Noise emitted from a fan is comprised of various mixed waves. People perceive this mixture of waves as sound pressure and sound quality. Sound pressure is a longitudinal wave of pressure fluctuations that is transmitted through the air. The unit of measurement used is decibel. Sound can also be described as a sine wave, where the loudness is dictated by the amplitude. In the figure here showing the red and blue sine waves, one can see that the red wave describes a louder sound than the blue wave.