For a progressive wave, State what frequency means.
A loudspeaker, microphone and cathode-ray oscilloscope (CRO) are set out as in Fig. 4.1. The loudspeaker produces a sound wave, which the microphone picks up and the CRO displays on its screen, as shown in Fig. 4.2. The time-base on the CRO is adjusted to $0.50\,\text{ms\,cm}^{-1}$ and the $y$-gain is adjusted to $0.20\,\text{V\,cm}^{-1}$.
the frequency of the sound wave.
the amplitude of the signal picked up by the CRO.
The intensity of the sound wave in (b) is lowered to one quarter of its initial value with no change in frequency. Assume that the amplitude of the signal received by the CRO is proportional to the amplitude of the sound wave. On Fig. 4.2, sketch the trace that is now visible on the CRO screen.
A metal sheet is now placed in front of the loudspeaker in (b), as shown in Fig. 4.3. A stationary wave is produced between the loudspeaker and the metal sheet. State the principle of superposition.
The microphone starts at a position where the CRO trace has an amplitude minimum. It is then moved a distance of $1.05\ \text{m}$ away from the loudspeaker along the line joining the loudspeaker and the metal sheet. During the movement, it passes through three positions where the trace has an amplitude maximum before ending at a position where the trace has an amplitude minimum. Determine the wavelength of the sound wave.
Use your results from (b)(i) and (d)(ii) to find the speed of sound in air.