Pressure at nodes and antinodes

[David Housden]

Hi Graham
An analysis of the Ruben's tube was published some time ago in AJP (1983).
Howard Lukefahr and John Hannah's site (www.physics.school.nz) make
reference to this on
http://www.vuw.ac.nz/scps-demos/demos/Light_and_Waves/SoundFlames/SoundFlame
s.htm 
The AJP paper says that both situations can occur depending on sound
intensity and gas flow.
Cheers
David Housden


-----Original Message-----
From: phys-teach-talk-bounces at nzip.org.nz
[mailto:phys-teach-talk-bounces at nzip.org.nz] On Behalf Of g.foster
Sent: Wednesday, 18 June 2008 6:41 p.m.
To: Phys-teach-talk at nzip.org.nz
Subject: Pressure at nodes and antinodes

Greetings Fellow Physies
We have just made a long metallic tube with a loud speaker
at one end and gas entry at the other. This enables us to
light the gas which comes through 1 mm holes 1.0 cm apart
and to produce single frequency sound at the other. The
flame pattern at resonance is one where we see high flames
at the antinodes and small flames at the nodes. Many of you
will have seen similar appartaus in photos. 
However we are discussing why we get high and low flames and
we would appreciate clarification. If the high flames are at
the antinode this is where there is maximum variation in gas
pressure, so sometimes there is low pressure and at other
times a higher pressure than normal. At the nodes the gas
pressure remains constant at normal pressure and we think
that is where we get lower flames.
Question: Is a high flame produced at the antinode? 
Our reasoning: 
For at least half the time there is higher pressure than
'normal'? Since the frquency for the best effect seems to be
1 kHz to 2 kHz this would imply that there is higher than
usual gas pressure for at least 50% of the time and this
pushes more gas out the holes.
Comments please.

Thanks
Graham Foster
EGGS Auckland

Graham Foster
Director of Science, EGGS
AMI Learned Society

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