Find the loudest note. Read its period on a c.r.o.
Sound experiments in Paper 5 use a loudspeaker driven by a signal generator and a microphone connected to a c.r.o. Here a loudspeaker sounds at the open end of a tube closed by a sliding piston, and you find the first resonant frequency f for each length L of the air column. Watch the set-up built step by step, with the variables and the graph beside it, then build it yourself and check it against the mark points.
Past-paper referenceOriginal experiment, wording and figures, written in the style of Cambridge AS & A Level Physics 9702 Paper 5, Question 1. It is not copied from a past paper. The “what the examiners said” notes are our paraphrase of the Principal Examiner Reports for Teachers.
Practise next: 9702/52 May/June 2015, Question 1 (intensity of sound reflected from a wall covered in foam) · official past papers
Practise next: 9702/52 February/March 2024, Question 1 (resonant frequency of a struck metal rod, detected with a microphone) · official past papers
Examiner reports used: 9702 June 2015 and March 2024.
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What a full-mark diagram shows
Making the sound
A signal generator connected to a loudspeaker, so the frequency can be changed. Never a d.c. supply.
Detecting the sound
A microphone connected directly to a c.r.o. No battery in that circuit, and not a computer.
Measuring L
A metre rule from the open end to the piston face, with the tube held in a clamp stand.
Using the c.r.o.
T from the length of one wave × time-base, then f = 1/T. Adjust the time-base so one wave is as long as possible.
Habits the examiner reports keep asking for
Write “kept constant”, not “controlled”, and name the quantity: the air temperature θ and the tube diameter.
Say ONE wave: the period is the length of one complete wave times the time-base, then f = 1/T.
Wait for a steady trace, and repeat f for the same L and average. “Repeat the experiment” on its own earns nothing.
A quiet room, not a soundproof room. Other sounds in the room still reach the microphone.
Safety with a loudspeaker is about your ears. Keep the output low and wear ear defenders.