Required Practicals / OCR A / Practical 12
12 AS PAG 12

Speed of sound using stationary waves (PAG 12)

Determine the speed of sound in air by finding two successive resonant lengths of a closed air column.

Apparatus

  • Resonance tube (adjustable length using inner sliding tube in water-filled outer tube)
  • Tuning forks of known frequencies
  • Metre rule
  • Thermometer

Safety

  • Strike tuning forks on a rubber pad only to avoid cracking.
  • Handle glass tube carefully to avoid breakage.

Method

  1. Hold a vibrating tuning fork of frequency f just above the open end of the tube.
  2. Adjust column length until first loud resonance at $L_1$, then second resonance at $L_2$.
  3. Wavelength: $\lambda = 2(L_2 - L_1)$. Speed: $v = f\lambda$.
  4. Repeat for at least four tuning fork frequencies.
  5. Plot $\lambda$ vs $1/f$: straight line through origin, gradient $= v$.

Key Variables

Independent Frequency f (via different tuning forks)
Dependent Wavelength lambda
Controlled Air temperature; Same tube diameter

Analysis and Results

  • From two resonances: $\lambda = 2(L_2 - L_1)$, eliminating end correction.
  • Plot $\lambda$ vs $1/f$: gradient $= v$.
  • Compare v with the expected value at measured temperature: $v \approx 331 + 0.6T$ m s$^{-1}$ where T is in degrees C.

Common Errors

  • Using only one resonance length (end correction introduces systematic error).
  • Not striking tuning fork hard enough for a clear resonance.
  • Not recording air temperature for comparison with the calculated speed.

Exam-style questions on this practical. Click Show mark scheme to reveal the answer after attempting each question.

Q1 4 marks

A 440 Hz tuning fork gives resonances at $L_1 = 18.2$ cm and $L_2 = 55.7$ cm. Calculate (a) the wavelength, (b) the speed of sound, and (c) the end correction.

Q2 2 marks

At 20 degrees C the expected speed of sound is approximately 343 m s$^{-1}$. The student measures 330 m s$^{-1}$. Suggest one reason for this discrepancy.