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.


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.


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)


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


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.