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The Trombone ForumCreation and PerformanceMusical Miscellany(Moderators: JP, BGuttman) An interesting experiment with helium and Sulfur hexaflouride!
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Author Topic: An interesting experiment with helium and Sulfur hexaflouride!  (Read 458 times)
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davdud101
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« on: Jan 02, 2018, 01:41PM »

Came across this video while looking up prank videos. Very interesting stuff!!!!

https://www.youtube.com/watch?v=bKt81ZSMADE
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« Reply #1 on: Jan 02, 2018, 01:46PM »

All based on the density of the medium.

But I'd think using either of them as a means to adjust pitch is a little expensive.  And probably dangerous.
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« Reply #2 on: Jan 02, 2018, 01:50PM »

So... a denser column of air resonates at a lower frequency, just as a heavier string resonates at a lower frequency.


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« Reply #3 on: Jan 02, 2018, 03:17PM »

Actually, it may relate to the speed of sound.  That is a function of the density of the medium.
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« Reply #4 on: Jan 02, 2018, 03:38PM »

Definitely, the time taken for the sound to reach the end of the bell changes, producing a change in pitch.

I'm guessing that based on what we know of how much air actually makes it through the trombone while playing (very little) that they are actively blowing a large volume of air through the horn while the guy plays to produce a change in pitch that quickly, and changing the proportion of blown room air to added helium/SF6 via valves.
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« Reply #5 on: Jan 02, 2018, 03:40PM »

And yes, that is dangerous. SF6, while not directly toxic, can displace breathable air if enough of it accumulates in a room.
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« Reply #6 on: Jan 02, 2018, 04:06PM »

And yes, that is dangerous. SF6, while not directly toxic, can displace breathable air if enough of it accumulates in a room.

So can helium.  Helium from the ceiling and SF6 from the floor.
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« Reply #7 on: Jan 02, 2018, 05:39PM »

Here's another interesting sulphur hexaflouride demonstration...

https://youtu.be/1PJTq2xQiQ0
<a href="https://www.youtube.com/v/1PJTq2xQiQ0" target="_blank">https://www.youtube.com/v/1PJTq2xQiQ0</a>

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« Reply #8 on: Jan 03, 2018, 08:27AM »

Actually, it may relate to the speed of sound.  That is a function of the density of the medium.
Don't think so on this one Bruce.  I think it is entirely due to the mass of the gas column.

I just noticed something and I could be wrong.  I have to take the pooch for a walk but will be back to research this a bit and do some calculations. Good!
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« Reply #9 on: Jan 03, 2018, 08:47AM »

Don't think so on this one Bruce.  I think it is entirely due to the mass of the gas column.

The speed of sound must be involved somehow because that governs how long it takes for the compression wave emitted form your embouchure to travel the length of the horn and reflect back from the open end.

Wiki says...

Quote
In low molecular weight gases such as helium, sound propagates faster as compared to heavier gases such as xenon.


Whether this is analogous to the variation of the weight of a string, I can't ascertain.
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« Reply #10 on: Jan 03, 2018, 08:56AM »

Whether this is analogous to the variation of the weight of a string, I can't ascertain.
Vibration of a string is a different kind of wave than those that would depend on the speed of sound in the medium (steel or air).  All we have to do is determine if the sound produced by a trombone is as a result of 'shaking' of the air column, that would be like a vibrating string (displaced mass and restoring force - weight on a spring kind of thing), or due to propagation of a pressure wave.  I'll do that in a bit and get back to all.
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« Reply #11 on: Jan 03, 2018, 11:17AM »

A Bb trombone measures at ~2.8M long.  However, it does not have the same resonance frequencies of a straight tube that length.  The bell, lead pipe, mouthpiece and oral cavity all contribute to the effective length, which is a bit more than the measured length and varies according to the frequency and those physical factors.  We can't pre-determine all these factors here so we'll just go with the measured length and 'expect' the result will be off by a bit.  Given that the speed of sound is about 330 M/sec and we assume that the sound produced by a trombone is due to a pressure wave traveling at the speed of sound (rather than a vibrating column of gas) then we should expect the frequency would therefore be f = speed/length.

Using the values for the measured length of a trombone and the speed of sound in air we get f = 330/2.8 = ~118Hz, which is in pretty good agreement with Bb2 = 116.6Hz

I can come up with no such calculation for the mass of the air column as I have no fixed restoring force.  So, gas density (or mass of the air column) has nothing to do with the situation and my fist 'impression' was wrong.  It is indeed the speed of sound in the gas that is the determining factor.  This is born out by the fact that, given a constant air temperature, trombones play in the same pitch in Denver as they do in Los Angeles despite the air being 12% less dense in Denver.

Unfortunately, the way they did the experiment they were not using pure gases.  Both the He and the SF6 are contaminated with air.  However, the correlation between the frequency and the speed of sound in those gases is closer than the correlation to density by a factor of better than 2.

The frequency for the Helium is 2.47 times the frequency in air, the Frequency for the SF6 = .67 times the frequency in air

The speed of sound in He is 2.94 times the speed in air, the speed of sound in SF6 is .40 times the speed in air

The density of Air is 6.9 times the density of He, the density of air is .20 the density of SF6.

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« Reply #12 on: Jan 03, 2018, 11:43AM »

This paper describes the velocity of sound in air and helium:

http://hyperphysics.phy-astr.gsu.edu/hbase/Sound/souspe3.html

When we do a ideal gas model the speed is the Bulk Modulus divided by the density.  For real gases it's sqrt(gamma*R*T/M) where Gamma is the restorative constant, R is the universal Gas constant, T is the absolute temperature, and M is the molecular mass of the gas (similar to density).
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« Reply #13 on: Jan 03, 2018, 12:07PM »

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and we assume that the sound produced by a trombone is due to a pressure wave traveling at the speed of sound (rather than a vibrating column of gas)

I would say that a pressure wave traveling the length of the column IS a vibrating column.  It's really the only way the column can vibrate and resonate.
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« Reply #14 on: Jan 03, 2018, 12:37PM »

I would say that a pressure wave traveling the length of the column IS a vibrating column.  It's really the only way the column can vibrate and resonate.
For a pressure wave or sound wave in a gas the net displacement is zero if measured along the entire tube.  In the case of standing waves, where the pressure amplitude is highest, there is no displacement whatsoever.  This is different than the whole column shaking back and forth as in a displacement wave.  In gases, because they are compressible, pressure waves do cause some displacement of the molecules, but in our case (the trombone) this is not very significant because the pressure amplitude (even at fff) is small and can be considered perturbations.  For incompressible fluids, like water, pressure waves cause no displacement at all.  It is also easier to see displacement waves in water too.  These are the waves we are familiar with that cause variations in water height and break onto sandy beaches.

If your math is good, you can have a look at this: http://farside.ph.utexas.edu/teaching/315/Waves/node30.html
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« Reply #15 on: Jan 03, 2018, 05:38PM »

Now this is my kind of thread!  :D
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« Reply #16 on: Jan 03, 2018, 08:04PM »

This paper describes the velocity of sound in air and helium:

http://hyperphysics.phy-astr.gsu.edu/hbase/Sound/souspe3.html

When we do a ideal gas model the speed is the Bulk Modulus divided by the density.  For real gases it's sqrt(gamma*R*T/M) where Gamma is the restorative constant, R is the universal Gas constant, T is the absolute temperature, and M is the molecular mass of the gas (similar to density).

The molecular mass of a gas is not the density.  There is a relationship, but it changes from substance to substance.  Even in ’ideal’ gases, the closest we have are the noble gases, the speed of sound  is dependant on the molecular mass as is given on the page you linked to, not the density.   I think you might be confusing molecular mass with molar mass.  Molar mass is density, molecular mass is not.
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« Reply #17 on: Jan 03, 2018, 08:26PM »

Molar mass is density, molecular mass is not.

There is some correlation none-the-less, right?

Helium does turn out to be the less dense and sulpher hexaflouride the more dense, no?
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« Reply #18 on: Jan 03, 2018, 10:34PM »

There is some correlation none-the-less, right?

Helium does turn out to be the less dense and sulpher hexaflouride the more dense, no?

Only in the crudest possible sense, and there is no 'simple' formulation of this.

For example, chlorine and and argon have almost the same molecular mass with chlorine being slightly lower than argon, however the density of chlorine is is 3.2 g/L and the density of argon is only 1.4 g/L.  Wildly in the opposite direction from what you might assume from their molecular masses.  And this is just an example of 'simple' elemental gases.  When we move to multi-element molecular gases, the variances are off the wall.  Any semblance of a usable correlation goes out the window.  It turns out that Helium and SF6 correlate well (actually really well) WRT molecular mass and density, but that is mostly due to their both being non-polar non-reactive gases.  An exception rather than a rule.

So, overwhelmingly the speed of sound in a gas is dependent on molecular mass (and temperature, and that adiabatic constant that is characteristic of the specific gas - being the gorilla in the room) rather than density.
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