Sound
Sound travels as distinct waves with two physical characteristics: amplitude, which is volume or loudness; and frequency, which is pitch. Amplitude is the height of a sound wave. Frequency represents how often a sound wave goes through a full cycle. As a sound wave travels across distance, the amplitude decreases. This is why we can't hear a quiet whisper across a room.
Information provided by Genetic Science Learning Center, University of Utah, http://gslc.genetics.utah.edu."