The effects of inhaled helium on the human voice were investigated in a recent article in The Physics Teacher.1 As mentioned in that article, demonstrations of the effect are a popular classroom activity. If the number of YouTube videos is any indication, the effects of sulfur hexafluoride on the human voice are equally popular. However, there appears to be little information available on the effects of either of these gases on musical instruments.2 We describe here the results of a student project that involved measuring the frequency shifts in an organ pipe, a trumpet, and a trombone as the result of filling the instruments with these two gases. The project was one of several possible end-of-semester projects required in an elective science of sound course for non-science majors.

1.
C. D.
Wentworth
, “
Helium speech: An application of standing waves
,”
Phys. Teach.
49
,
212
215
(
April 2011
).
2.
B.
Holmes
, “
The helium-filled organ pipe
,”
Phys. Teach.
27
,
218
219
(
March 1989
).
3.
T. D.
Rossing
,
F. R.
Moore
, and
P. A.
Wheeler
,
The Science of Sound
(
Pearson Education Inc.
,
2002
), p.
65
.
4.
Audacity: The Free, Cross-Platform Sound Editor
, http://audacity.sourceforge.net/.
5.
N. H.
Fletcher
and
T. D.
Rossing
,
The Physics of Musical Instruments
, 2nd ed. (
Springer Science
,
1998
), p.
196
.
6.
M. J.
Ruiz
, “
Boomwhackers and end-pipe corrections
,”
Phys. Teach.
52
,
73
(
Feb. 2014
).
7.
Ref. 5, p. 560.
8.
In fact the velocity of sound in a gas mixture can be used to determine the ratio of gases in a mixture of two gases. For an interesting example, see
A.
Haggermann
 et al. “
Speed of sound measurements and the methane abundance in Titan's atmosphere
,”
Icarus
189
,
538
543
(
Aug. 2007
).
9.
Sulfur hexafluoride, although inert, is a powerful greenhouse gas. We encourage anyone using this gas in demonstrations to make a nice film of the experiment, post it on a website such as YouTube so we can all see it, and show the film in class instead of repeatedly releasing the gas into the atmosphere.
10.
The speed of sound in SF6 is reported in various places on the Internet as 150 m/s, which is incorrect. We verified our calculated value with the National Institute of Standards and Technology website: http://webbook.nist.gov/chemistry/fluid/.
11.
M. C.
LoPresto
, “
Mouthpiece and bell effects on trombone resonance
,”
Phys. Teach.
52
,
26
29
(
Jan. 2014
).
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