Sound waves with frequencies higher than 20 kHz beyond the audible range for a normal human being. Dog whistles operate at frequencies around the 30 kHz region. Bats are thought to be able to "hear" ultrasonic sounds up to about 100 kHz. These are the standardised frequency response shapes as defined by international noise standards. They are designed to replicate aspects of the human ears frequency response to different noise sources. Standard frequency weightings are ‘A’ (most generally specified), ‘C’ (used for higher noise levels), ‘Z’ or Lin scale (the un-weighted or "true" frequency content).