system
Given the following post, respond as if you're commenting on Reddit:
human
What is the lowest speed of sound a material may have?
gpt
In liquid helium there is something called "second sound" where heat propagates through a material like a sound wave. Very close to and below the temperature at which helium becomes a superfluid, this second sound can propagate at arbitrarily low speeds, but speeds up as it gets colder. In terms of "normal" sound propagation, in gaseous helium close to its melting point the speed of sound is nominally just above 100 m/s.