Rarefaction is the reduction of an items density. Rarefaction is the opposite of compression. It is a technique to look at a species richness.
When something vibrates, like a loudspeaker, it compresses the air and increases the pressure of the air when it moves out. When the loudspeaker moves in it creates a partial vacum and a decrease in air pressure. The increase/decrease in air pressure is propagated thru the air (or any medium),as a longitudinal wave, by pushing the air in front of it. The part of the cycle which produces the decrease in pressure is called a "rarefacation" of the air.
When something vibrates, like a loudspeaker, it compresses the air and increases the pressure of the air when it moves out. When the loudspeaker moves in it creates a partial vacum and a decrease in air pressure. The increase/decrease in air pressure is propagated thru the air (or any medium),as a longitudinal wave, by pushing the air in front of it. The part of the cycle which produces the decrease in pressure is called a "rarefacation" of the air.