free notes are just the start.

Unlock every Workbook and Chapter at a Glance, and generate your own worksheets and predicted papers.

Explore plans

Related chapters

Physical quantities and measurement techniquesMotionMass and weightDensityForcesMomentum