Required Practicals / Edexcel / Practical 5
5 AS CP5

Speed of sound in air using stationary waves (CP5)

Determine the speed of sound in air by finding resonant lengths of a closed air column at known frequencies.

Apparatus

  • Resonance tube (glass tube that slides into a water-filled outer tube to adjust length)
  • Set of tuning forks (at least four different frequencies)
  • Ruler
  • Thermometer

Safety

  • Strike tuning forks against a rubber pad only; striking against a hard surface can crack the fork.
  • Handle the glass resonance tube carefully to avoid breakage.

Method

  1. Strike the tuning fork of frequency f and hold it just above the open end of the resonance tube.
  2. Slowly raise the inner tube (lengthening the air column) until the first loud resonance is heard. Record this length $L_1$.
  3. Continue raising until a second loud resonance is heard at length $L_2$ (third harmonic).
  4. Wavelength: $\lambda = 2(L_2 - L_1)$. Speed: $v = f\lambda$.
  5. Repeat for each tuning fork frequency. Plot $\lambda$ against $1/f$: straight line through origin, gradient $= v$.

Key Variables

Independent Frequency f (via different tuning forks)
Dependent Wavelength lambda (from resonant lengths)
Controlled Air temperature (hence v); Same tube diameter (end correction constant)

Analysis and Results

  • First resonance: $L_1 + c = \lambda/4$ (c = end correction).
  • Second resonance: $L_2 + c = 3\lambda/4$.
  • Subtracting: $L_2 - L_1 = \lambda/2$, so $\lambda = 2(L_2 - L_1)$ (end correction cancels).
  • Plot $\lambda$ vs $1/f$: gradient $= v$ (speed of sound in air at that temperature).

Common Errors

  • Using only the first resonance; subtracting two resonant lengths eliminates the end correction.
  • Not striking the tuning fork hard enough to maintain a clear tone near the tube.
  • Measuring L from the wrong reference point (measure from the open end of the tube).
  • Not recording air temperature; speed of sound changes with temperature ($v \approx 331 + 0.6T$ m s$^{-1}$ where T is in degrees C).

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

Q1 4 marks

A tuning fork of frequency 512 Hz produces first and second resonances at air column lengths of 16.0 cm and 49.5 cm. Calculate the speed of sound and the end correction.

Q2 3 marks

Explain why using two successive resonances rather than just the first resonance gives a more accurate value for the speed of sound.