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3.4.2Waves

Measuring the Speed of Sound — Cambridge IGCSE Physics 0625 Revision Notes & Topical Questions

5 min read 9 parts Lesson 33 of 35 1 quiz

📚 Cambridge IGCSE Physics 0625 Hub / Topical Revision
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Curriculum Equation Key Wave Speed & Doppler Effect FormulasFast Exam Solution
v = f × λ | f_obs = f × (v ± v_o) / (v ∓ v_s) | T = 1 / f
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Lesson summary

3.4.2 Measuring the Speed of Sound — IGCSE PHYSICS notes: Describe and evaluate experiments that measure the speed of sound in air.Air — varies from 330 to 350 m/s and is greater at higher air temperatureOther media — about 1500 m/s in liquids and about 5000 m/s in solidsEquation…

  • Expected result
  • Independent variable
  • Control variable
  • Timing in experiment 1
  • Reaction time
  • Time base in experiment 2

Part 1 of 5Speed of sound

Air
  • Air varies from 330 to 350 m/s and is greater at higher air temperature
Other media
  • Other media about 1500 m/s in liquids and about 5000 m/s in solids
Equation
  • Equation speed = distance ÷ time
Expected result
  • Expected result about 340 m/s in air

Part 2 of 5Equipment

EquipmentPurpose
Trundle wheelMeasure the distance travelled by the sound
Wooden blocksCreate a sound when banged together
StopwatchTime the sound's journey
OscilloscopeDisplay the sound wave electronically
Two microphonesDetect sound and turn it into an electrical signal
Tape measureMeasure the distance between the microphones

Part 3 of 5Errors and improvements

Errors and improvements
Timing in experiment 1
  • Timing in experiment 1 the main error is the measurement of time, so repeat and average
Reaction time
  • Reaction time up to 0.2 s, which is large when the travel time is as little as 0.3 s
Distance
  • Distance maximise the distance between the two people so the time is longer and the error smaller
Time base in experiment 2
  • Time base in experiment 2 the scale is small, for example milliseconds, so account for it in the calculation
Accuracy
  • Accuracy experiment 2 is the most accurate because the timing is automatic, and experiment 1 the least accurate

Part 4 of 5Other methods

Echo method
  • Echo method stand a distance d from a wall and time the echo, v = 2d ÷ t
  • Clap in rhythm with the echoes and time many claps
  • Divide the total time by the number of claps
Wind
  • Wind repeat in the opposite direction and average
Light
  • Light travels so fast its time is negligible when an observer watches for the flash

Part 5 of 5More from the specification

1Longitudinal Nature of Sound Waves
  • Sound waves propagate through media as longitudinal waves composed of alternating series of high-pressure compressions and low-pressure rarefactions;
    • frequency determines the perceived pitch and amplitude dictates loudness.
2Speed of Sound Measurement in Air
  • The speed of sound in air is determined by measuring the distance between a sound generation event and a detector, divided by the time taken for the sound waves to travel that path (v = d/t).
3Ultrasound Echo Sounding and Defect Detection
  • Ultrasound consists of high-frequency sound waves (> 20 kHz);
    • by measuring the time taken for reflected pulses to return, instruments can accurately calculate ocean depth or detect an internal structural crack in solid materials.
4Longitudinal Nature of Sound Waves
  • Sound waves propagate through media as longitudinal waves composed of alternating series of high-pressure compressions and low-pressure rarefactions;
    • frequency determines the perceived pitch and amplitude dictates loudness.
5Speed of Sound Measurement in Air
  • The speed of sound in air is determined by measuring the distance between a sound generation event and a detector, divided by the time taken for the sound waves to travel that path (v = d/t).
6Ultrasound Echo Sounding and Defect Detection
  • Ultrasound consists of high-frequency sound waves (> 20 kHz);
    • by measuring the time taken for reflected pulses to return, instruments can accurately calculate ocean depth or detect an internal structural crack in solid materials.
7Ultrasound
  • Sound with a frequency above 20 kHz
8Compression
  • Decrease in length under a squashing force.
9Rarefaction
  • Region of a longitudinal wave where particles are spread apart
10Loudness
  • How loud a sound is, set by its amplitude
11Pressure
  • Force per unit area, P = F/A.
    What the symbols mean: p = pressure (Pa = N m−2) · F = force acting at right angles to the surface (N) · A = area (m2)
12Frequency
  • Revolutions per second, f = 1/T.
    What the symbols mean: T = period, the time for one complete wave (s) · f = frequency (Hz)
13Boundary
  • Surface between two media at which waves are partially reflected
14Longitudinal wave
  • Wave whose vibrations are parallel to the direction of travel

Key terms

TermMeaning
Speed of soundAbout 340 m/s in air
OscilloscopeInstrument that displays electrical signals against time
MicrophoneDevice that converts sound into an electrical signal
Reaction timeDelay in a person starting or stopping a stopwatch
Time baseOscilloscope setting that sets the time scale of the display

Avoid these mistakes

1
Using a stopwatch over a short distance — reaction time then causes a very large error.
2
Forgetting to convert the time base — oscilloscope times are often in milliseconds.
3
Blaming vague human error — name reaction time in starting and stopping the stopwatch.
4
Saying one reading is enough — repeat and average.

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