Measuring the Speed of Sound

• You already know $v = f\lambda$ from the Waves chapter: speed equals frequency times wavelength.
• The frequency $f$ is set by your source (a fork or a signal generator), so you already know it. The one thing you must measure is $\lambda$.
• For accuracy, do not measure just one gap. Measure across several node gaps at once, then divide. That makes the error smaller.

• Resonance tube. Hold a vibrating tuning fork over a tube standing in water. Raise or lower the water until the sound is loudest. That loudest point is resonance. There the air column is $\lambda/4$, so $\lambda = 4 \times$ that length.

• Kundt's dust tube. Send sound down a tube with fine dust in it. At the antinodes the air moves the most, so the dust is shaken away. It gathers at the still nodes. Measure the pile spacing: that is $\lambda/2$.

• Speaker and reflector. Point a speaker at a hard board. The reflection makes a stationary wave. Move a probe to find the nodes: they are $\lambda/2$ apart. Microwaves work the same way: point the transmitter at a metal sheet and move the probe between them.

Every method does the same thing: use the stationary wave to get $\lambda$, then $v = f\lambda$. Measure across several gaps for accuracy.