When a loudspeaker moves back and forth, sound is generated. The moving of the loudspeaker diaphragm causes condensation of air when it moves outward, and rarefaction of air when it moves inward. When this movement is happening at a certain frequency, we hear a sound. See for example this illustration: https://www.physicsforums.com/attachments/sound-gif.84528/?temp_hash=012fb293b7e650bf392ecc98c949c953 [Broken] I was thinking about this and I understand we perceive the condensation/rarefaction as sound. But what happens if we have only one or the other? I.e. if we have a loudspeaker only moving from it's initial position outward ONCE, only condensation of air happens. In the illustration above, this would correspond to only one crest of the wave, but no trough. Same question goes if the loudspeaker only moves backwards from it initial position once, so there's only rarefaction (one trough, no crest). So my questions are: 1. If ONLY ONE condensation (one "crest") is happening, will we still perceive it as sound? 2. If ONLY ONE rarefaction (one "trough") is happening, will we still perceive it as sound? 3. If the answer is yes to any or both of question 1 and 2, are the sounds in 1 and 2 different from each other? If so, how? 4. If the answer is yes to any or both of question 1 and 2, are the sounds different from a sound that contains both condensation and rarefaction? If so, how?