Measuring the Speed of Sound with a Tuning Fork and a Tube

Measuring the speed of sound doesn’t need specialist lab equipment. A simple tuning fork, a resonance tube, and a beaker of water allow students to determine the speed of sound in air with impressive accuracy. This classic physics experiment links frequency, wavelength, and resonance — all central ideas in waves and acoustics.

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The Experiment

Equipment:

• Tuning forks of known frequency (e.g. 256 Hz, 320 Hz)

• Hollow resonance tube or a cardboard/plastic tube

• Large beaker or cylinder of water

• Metre ruler

Method:

1. Fill the beaker with water and place the tube vertically inside so that the bottom end is submerged.

2. Strike the tuning fork and hold it just above the top of the tube.

3. Slowly raise the tube to change its effective air column length.

4. At a certain point, the tube will resonate — the sound becomes much louder.

5. Measure the length of the air column at this point.

6. Use this length to estimate the wavelength of the sound.

Why does this work?
The tube acts as a pipe closed at one end (the water surface). The first resonance occurs when the air column is one quarter of the wavelength:

$$L = \frac{\lambda}{4}$$

So:

$$\lambda = 4L$$

Once the wavelength is known:

$$v = f\lambda$$

where

• $v$ = speed of sound

• $f$ = frequency of tuning fork

• $\lambda$ = wavelength

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Typical Results

For a 256 Hz tuning fork:

• Resonance length measured: 33 cm (0.33 m)

• Estimated wavelength:

  $$\lambda = 4 \times 0.33 = 1.32\text{ m}$$
  

• Speed of sound:

  $$v = 256 \times 1.32 \approx 338\text{ m/s}$$
  

This is very close to the accepted value of around 343 m/s at room temperature.

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Why It Works in Teaching

This method gives a loud, unmistakable resonance that makes wavelength and frequency feel real.
Students hear the physics, measure the physics, and calculate the speed of sound themselves.

It’s ideal for linking experimental method with wave theory, resonance, and the relationship $v = f\lambda$.

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Skills Highlight

• Identifying resonance experimentally

• Measuring air column lengths accurately

• Calculating wavelength and wave speed

• Understanding closed-pipe harmonics