# To measure the velocity of sound in air by means of a resonance tube

To measure the velocity of sound in air by means of a resonance tube

We saw on page 315 that the velocity of sound is given by the formula and hence if we can find the wavelength, A, of the sound wave emitted by a standard tuning fork of frequency, f, we may easily calculate the velocity, v.

The resonance tube suggests a method of doing this. In the first position of
resonance the length of the tube is A/4, and therefore it would appear that A can be found by multiplying the tube length by 4. However, there is a difficulty. The antinode at the top does not coincide exactly with the top of the tube, but projects slightly above it by an amount which is called the end correction, c. Fortunately the difficulty may be overcome by measuring the length of the tube for the second position of resonance as well as the first (see Fig. 29.8). By subtracting one from the other, the end correction is eliminated and we obtain a correct value for half a wavelength.

Thus, if length of tube for 1st position of resonance = II
and length of tube for 2nd position of resonance = 12