Sounds produced by animals that may or may not (ultrasoundUltrasound Sound with a wavelength outside the audible range of the human ear. A ~20 kHz frequency is associated with possible discomfort, a 50 kHz frequency is considered environmentally acceptable. Ultrasound is associated with a state of animal welfare in that animals may emit more when they are stressed., infrasound) be audible to humans. The rat associates 20 kHz sounds with fear or painPain The negative sensory or emotional experience that indicates awareness in the animal of injury or the threat of injury to the body. This negative experience induces changes in an animal’s behavior and physiology, intended to limit the effect of, or avoid the injury, to reduce the chances of repeated injury and to promote recovery. and 50 kHz sounds with pleasant situations.