The spinning wheel, which is used to represent frequency, amplitude and phase of a vibration, is also called phasor diagram in the literature. It is extremely useful for the calculation and visualization of vibrations. A phasor is rotating inside the spinning wheel.
The length of the phasor corresponds to the radius of the wheel. The number of circular revolutions per second represents the frequency of the sound wave. The radius corresponds to the amplitude of the sound wave.
The wavelength lambda is calculated from the ratio of the wave propagation rate c and the frequency f. Let us consider the red dot on the wave, and let us think of the best way to clearly describe its position, or more precisely, the phase of the wave. One option is to use the dot's distance delta to the next zero crossing of the wave. The distance delta can be used in the following formula. Phase shift phi equals 2 pi times delta divided by lambda.
That is precisely the phase of the sound wave we have already seen in the previous station.