# 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) 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. What is the frequency of the sound wave?

A    2.4 Hz          B     24 Hz             2.4 kHz             24 kHz

#### 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:
• 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) ### Author: Ashika

Ashika graduated with a first-class Physics degree from Manchester University and, having worked as a software engineer, focused on Physics education, creating engaging content to help students across all levels. Now an experienced GCSE and A Level Physics and Maths tutor, Ashika helps to grow and improve our Physics resources.
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