Voiceless Nasal Sounds in Three Tibeto-Burman Languages : This paper focuses on two types of voiceless nasal sounds in Xumi, a Tibeto-Burman language: (1) the voiceless aspirated nasals /m̥/ [m̥h̃] and /n̥/ [n̥h̃]; and (2) the voiceless nasal glottal fricative [h̃]. We provide a synchronic description of these two types of sounds, and explore their similarities and differences. Xumi voiceless nasal consonants are described with reference to the voiceless nasal consonants /m̥/ and /n̥/ in Burmese and Kham Tibetan, because Burmese voiceless nasals are the best described type of voiceless nasals, and are therefore used as a reference point for comparison; whereas voiceless nasals in Kham Tibetan, which is in close contact with Xumi, represent a characteristic regional feature. The synchronic description is based on acoustic and aerodynamic measurements (the total duration of the target phonemes, the duration of the voiced period during the target phonemes, mean nasal and oral flow). Our study (i) contributes to a better understanding of voiceless nasals as a type of sound; (ii) provides a first-ever instrumental description (acoustic and aerodynamic) of the voiceless nasal glottal fricative [h̃], as attested in a number of Tibeto-Burman languages of Southwest China; and (iii) suggests a possible phonetic basis for the observed dialectal and diachronic variation between voiceless nasals and [h̃] in some Tibeto-Burman languages.