Waves can also add together in a way which cancels out or lessens their amplitude resulting in a quieter combined sound. When the crests of one wave superimpose upon the troughs of the other wave the amplitude lessens. This is known as destructive interference especialy if the sounds totally cancel each other out. Follow the links below to listen to and visualize the process.
Notice how the Crest and Troughs of the upper two waves coincide or overlap each other. The third wave is picture of the top two waves added together. Notice how its amplitude is smaller. Listen to these waves when added together. This superposition has "destroyed" some of the amplitude or volume.
You should notice that the sound is quieter than either of the other two waves.