The simplest meaning for duration is the length of time that a note is sounded. This term can also refer to the notation of such a length or the length of time that a rest should be observed (silence). Duration can also refer to the interval of time for a phrase, section or entire composition. When creating electronic music, duration may refer to the length of decay, attack, sustain, delay or other time based effects.<br><br>Human hearing is capable of detecting very short sounds, even those that contains only a few cycles of air pressure oscillation. We can distinguish between different types of short sounds such as a click or a pop. If such sounds are repeated often and quickly enough, we lose the ability to distinguish individual pops and clicks. Sounds that were once perceived as being short in duration now sound like an everlasting hum. Frequencies of 20-30 Hz are generally where the perception changes. You can compare this to the perception of motion in movies. Still images that are changed at the right frequency are indistinguishable from the individual still scenes and instead motion is perceived.<br>