Waves can be combined together such that, at a given location and time, they may cause either constructive effects, in which  the amplitude is strengthened, or destructive effects in which the amplitude is reduced. A remarkable property of waves is that they can pass through one another without being disturbed.  While they are passing through, the resulting acoustic pressure is always just the sum of the acoustic pressures of each individual wave at that moment and location. 

The following example shows two wave pulses travelling in opposite directions. See for yourself that the resulting displacement is the sum of the individual wave displacements.