Disclaimer:

Reprinted by permission of Dick Berg, University of Maryland, for use on this website.

The demonstrations contained and referenced herein are listed for the purposes of cataloging and describing physics demonstrations which should be conducted only under the direction of a trained instructional support professional or physicist. These demonstrations are not presented for the purpose of being conducted by persons unconnected to this Facility and/or persons not consulting with or being supervised by the recognized instructional support professional or physicist and his/her staff. The University is responsible only for those demonstrations carried out using its own equipment using established safety and scheduling policies, and bears no responsibility for those choosing to use this source material for their own purposes. All demonstrations described and contained herein are public domain, and can also be found in reference materials in libraries, bookstores, and electronic sources.

Further information regarding legal liability in use of demonstrations and labs will be found on the web site Injuries in School/College Laboratories in USA.

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When breathing helium your vocal tract acts as a filter for the sounds produced by the vocal folds.  In effect, the higher frequencies are enhanced and the intensity of the lower frequencies is lessened. 

To show that the frequency does not change when replacing air with helium, use a Whoopee-Cushion.  The Whoopee-Cushion will make the same sound when filled with air or helium.  To simulate the vocal tract, add a plastic tube to the Whoopee-Cushion.  The tone quality will change due to the resonance effect.  The Whoopee-Cushion frequency that corresponds to the natural frequency of the chosen tube will be amplified.