Sound with a wavelength outside the audible range of the human ear. A ~20 kHz frequency is associated with possible discomfortDiscomfort A state of pain and distress in laboratory animals., a 50 kHz frequency is considered environmentally acceptable. 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. is associated with a state of animal welfareWelfare A complex and dynamic internal state that includes: successful biologic functioning, positive experiences, and absence of negative conditions. The animal adapts effectively to internal needs and external stimuli and maintains a good mental state. in that animals may emit more when they are stressed.