Graphs of Sound Wave

Pressure-Distance Graph (for all particles)

Compression, C: highest pressure of sound wave;
Rarefaction, R: lowest pressure of sound wave;
Amplitude, A: the maximum pressure change from surrounding pressure in either direction;
Wavelength, λ: the distance between two consecutive C or R or any two points that are in phase. 

Displacement-Distance Graph (for all particles)

Both C and R are at position of zero displacement, which are at rest position.
Amplitude, A: maximum distance from the rest or central position, in either direction.
Wavelength, λ: the distance between two consecutive C or R or any two points that are in phase.

Note that Compressions and Rarefactions in a longitudinal wave (sound) are NOT analogous to the crests and troughs in a traverse wave although some textbooks stating otherwise.

Displacement-Time Graph (for individual particle)

Displacement-Time Graph is for an individual particle in the vibrating medium. It shows the displacement of a particle from the rest position at a particular time. 

At t = 0 s, particle is at displacement = 0 m;
At t = T/4 s, particle is at displacement = – A m;
At t = T/2 s, particle is at displacement = 0 m;
At t = 3T/4 s, particle is at displacement = +A m;
At t = T s, particle is at displacement = 0 m.
Both C and R are at position of zero displacement.
Amplitude, A: maximum distance from the rest or central position, in either direction.
Period, T: time taken for each particle to complete one oscillation.

Prediction on Particle Movement (Optional)

Step:

Step 1: Locate positions of C and R. If we cant locate them, we cant predict.
Step 2: Draw a new wave in the wave direction and locate new position of C and R.
Step 3: Trace out a vertical line from new C and R. This line will help us to determine the movement.
Remember that in longitudinal wave, particles always move towards compression region! So,
For particles lie on the wave from A to B, they will move to left;
For particles lie on the wave from A to D, they will move to right.
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