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Video: Speed of sound

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 longitudinal 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.