Last year, a team composed of scientists from the USA and the Philippines observed calls of the Philippine tarsier that is purely within the ultrasonic domain . They said that these “are among the highest recorded for any terrestrial mammal, and a relatively extreme example of ultrasonic communication.”
Listen to the Philippine tarsier’s ultrasonic voice with 8 times slower frequency here  (but I have to warn you that you may find it irritating):[audio http://now.dartmouth.edu/wp-content/uploads/2012/02/tarsier.mp3]
This year one of the researchers in  observed ultrasonic calls that differ in frequencies.
The chirp which is as short as a fraction of one thousandth of a second has an average audio frequency of 22 kHz. The twitter has a duration of about 120 millisecond at an average audio frequency of 38 kHz. The whistle has the longest duration at an average audio frequency of 76 kHz . For comparison, the audible frequency of an average human is between 0 to 20 kHz.The researcher said that these ultrasonic calls most likely serve as distress calls. However, the whistle according to the author may have a specific purpose as it can only be ‘heard’ on certain circumstances during the study.
 Ramsier MA, Cunningham AJ, Moritz GL, Finneran JJ, Williams CV, Ong PS, Gursky-Doyen SL, & Dominy NJ (2012). Primate communication in the pure ultrasound. Biology letters, 8 (4), 508-11 PMID: 2319094; http://now.dartmouth.edu/2012/02/tiny-primate-is-ultrasonic-communicator-dartmouth-professor-finds/
 Gursky-Doyen S (2013). Acoustic characterization of ultrasonic vocalizations by a nocturnal primate Tarsius syrichta. Primates; journal of primatology, 54 (3), 293-9 PMID: 23549838
 Tarsier and shadow images are from wiki commons.