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3.4CH3 Waves

Sound — Cambridge O Level Physics 5054 Revision Notes & Topical Questions

10 min read 12 parts Lesson 4 of 4

📚 Cambridge O Level Physics 5054 Hub / Topical Revision
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Part 1 of 10Production and nature of sound

  • Source of sound
    • sound is produced by a vibrating object, such as a guitar string, a loudspeaker cone or vocal cords.
  • Longitudinal wave
    • sound is a series of compressions and rarefactions travelling through a medium.
    • Particles vibrate parallel to the direction in which the energy moves.
  • Medium required
    • sound needs a medium of particles and cannot travel through a vacuum.
    • A bell ringing inside a jar fades away as the air is pumped out.
  • Compressions and rarefactions
    • a vibrating source pushes particles together then pulls them apart (lower pressure).
      • (higher pressure)
  • Transfer of energy
    • the energy passes from particle to particle, but the particles only oscillate about fixed points.

Part 2 of 10Speed of sound in different media

  • Speed in air
    • about 330 to 350 m/s at room temperature, taken as 340 m/s in most questions.
  • Speed depends on the medium
    • sound is slowest in gases, faster in liquids and fastest in solids.
    • Particles in solids are closer and more strongly linked, so vibrations are passed on faster.
MediumApproximate speed (m/s)
Air340
Water1500
Wood3000-4000
Steel5000
  • Effect of temperature
    • sound is a little faster in warmer air.
  • Frequency does not change the speed
    • in one medium all frequencies travel at the same speed, so high and low notes arrive together.
  • Using the wave equation
    • v = fλ also applies to sound, with v being the speed in the medium.

Part 3 of 10Measuring the speed of sound

  • Direct timing method
    • use an echo or a two-person method over a measured distance.
  1. Stand a known distance, such as 100 m, from a large wall.
  2. Make a sharp sound such as a clap or a starting gun.
  3. Start the stopwatch at the clap and stop it when the echo is heard.
  4. The sound travels there and back, so v = 2d ÷ t.
  5. Repeat the measurement several times and take a mean.
  • Alternate method with clapping
    • adjust the clapping rate to coincide with the echoes and time many claps, which reduces reaction-time error.
  • Two microphones and a timer
    • place two microphones a known distance apart, connected to an electronic timer.
    • Measure the time for the sound pulse to pass from one to the other and calculate v = distance ÷ time.
  • Sources of error
    • wind, human reaction time and a short distance make the time too small to measure accurately.

Part 4 of 10Pitch, loudness and quality

  • Pitch
    • how high or low a note sounds, determined by its frequency.
    • High frequency gives a high pitch, and low frequency gives a low pitch.
  • Loudness
    • determined by the amplitude of the wave, with a larger amplitude meaning more energy and a louder sound.
  • Quality (timbre)
    • the character of a note, determined by the shape of the waveform, so a violin and a flute differ at the same pitch and loudness.
Property of soundWave featureGraph on an oscilloscope
PitchFrequencyMore waves across the screen
LoudnessAmplitudeGreater height
QualityWave shapeDifferent pattern
  • Oscilloscope trace
    • a microphone connected to a cathode-ray oscilloscope shows the waveform of the sound,
    • and the period read from the time base gives f = 1 ÷ T.
    • If one wave takes 5.0 ms on the screen, f = 1 ÷ 0.0050 = 200 Hz.
  • Range of human hearing
    • about 20 Hz to 20 000 Hz, and the range narrows with age.
  • Infrasound and ultrasound
    • frequencies below 20 Hz are infrasound and above 20 000 Hz are ultrasound.

Part 5 of 10Reflection and echoes

  • Reflection
    • hard, smooth surfaces reflect sound well, and soft, rough surfaces absorb it.
  • Echo
    • a reflection of sound heard after the original sound.
    • An echo is noticeable if the surface is at least 17 m away, so the delay exceeds about 0.1 s.
  • Calculating distance
    • the sound travels to the reflector and back, so the distance to the reflector is d = vt ÷ 2.
  • Reverberation
    • the many overlapping echoes in a large hall; soft furnishings and acoustic tiles reduce it.
  • Sonar
    • ships send out sound pulses and time the echoes from the sea bed or fish shoals.
    • With v = 1500 m/s in water, an echo returning after 0.20 s means a depth of 150 m.

Part 6 of 10Ultrasound and its uses

  • Ultrasound
    • sound with a frequency above the upper limit of human hearing, above 20 kHz.
  • Medical imaging
    • pulses of ultrasound are reflected at boundaries between tissues and the echo times build up an image of a fetus or organs.
    • It is safe because it is non-ionising.
  • Cleaning
    • ultrasonic baths shake dirt from delicate objects such as jewellery and surgical instruments.
  • Industrial uses
    • ultrasonic detectors find cracks in metal, and pulse echo measures the thickness of materials.
  • Animals
    • bats, dolphins and some other animals use ultrasound echoes to navigate and find prey.
  • Why ultrasound is useful
    • it has a short wavelength and so can be directed in narrow beams and can detect small features.

Part 7 of 10Everyday sound ideas

  • Hearing
    • sound waves enter the ear, vibrate the eardrum, and the vibrations are passed through bones to the cochlea and then as signals to the brain.
  • Loud sounds
    • noise above about 85 dB can damage hearing, so ear protection is needed in noisy workplaces.
  • Reducing noise
    • absorb it with soft materials, increase the distance, or isolate the source with double glazing.
  • Musical instruments
    • the pitch of a string depends on its length, tension and thickness, and a shorter, tighter, thinner string gives a higher pitch.

Part 8 of 10Sound in practice

  • Sound intensity and distance
    • a sound gets quieter with distance because its energy spreads out over a larger area.
    • Doubling the distance from a small source greatly reduces the loudness.
  • Beats and tuning
    • two close frequencies together produce a regular rise and fall in loudness, which musicians use to tune instruments.
  • Resonance
    • an object vibrates strongly when driven at its natural frequency.
    • A singer can break a glass by sounding a note at the glass's natural frequency.
  • Absorbing sound
    • soft, porous materials such as carpets, foam and curtains absorb sound, while hard surfaces reflect it.
  • Sound and hearing damage
    • very loud sound can harm the ear because the large amplitude damages the delicate cells.
    • Hearing protection reduces the amplitude that reaches the eardrum.

Part 9 of 10Experiments with sound

  • Sound cannot travel through a vacuum
    • a bell in a bell jar shows it.
  1. Suspend an electric bell inside a sealed bell jar on a foam pad.
  2. Switch on the bell and listen to the clear ringing.
  3. Pump air out of the jar with a vacuum pump.
  4. The sound becomes fainter until it can hardly be heard, although the clapper can still be seen moving.
  5. Let air back in and the sound returns.
  • Reflection of sound
    • it reflects from hard, smooth surfaces such as a wall or board.
    • Place a ticking clock at the end of a long tube, with another tube and a hard board angled the same way on the other side of the normal,
      • and the sound is loudest when the angle between the tubes and board follows the law of reflection.
    • Soft materials such as cloth placed in the path absorb the sound and the reflected note becomes quieter.
  • Comparing speeds
    • sound travels fastest in solids, slower in liquids and slowest in gases.
    • This is why you can hear an approaching train through the rail before you hear it through the air.
  • Timbre on an oscilloscope
    • two instruments playing the same note give waves of the same frequency and amplitude,
    • but different shapes,
    • which gives them different qualities.
    • A tuning fork gives a smooth sine wave and a violin gives a more complex pattern.

Part 10 of 10Worked examples

  1. A man claps his hands and hears an echo from a cliff 1.2 s later. Calculate the distance to the cliff (speed of sound 340 m/s). [3]3 marks

  2. A note has a frequency of 680 Hz. Calculate its wavelength in air (v = 340 m/s). [2]2 marks

  3. An oscilloscope shows one complete wave of a sound over 4.0 ms. Calculate the frequency and say whether it is audible. [3]3 marks

  4. A ship's sonar sends a pulse and receives the echo from the sea bed after 0.80 s. Sound travels at 1500 m/s in water. Find the depth. [3]3 marks

  5. Describe how the sound on an oscilloscope changes when a note becomes louder and higher pitched. [3]3 marks

  6. A student stands 85 m from a wall and claps repeatedly until the claps match the echoes, and counts 30 claps in 15 s. Find the speed of sound. [4]4 marks

Key terms

TermMeaning
Sound waveA longitudinal wave of compressions and rarefactions in a material.
PitchHow high or low a note sounds, set by its frequency.
LoudnessHow strong a sound seems, set by its amplitude.
EchoA reflected sound heard after the original sound.
UltrasoundSound with a frequency above 20 000 Hz.
InfrasoundSound with a frequency below 20 Hz.
SonarUsing echoes of sound pulses to find distance or depth.

Avoid these mistakes

  1. Saying sound travels in a vacuum — it needs a medium.
  2. Thinking sound is faster in air than in solids — it is fastest in solids.
  3. Using the full echo distance as the distance to the wall — halve it because the sound travels there and back.
  4. Mixing up pitch and loudness — pitch is frequency and loudness is amplitude.
  5. Saying a higher pitch travels faster — all frequencies have the same speed in one medium.

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