State what is meant by the principle of superposition.
Fig. 6.1 shows an arrangement that can be used to find the speed of sound in air. Sound waves of constant frequency are sent out by the loudspeaker $L$ and reflected from a point $S$ on a hard surface. The loudspeaker is moved away from $S$ until a stationary wave is formed. Explain how the sound waves from $L$ produce a stationary wave between $L$ and $S$.
A microphone linked to a cathode ray oscilloscope (c.r.o.) is placed between $L$ and $S$ as shown in Fig. 6.1. The trace on the c.r.o. is shown in Fig. 6.2. The time-base setting on the c.r.o. is $0.10\,\text{ms cm}^{-1}$.
Calculate the frequency of this sound wave.
The microphone is now moved towards $S$ along the line $LS$. When it has moved $6.7\,\text{cm}$, the c.r.o. trace changes from maximum amplitude to minimum amplitude and then back to maximum amplitude. Use the properties of stationary waves to explain these changes in amplitude.
Calculate the speed of sound in air.