School of Physics Lecture Demonstration Manual

Wa-5 Speed of Sound (Density Dependence)

Published: Tuesday 14 March 2006 - Updated: Wednesday 30 March 2011


To demonstrate the dependance of the velocity of sound upon the density of the carrier gas.


Helium Gas Balloon (Mixture of Helium and Oxygen)


This dramatic and fun filled demonstration requires the lecturer to inhale a quantity of Helium gas. The lungs are first exhaled and then a deep breath of helium is taken to completely fill the lungs. To satisfy the velocity wavelength relationship with the lighter gas the pitch of the lecturers voice is raised to a degree similar to that of Donald Duck. Please Note: This demonstration should not be attempted without prior consultation with the lecture demonstration staff. Balloon gas should not be used and BOC gases warns of the danger due to the lack of oxygen if pure Helium is used.

Safety notes

Personal safety


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