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
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
Equipment
Purpose
Several tuning forks of known frequency
Produce a clean tone at the fork's stamped frequency
Rubber striking pad
Strike the fork on something soft to avoid metallic ringing overtones
Microphone
Pick up the fork's tone and turn it into a varying voltage
Oscilloscope
Display the voltage against time and measure its period
Connecting leads
Wire the microphone to the oscilloscope's channel-1 input
Method
Connect the microphone to channel 1 of the oscilloscope and switch the oscilloscope on
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)
Strike the first tuning fork sharply on the rubber pad and immediately hold it about 5 cm from the microphone
Freeze the oscilloscope display, or take a quick photograph of the screen, while the trace is steady
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)
Repeat the strike-and-freeze cycle a further two times for the same fork; take the average number of divisions
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