Parnell’s mustached bat,Pteronotusparnellii, uses echolocation to hunt for insects in the New World tropics. The sonar signals, which are emitted from the mouth, are long, constant‐frequency types with a ‘‘missing’’ fundamental, so that the second and third harmonics contain most of the pulse energy. This pulse structure implies that the supraglottal vocal tract acts as an acoustic notch filter for the fundamental. To investigate this possibility, light and heavy gases were used to obtain the vocal tract transfer function. The results show that a fundamental notch filter is present, and anatomical manipulations of the vocal tract suggest that this filtering is achieved by a combination of antiphasic reflections from the nasal passages and an area‐function filter in the throat. During these experiments, tracheal sound was also monitored to assess the acoustical role of the tracheal sac. This is an elastic section of the trachea, situated immediately posterior to the larynx, which inflates during vocalization. While demonstrating that this structure plays a significant role in determining the acoustics of the subglottal system, our results neither confirm nor dismiss the hypothesis that it reflects sound back to the larynx to improve vocal efficiency. As part of the investigation, the radiation pattern for the mustached bat was also obtained and was compared with various model sources. At the second and third harmonics, sound radiation is highly directional and consistent with the idea that the mouth acts as a conical horn.