# Equation of variation of displacement and pressure of sound wave

## Homework Statement

i have attached the notes from 2 books below, i know that the graph of pressure of sound waves lag behind the displacement grpah by 90 degree. so it should be p=p max sin (wt-kx-(pi/2)) am i right? why the another book gives p=p max sin (wt-kx+(pi/2)) ? which is correct? the second and third photo are from the same book.

## The Attempt at a Solution

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No, I think p=p max sin (wt-kx+(pi/2)) is right. Either that or p=p max sin (-wt+kx-(pi/2)). And I wouldn't describe that as lagging behind. I think the displacement is the one that's lagging.

No, I think p=p max sin (wt-kx+(pi/2)) is right. Either that or p=p max sin (-wt+kx-(pi/2)). And I wouldn't describe that as lagging behind. I think the displacement is the one that's lagging.
why do u say displacement is the one that's lagging ? i have drawn the phasor diagram at t=o. it shows that dispacement is leading am i right?

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