4PH1

Sound

Waves

Exam Frequency Analysis

Past paper frequency (2018 to 2024)

This topic accounts for approximately 7% of your exam marks.

stable
Low
Stable7%

Speed of sound calculations, echo timing and ultrasound uses are standard shorter questions.

Aim

  • Measure the of the sound produced by tuning forks of different pitches by reading the period off an oscilloscope screen
Apparatus for measuring frequency: a struck tuning fork vibrates next to a microphone, which is wired to an oscilloscope that displays the resulting sound wave as a trace on its screen.
Source: Oscilloscope experiment by Save My Exams

Variables

  • Independent variable: the tuning fork being used (each fork is stamped with its rated frequency)
  • Dependent variable: the period T read from the oscilloscope trace (s), from which the measured frequency is calculated
  • Control variables: the same microphone and oscilloscope, the same time-base setting (kept constant during one measurement, and noted so it can be converted), the same fork-to-microphone distance, the same quiet room

Equipment

EquipmentPurpose
Several tuning forks of known frequencyProduce a clean tone at the fork's stamped frequency
Rubber striking padStrike the fork on something soft to avoid metallic ringing overtones
MicrophonePick up the fork's tone and turn it into a varying voltage
OscilloscopeDisplay the voltage against time and measure its period
Connecting leadsWire the microphone to the oscilloscope's channel-1 input

Method

  1. Connect the microphone to channel 1 of the oscilloscope and switch the oscilloscope on
  2. Set the time base so that a few full cycles of a typical 400–500 Hz fork will fit across the screen (a starting value of around 0.5 ms per division works well)
  3. Strike the first tuning fork sharply on the rubber pad and immediately hold it about 5 cm from the microphone
  4. Freeze the oscilloscope display, or take a quick photograph of the screen, while the trace is steady
  5. Read off how many horizontal divisions make up one full cycle of the wave by tracking from a peak to the next peak (this gives the cleanest reading)
  6. Repeat the strike-and-freeze cycle a further two times for the same fork; take the average number of divisions
  7. Repeat steps 3–6 for the other forks, adjusting the time base if the higher-frequency forks pack too many cycles into the screen

Analysis

  • Convert divisions to seconds using the time base:

T = (divisions per cycle) × (time-base scale per division)

  • Convert period to frequency:

f = 1 / T

  • Compare the measured frequency with the value stamped on the side of the fork

Sources of error and safety

  • Systematic error, wrong time-base reading. The scale on most oscilloscopes is in milliseconds per division; misreading it as seconds per division throws the answer out by a factor of 1000. Always note the scale at the start of each measurement
  • Random error, background noise. Other sounds in the room add fuzz to the trace and make division-counting harder. Carry the practical out in a quiet room and ask others not to talk while a measurement is being taken
  • Safety. Strike the fork on a rubber pad rather than a hard surface; striking glass or concrete can chip the fork and send fragments flying