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Frogs and toads mostly call with their mouths shut, unlike many other vertebrates. Sound is generated when air crosses the larynx, but there is no direct airflow to the external environment and radiation occurs at the skin. This study directly compares the acoustic output obtained from euthanized frogs with the mouth open against the output obtained with the mouth closed during activation of the larynx by airflow. With the mouth closed, the vocal sac was inflated and the acoustic energy was concentrated in the same harmonics as in the advertisement call, whereas with the mouth open, energy was spread in a wide range of harmonics. The acoustic output at the dominant frequency was more intense with the mouth closed than with the mouth open. More sound was radiated through the vocal sac and head than through the rest of the body. The spectral differences between open and closed mouth treatments matched the differences observed between natural advertisement calls, produced with the mouth closed, and distress calls, produced with the mouth open. By calling with the mouth closed, treefrogs can potentially produce advertisement calls with the energy concentrated in a narrower frequency range than with the mouth open.

Most divers can speak easily but cannot whistle at elevated pressure. Suggestively, sperm whales are not known to whistle. However, rigid flutes or bird calls remain easy to blow “at depth”; frequency changes little. The author can just whistle tunes at 4 atm (equivalent water depth of 30 m). At one atmosphere he could whistle with a breath of half air and half sulfur hexafluoride but not with pure SF6 (density relative to air of 5.1); alternate whistling in and out aids the observation. Possibly this is due to increasing gas density yielding damping vibrations in the soft tissues shedding vortices. Caution: Limiting duration prevents suffocation; possible impurities in SF6 are toxic and corrode metal. The SF6 lowers whisper and speech tones but not hum and whistle frequencies; reflex changes in the throat can confuse the latter. A colleague, Jon Pegg, can whistle tunes normally as well as through his nose with mouth closed and vocal cords constricted to a slit. His nose whistle quenches at 20 m and the normal one at 10 m, suggesting sites of different stiffness. It is said to be impossible for organ pipes to sound if the stream is denser than the surroundings. Several blew well (frequency lowered) with octafluoropropane (density 6.5) in air.