Astronauts cannot whistle inside their spacesuits because the air pressure is too low to create sound vibrations.

The physics of whistling depends on creating a resonant standing wave within a cavity of air. To produce a clear tone, air molecules must be dense enough to create significant turbulence as they pass over the lips or tongue. Inside a NASA Extravehicular Mobility Unit (EMU), the internal pressure is maintained at approximately 4.3 pounds per square inch (psi), compared to the 14.7 psi found at sea level on Earth.This specific pressure was chosen to provide enough oxygen for the astronaut while keeping the suit flexible enough for movement. However, this low pressure means there are fewer air molecules available to vibrate. When an astronaut attempts to whistle, the low-density air fails to generate the necessary acoustic feedback loop required to sustain a musical note.Astronaut Dan Barry famously attempted to whistle during his STS-72 mission in 1996 and found it nearly impossible. He noted that while he could move the air, the sound produced was more of a soft 'whoosh' than a distinct whistle. This phenomenon is a direct result of the Reynolds number of the airflow being too low to trigger the fluid-dynamic instability needed for sound.NASA engineers and researchers have confirmed that the speed of sound is also slightly affected by the composition of the air in the suit, which is 100% oxygen. While sound still travels through this medium, the lack of atmospheric mass makes the energy transfer for high-frequency sounds like whistling extremely inefficient. This remains one of the many strange sensory changes humans experience when working in the vacuum of space.