4. The figure below shows the pressure variation for a particular sound moving to the right. The speed of the sound is 330 m s. Pressure a. Direction of propagation of sound 0.1 0.3 0.5 0.7 Distance (m) On the axes provided above, draw the pressure variation of the sound three-quarters of a period later than the instant shown in the figure. (2 marks b. Determine the frequency of this sound wave. (2 marks