The longitudinal displacement s(x,t) along the x-axis is plotted along the
y-axis.
s(x,t)
s(x,t)
s(x,t) denotes the longitudinal displacement of of the particles from their
equilibrium positions at x.
When you moves the piston in a sinusoidal manner, the air next to the piston
expands and contracts in a sinusoidal manner.
Because of the elasticity of air, a longitudinal sinusoidal traveling wave is
generated.
Displacement of sound waves
The
displacement amplitude sm is the maximum displacement of the air
particles from their equilibrium positions.
Displacement of the air particles form their equilibrium positions
Piston
Air particles
When you push the piston forward, it compresses the air next to it.
Consequently, the density and the pressure of the air next to the piston rise above
the normal values. Because of the elasticity of air, the region with higher pressure moves forward.
When you push the piston backward, it expands the air next to it.
Consequently, the density and the pressure of the air next to the piston drop
blow the normal values. Because of the elasticity of air, the region with
lower pressure moves forward.
Since it is difficult to plot s(x,t) along the x-axis, we will plot the
longitudinal displacement along the y-axis. This is just to help us to
visualize the displacement and it does not mean that the displacement is along
the vertical axis.