20 kHz range: Rats emit long (> 1 second) 20 kHz vocalizations when they are unhappy or stressed. These calls are emitted when an adult or juvenile is defeated socially, sees a predator, experiences pain or anticipation of pain (Antoniadis 1999), or when an untame rat is handled. This is sometimes referred to as a 22 kHz vocalization, and sometimes the range of 20-30 or 20 - 35 kHz is mentioned for these vocalizations. The larger range is especially applicable to infants.
30 to 50 kHz range: Infant rats produce very high pitched distress calls. These cries elicit maternal care such as retrieving the infants to the nest. Juveniles also vocalize in this range (and up to 70 kHz) during play and aggression.
The range of frequencies for a given call varies not only with the age of the rats, but also between individuals.