The Kundt’s tube experiment enables us to study **stationary sound waves**.

Remember that a stationary wave is the sum of two** progressive** waves of **equal** frequency and amplitude, but moving in **opposite** directions.

The resultant resembles a lone** vibration** more than a wave but it is really a **superposition of waves.**

Sound is a l**ongitudinal** wave -- that is, the displacements experienced by masses of air are **parallel** to the direction of the wave’s propagation.

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The curve “s of x, t” measures this displacement.

Here we can read , (in ordinate), the horizontal displacement around the equilibrium position of a layer of air, along the x axis.

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For any ordinary frequency of excitation, the curve “s of x, t” shows a disorderly motion of low amplitude.

But, for certain frequencies, a **stationary wave system** sets in. The curve clearly shows the appearance of **antinodes** of vibration (areas where the air molecules vibrate with maximum amplitude)

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And **nodes** of vibration (areas where they do not vibrate at all)

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Note that the distance between two consecutive nodes is half the **length of the wave**: l/2 (read lambda the greek letter)

Knowing the length of the tube and the resonant frequency,** f**, we can determine the **speed of sound**, which is equal to l*f (lambda times f).

For what is currently under observation, we measure **half** a wavelength as 25 cm, and thus a wavelength is 50 cm.

Since the frequency is 680 Hz, we find that the **speed of sound** is 340 meters per second.