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14.1Sound

Properties and Detection of Sound — American System Physics Revision Notes & Topical Questions

10 min read 7 parts Lesson 3 of 5 1 quiz

📚 American System Physics Hub / Topical Revision
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Lesson summary

Describe sound as a longitudinal wave requiring a medium.Speed of soundv = d/t (m/s = m ÷ s).Echosound travels there and back, so v = 2d/t or d = vt/2.Wave equation for soundv = fλ.Human hearing range20 Hz to 20 kHz; ultrasound > 20 kHz.Order of speedsolids > liquids > gases; sou…

  • Intensity falls with distance from a point source
  • The ear detects sound
  • The Doppler effect
  • Sound wave
  • Compression
  • Rarefaction

Part 1 of 7Key ideas

1Speed of sound
Speed of sound v = d/t (m/s = m ÷ s).
2Echo
Echo sound travels there and back, so v = 2d/t or d = vt/2.
3Wave equation for sound
Wave equation for sound v = fλ.
4Human hearing range
20 Hz to 20 kHz; ultrasound > 20 kHz.
5Order of speed
Order of speed solids > liquids > gases; sound cannot pass through a vacuum.
610× intensity
A 10 dB increase means 10× intensity; 20 dB means 100× intensity. Prolonged exposure above about 85 dB can damage hearing.
7
Doppler effect for a moving source and stationary observer: fobs = fs × v/(v − vs) approaching, v/(v + vs) receding (v = speed of sound).
8Intensity falls with distance from a point source
I ∝ 1/r².

Part 2 of 7Learning objectives

1
Describe sound as a longitudinal wave requiring a medium.
2
Relate pitch to frequency and loudness to amplitude.
3
Measure the speed of sound in air and use echo calculations.
4
Explain the Doppler effect and describe uses of ultrasound.

Part 3 of 7Essential knowledge

Essential knowledge
14.1 Properties and Detection of Sound concept map: Speed of sound, Echo, Wave equation for sound, Human hearing range, Order of speed
Visual guide to the key ideas explained in this lesson.
Sound is produced by vibrating objects such as vocal cords, guitar strings and loudspeaker cones. The vibration pushes and pulls neighbouring air particles, creating compressions and rarefactions that travel outwards.
Because sound relies on particles passing on vibrations, it cannot travel through a vacuum. The bell-jar experiment shows this: as air is pumped out, the ringing bell becomes fainter although it can still be seen vibrating.
Sound travels fastest in solids and slowest in gases, because particles in solids are close together and strongly bonded, so vibrations are passed on quickly. Temperature also matters: sound is faster in warmer air.
The frequency of a sound determines its pitch and the amplitude determines its loudness. On an oscilloscope trace, closer waves mean higher frequency; taller waves mean greater amplitude.
Humans hear roughly 20 Hz to 20 kHz. Ultrasound (above 20 kHz) is used for prenatal scans (partially reflected at boundaries between tissues; the time of echoes gives depth), sonar depth-finding, detecting cracks in metals and cleaning delicate objects.
An echo is reflected sound. Hard, smooth surfaces reflect sound well; soft, porous surfaces absorb it, which is why concert halls use soft materials to reduce reverberation.
The ear detects sound
The ear detects sound the eardrum vibrates in response to pressure variations, the small bones (ossicles) amplify the vibrations, and the cochlea converts them into electrical nerve signals sent to the brain.
Sound level is measured in decibels, a logarithmic scale. 0 dB is the threshold of hearing; normal conversation ≈ 60 dB; a rock concert ≈ 110 dB; pain threshold ≈ 120–130 dB. Each +10 dB is ten times the intensity.
The Doppler effect
The Doppler effect when a source moves towards you, wavefronts are bunched up in front of it, so the wavelength you receive is shorter and the frequency (pitch) higher; when it moves away, wavefronts are spread out, giving lower pitch. It is used in police radar, weather radar and blood-flow measurements.
Microphones detect sound by converting pressure variations into electrical signals, which can be displayed on an oscilloscope.

Part 4 of 7Process & sequence

1
Two observers stand a measured distance apart (at least 100 m, measured with a trundle wheel) in an open field.
2
Observer A bangs two wooden blocks together (or fires a starting pistol) above the head so the action is visible.
3
Observer B starts a stopwatch on seeing the action (light arrives almost instantly) and stops it on hearing the sound.
4
Repeat several times and calculate the mean time to reduce random errors from reaction time.
5
Calculate speed of sound v = distance ÷ mean time.
6
Alternative echo method: stand a known distance d from a large wall, clap in rhythm with the echo, time 20 claps; v = 2d × 20 ÷ total time.

How an ultrasound scan builds an imagewrite these steps in this order; each step is a marking point.

1
Probe emits a short pulse of ultrasound into the body through coupling gel.
2
At each boundary between tissues, part of the pulse is reflected back.
3
Probe detects the echoes and records the time for each to return.
4
Computer uses depth = speed × time ÷ 2 to locate each boundary.
5
Many pulses from different directions are combined into an image on screen.

Part 5 of 7Worked examples

340 m/s
A student stands 170 m from a cliff and hears the echo of a clap 1.0 s later. Calculate the speed of sound.
  • Total distance = 2 × 170 m = 340 m. v = d/t = 340 m ÷ 1.0 s = 340 m/s.
    What the symbols mean: v = speed (m s−1) · d or s = distance (m) · t = time (s)
600 m
A ship sends an ultrasound pulse to the seabed; the echo returns after 0.80 s. Sound travels at 1500 m/s in seawater. Find the depth.
  • Distance travelled = vt = 1500 × 0.80 = 1200 m (there and back). Depth = 1200 ÷ 2 = 600 m.
863 Hz
An ambulance siren of 800 Hz approaches a stationary listener at 25 m/s. Speed of sound 343 m/s. What frequency is heard?
  • fobs = fs × v/(v − vs) = 800 × 343/(343 − 25) = 800 × 343/318 = 863 Hz (3 s.f.).

Part 6 of 7Summary

1
Sound is a longitudinal wave of compressions and rarefactions that needs a medium.
2
Frequency → pitch; amplitude → loudness.
3
Humans hear 20 Hz – 20 kHz; above this is ultrasound.
4
For echoes use distance = speed × time ÷ 2.
5
Doppler effect: approaching source sounds higher in pitch.

Part 7 of 7Compare: Compare

Speed of sound in different media
MediumTypical speed (m/s)Reason
Air (20 °C)343Particles far apart; weak interactions
Water≈1500Particles closer; liquid less compressible
Steel≈5000–6000Particles strongly bonded; very stiff
Vacuum0 (cannot travel)No particles to pass on vibrations

Sound speed in air increases with temperature (roughly +0.6 m/s per °C).

Wave property vs what you hear
Wave propertyPerceptionChange
FrequencyPitchHigher f → higher pitch
AmplitudeLoudnessLarger A → louder
Waveform shapeQuality (timbre)Different mix of harmonics

Key terms

Command terms

Command terms
Identify / Describe (FRQ)
A short, specific statement; describing requires relevant characteristics, not a single word.
Explain / Justify (FRQ)
Give the cause-and-effect reasoning or evidence; a restated fact does not score.
Calculate
Show setup, substitution and the answer with units.
Predict
State the expected outcome; add a reason only if asked to justify.
MCQ technique
Read every option; eliminate distractors that are true statements but do not answer the question; watch for NOT/EXCEPT.

Exam technique

Keywords the mark scheme looks for in this lesson

Keywords the mark scheme looks for in this lesson
Sound wave · Compression · Rarefaction · Speed of sound · Pitch · Loudness · Audible range (human hearing) · Ultrasound · Infrasound · Echo · Sound level (decibel, dB) · Doppler effect · Sound intensity
Learn each definition in the Key words tab exactly; a missing keyword in a definition usually loses the mark.

How to get full marks (examiner guidance)

1
Answer exactly what is asked; extra correct material does not earn credit, and contradictory statements can cancel credit (AP FRQ).
2
Use precise vocabulary, never “it/stuff/things”.
3
Show units and working in calculations.
4
In MCQs, predict the answer before reading the options, then eliminate.

Words to avoid

1
sound travels faster in air because it is less densesound travels fastest in solids because particles are closely bonded.
2
loudness depends on frequencyloudness depends on amplitude; pitch depends on frequency.
3
distance = speed × time for an echodistance to the reflector = speed × time ÷ 2.
4
sound can travel through spacesound needs a medium and cannot travel through a vacuum.
5
the source's frequency changes in the Doppler effectthe observed frequency changes; the source frequency is unchanged.

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