Pressure equation of sound wave

[abhineetK]

Let us consider sound (longitudinal) wave
y=A.sin(\omegat-kx)

See attachment for diagram

Consider air column of length \Deltax and cross-sectional area S at distance x from source.
I want to derive the pressure equation of the sound wave using
Bulk modulus, B=-\DeltaP/(\DeltaV/V)
implying \DeltaP=-B.\DeltaV/V

But the trouble is that I am not able to select the expression for \DeltaV
1) \DeltaV=S.y
2) \DeltaV=S\Deltay
TELL ME WHICH EXPRESSION OUT OF THE TWO SHOULD I USE with EXPLANATION.

 

[abhineetK]

I give you a hint about what I was actually thinking.

1)\DeltaV=S.y
It would mean that \DeltaP is the difference between the initial pressure of the volume considered and the final pressure of the same column (now volume is reduced to (V-\DeltaV) but mass(amount) remains same).

2)\DeltaV=S.\Deltay
It would mean that \DeltaP is the change in pressure for a change \Deltay
in the length of the column.

Which of the above does the pressure equation of sound wave denotes? Please, tell.

NOTE: If you find any problem in understanding what I want to say, please tell.