CIE A Level Physics (9702) 2019-2021

Revision Notes

8.3.1 Sound Waves

Cathode-Ray Oscilloscope

  • A Cathode-Ray Oscilloscope is a laboratory instrument used to display, measure and analyse waveforms of electrical circuits
  • An A.C. current on an oscilloscope is represented as a transverse wave. Therefore you can determine its frequency and amplitude
  • The x-axis is the time and the y-axis is the voltage (or y-gain)

CRO diagram, downloadable AS & A Level Physics revision notes

Diagram of Cathode-Ray Oscilloscope display showing wavelength and time-base setting


  • The period of the wave can be determined from the time-base This is how many seconds each division represents measured commonly in s div-1 or s cm-1
  • Use as many wavelengths shown on the screen as possible to reduce uncertainties
  • Dividing the total time by the number of wavelengths will give the time period T (Time taken for one complete oscillation)
  • The frequency is then determined through 1/T


Worked Example

A cathode-ray oscilloscope(c.r.o.) is used to display the trace from a sound wave. The time-base is set at 7 µs mm-1.

WE - CRO question image, downloadable AS & A Level Physics revision notes

What is the frequency of the sound wave?

A    2.4 Hz          B     24 Hz             2.4 kHz             24 kHz

WE - CRO answer image, downloadable AS & A Level Physics revision notes

Exam Tip

The time-base setting varies with units for seconds (commonly ms) and the unit length (commonly mm). Unit conversions are very important for the calculate of the time period and frequency

Measuring Wavelength

  • Stationary waves can be produced inside air columns from sound waves
    • This is how musical instruments, such as clarinets and organs, work
  • This can be demonstrated by placing a fine powder inside the air column and a loudspeaker at the open end:

Wavelength of sound wave, downloadable AS & A Level Physics revision notes

Wavelength of stationary sound wave in an air column

  • At certain frequencies, the powder forms evenly spaced heaps along the tube, showing where there is zero disturbance as a result of the nodes of the stationary wave
  • In order to produce a stationary wave, there must be a minima (node) at one end and a maxima (antinode) at the end with the loudspeaker
  • The wavelength is then determined by the distance between the two antinodes with two nodes between them (or two nodes with two antinodes between them)

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