How does one sound drown out another? If I'm laying in bed at night with the fan on, I cannot hear nearby traffic. If the fan is off, I can hear the traffic. What is/are the principle(s) involved in this phenomenon?
 A: This is called masking. Here's how it works.
Sound power levels are measured in decibels (relative to a standard reference level) where to cover the dynamic range response (loudest versus softest perceivable sound level) of the human ear, the scale is logarithmic where each increase in dB of +3 units represents a doubling of the sound power level- which is just barely perceptible to the ear. For a sound source to appear twice as loud on this scale as another requires it to be ten times as powerful.
This means in practical terms that given two sound sources going at the same time, if one is +6dB or more louder than the other (that means greater than or equal to 4 times more powerful), your ear will respond to only the louder one and you will be almost completely unable to hear the softer one at all. We say the softer sound is being masked by the louder one.
Sound masking is commonly used to "hide" an annoying sound source (like a busy city street or a noisy office environment) with a source of random noise (like that produced by the blades of an electric fan).
