Has this ever happened to you? When your friend says something to you, but you were distracted so you didn't quite "hear" them, but then you understood it 2 seconds later. This is a perfect example of echoic memory. I wrote "hear" in parenthesis because your auditory system heard what your friend said perfectly. You just didn't pay attention to it completely to send the information to short term memory. Echoic memory has a duration of ~3 seconds and is one of the five components of sensory memory. So next time this happens to you, don't feel like you are rude because you interrupted them, blame it on your echoic memory.