Describe two ways that standing waves are different from travelling waves.

An experiment is carried out to measure the speed of sound in air, using the apparatus shown below.

A tube that is open at both ends is placed vertically in a tank of water, until the top of the tube is just at the surface of the water. A tuning fork of frequency 440 Hz is sounded above the tube. The tube is slowly raised out of the water until the loudness of the sound reaches a maximum for the first time, due to the formation of a standing wave.

(i) Explain the formation of a standing wave in the tube.

(ii) State the position in the tube that is always a node.

The tube is raised until the loudness of the sound reaches a maximum for a second time. Between the two positions of maximum loudness, the tube has been raised by 36.8 cm. The frequency of the sound is 440 Hz. Estimate the speed of sound in air.

energy is propagated by travelling waves / energy is not propagated by standing waves;
amplitude constant for travelling waves / amplitude varies with position for standing waves;
phase varies with position for travelling waves / phase constant for standing waves;
travelling waves do not have nodes and antinodes / standing waves do have nodes and antinodes;
travelling waves can have any wavelength/frequency / standing waves can only have certain wavelengths/frequencies (to fit boundary conditions);

(i) wave from tuning fork travels down tube and is reflected;
incident and reflected waves interfere/superpose/combine/add together to give a standing wave (that fits the boundary conditions);

(ii) the surface of the water (in/at the bottom of the tube);

\(\frac{\lambda }{2} = 0.368 \Rightarrow \lambda  = 0.736{\rm{m}}\);
v=ƒλ=440×0.736=320ms⁻¹;