Introduction: Music Controlled Fog Curtain! DIY Ruben's Fog Tube!
You may or may not have seen the Instructable and YouTube video for my flexible/adjustable Rubens Tube, but after receiving some feedback it is clear that there were a few unwanted holes in the tube itself causing fires and in my explanation that kept it from resonating with people. So lets kick the tires and light the fires to see if we can clear up some of the smoke around how this phenomenon works.
Now in saying that you really walked into this one because ironically we are going to be using smoke/fog to explain what is going on inside the tube to create this phenomenon. So what were the biggest disconnects that I received feedback on about the original tube? They were centered around the standing wave and what is happening in the tube.
- What is a standing wave and how does it happen inside the tube?
- What is it doing to the inside of the tube?
I will break these two questions down in the first few steps of this instructable but first lets explain what motivated this project then we will get to the material and tools used to make this project happen in case you want to make one for yourself.
One of the primary goals is to add a new dimension to our understanding about what is happening inside the Ruben's Tube to make the flames of different heights and the easiest way that I could think of to see inside something is to make it clear. However making it clear will not be enough by itself. What good is being able to see inside something if what you are trying to see is not visible? I do not have synesthesia so I can not see sound so to make this happen I need something that I can see to interact with the sound waves. This is where the fog comes into play, hopefully we can see variations in the air that we otherwise can not see.
I hoped to build this project in two stages to learn two things about how the fog interacts in a standing wave tube.
First, I tested using a completely sealed tube with no vent holes to see if we can observe the compression and rarefaction in the tube that causes the different flame heights. This will only require a small amount of fog to be present in the tube because if there is too much then I might not be able to tell where rarefaction and compression are taking place.
Second, because Explosions in the Sky is one of my favorite bands and I think it would be awesome to make some effects for a future tour (NOTE: if anyone reading this knows anyone please send me a direct message) I am going to test to see if we can modulate a fog curtain using only sound!
Now onto the hardware needed to make this project a reality.
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- Clear Tube
- I went with a plastic tube for this first one but Glass could be really interesting to work with.
- Rigid End Cap
- Bluetooth enables Audio Amp
- Acoustic Exciter
- 4" x 12' Flexible Corrugated Tubing (Tubing to feed the fog into the clear tube)
- 4" Drain Coupler (Used to help attach one of the end caps)
- Two 4" Drain End Cap (Home Depot has these)
- 3/4" GHT Male x 1/2" MIP Male Brass Garden Hose connection
- 3/4" GHT Female x 1/2" MIP Female Brass Garden Hose Connection
- Fog Machine
- Party Balloon
Step 1: What Is a Standing Wave and How Does It Happen Inside the Tube?
Just like those classic commercials say, "Everyone wants a Slinky" ... to explain standing waves. If you set a slinky on the ground and hold one end fixed and wiggle the other end with your hand at just the right speed you will see an interesting formation occur. It will look like there is a spot in the middle of the slinky that is not moving at all while each half on either side is swinging back and forth wildly. This is a standing wave with a wavelength that is the length of the slinky. Unfortunately I could not find an untangled Slinky so I resorted to using the whiteboard above my desk.
What causes this is two sources of energy that are put into the slinky. The first one is your hand wiggling the slinky and the second is the echo from the fixed point at the other end. If the phase of the the energy inputted from your hand and the energy reflected from the fixed end are the same they will add to each other and line up to create a standing wave in the slinky through constructive interference.
Sound is also wave that travels through matter much like the wave travels through a slinky. It has points of high pressure and low pressure that occur at a rate that matches the frequency of the sound. This means that if you have a 100 Hertz tone it is emitting 100 wavelengths or pulses each second, Hertz is a unit of 1 / seconds. If we know how fast sound travels we can even figure out the length of the wave or wavelength. Taking the standard speed of sound in air to be 343 meters per second (m/s) for our 100 hertz tone we can take the following equation to find out how long the wave is:
Wavelength = Speed of Sound / Frequency
Wavelength = 343 m/s / 100 1/s [Now when we reduce this fraction and the units along with it]
Wavelength = 3.43 m
This means that if we want to make a standing wave for our 100 Hertz tone we will need something that is designed to accommodate the 3.43 meters long wavelength.
If we look inside our tube where we have a speaker on one end that replaces our hand in the slinky analogy and a fixed cap that serves as the echoing surface we can generate a standing wave. Now this is where our tube analogy departs from the slinky. With the slinky the fixed end that echos that energy back is a node, a point where the wave looks fixed and also our hand looks fixed like a node also. However with our speaker in the tube it will be an anti-node and look like the wildly swinging part of the slinky. This means that when we are calculating our standing waves in the tube we will need to be thinking about it where the echo point is a node and the speaker is an anti-node. This would make the tube that we need to make a 100 Hertz standing wave only 3/4 of the wavelength long instead of a full wavelength.
Step 2: What Is Going on Inside the Tube?
Now that we understand how a standing wave can be created it is important to understand what it is doing inside the tube. If we look at our standing wave there are two extremes.
- Nodes: These are the seemingly fixed points in our wave. What this "fixed" characteristic means is that there is very little energy difference at this point in the wave and therefore very little pressure change. It will be a point in our standing wave that is consistently close to the regular pressure in the tube if there was no standing wave at all. This would look like a rather calm part in our standing wave
- Anti-Nodes: These are those crazy kids that are just running all over the place literally bouncing off the walls. This points swing between higher pressure and lower pressure. If we turn the sound energy up high enough we get to a point where the low pressure taking that standard pressure in that section of the tub low enough that it is creating a small partial vacuum which would pull in air from the area around it. When there are vent holes this results in air being pulled in which reduces the concentration of the fog. This will look like there is a considerable amount of turbulence.
This website does a great job of looking at what happens to the contents of the tube depending on how much sound pressure exists in the tube compared to the gas pressure. Definitely worth a read if you are interested.
Step 3: Constructing the Fog Collection Tube and Fog Machine Connection
This project has me thinking that I need to make my own fog machine especially after seeing how simple they look and their limitations, but that is an idea for another day. (Leave me a comment if this would be something you would be interested in seeing and follow me on Instructables and YouTube to stay updated for when this project hits)
That said to make this project work I will still need a fog machine so I searched online and found a few reasonable options but thankfully my neighbor was kind enough to lend me his fog machine that he uses in his yard for Halloween but after looking it over the output nozzle on the machine did not readily fit any sort of connector that I could find.
However that ends up working rather well because many of the fog curtain or fog piping systems I have seen online suggest that it is important to let regular air mix with the fog after it exits the machine to let the fog cool and expand. What I have as a solution I think is a good compromise. I am using 12' section of 4" flexible drainage ducting. This lets me use a cheap and easy to work with 4" end caps. Measuring the output nozzle of the fog machine it is ~2" in diameter so using a simple hole saw into the center of the cap gives me the desired hole to put the fog machine nozzle into. The rest of the work that I did to this end cap was to drill a collection of holes around this input hole to allow for regular air to be pulled into the flexible tube to mix with the fog as it expands. I did not attach this end cap very tight because I want to be able to open up the tube to let it dry out between uses since I know that there will be condensation inside the tube after each use.
The output end of the fog collection tube was a little bit trickier to figure out about how best to attach it to the pipe. I wanted something that would offer a large connection to not restrict the fog but would still be simple to install and operate. I settled on using garden hose connectors which offer about a ~3/4" cross section and a good seal.
Putting the garden hose connector on the other end of the flexible drainage pipe was pretty simple as well. The threading will fit tightly in a 7/8" hole in the end cap allowing the brass fitting to be threaded directly into the soft plastic making an effective connection. Using the #4 Step Drill pit I made my 7/8" hole and then using a pliers to get a better grip in the Brass Fitting I threaded the fitting into the other end cap.
Now that I had both of my end caps completed I placed them onto my flexible drainage tube. One end is configured to receive an end cap directly while the other needs a female-female adapter. I decided to put the end that needs the adapter on the end that connects to Fog machine in-case in my testing I want to drill a few more air inlet holes without ruining the 12' tube.
Step 4: Creating the Acoustic Exciter End Cap
This step is going to look a lot like the my last instructable for the flexible Rubens Tube where we will take a flexible membrane and connect an acoustic exciter. I actually burned the acoustic exciter used in the picture out and had to move to the 24 watt exciter that I have listed in the materials list. That said the construction is the exact same.
Using a party balloon make a clean cut closer to the mouth piece to help facilitate a tighter fit onto the clear tube. Carefully stretch the balloon membrane across the clear tube ensuring that it is tight on the end. This is important to help the acoustic exciter to be effective by having a semi rigid diaphragm to connect to.
Next taking the acoustic exciter I applied it to the membrane. I started with a smaller 19mm exciter to see if that would work using my higher powered driver board, but like I said above this ended up burning out so I switched to a larger exciter. This was simple wiring taking leads from the exciter and wiring them to the speaker output on the bluetooth driver board.
This should finish up the acoustic exciter cap.
Note: Through my testing I actually burned the 19mm driver up and I moved to a larger 24W driver which is why the 24W driver is the one in the parts list at the beginning of the instructable.
Step 5: Rigid End Cap
The rigid end cap is also where the fog is injected. I am hoping that the fog inlet on this end cap does not disrupt the sound too much undermining our desired effect of achieving a standing wave.
I ordered some matching tube caps from the internet that fit perfectly into the end of my 2 1/2" clear tube to get a strong seal. Then using my handy Step Drill bit I drilled to 3/4" to accomplish the same threading solution that I had used for the other brass fitting from the fog sourcing tube.
With this hole drilled I grabbed my pliers again and threaded the brass fitting in the end cap and pushed the end cap into the tube.
This completes the clear tube before I drilled any holes so we can see if there is visible compression and rarefaction in the tube. If you just want to make a modulated fog curtain then skip the next step.
I did not need to apply any sort of sealant to the hose adapter or to the contacts of the cap with the tube since this is a very low pressure connection friction fitting this components provided a sufficient seal.
Step 6: Testing a Sealed Tube
The first thing I did was warm up the fog machine and make sure that we are getting the smoke out of it that we are interested in. This will take a few minutes since it is having to warm up the internal coil that it uses to evaporate the liquid fog precursor. Once this is all warmed up I ran through a few testing steps of running the fog machine stand alone and then testing the fog collection tube to see how well the fog passes through this tube and the garden hose opening at the end.
Fog flowed smoothly from the collection tube out of the hose connector so I moved on to testing it with the tube and seeing if I could see the impact of the standing wave inside the tube.
Sweeping through the frequencies on the 36" tube which equates to ~0.914 meters we should be able to determine approximately what frequencies will result in standing wave. One thing that we I needed to account for was that I am at a relatively high elevation of ~5400 feet. This actually moves the speed of sound for me from 343 m/s to something closer to 320 m/s. Looking back at that first step and what wavelength multiples will most likely generate standing waves in this system. We are looking at one of the following wavelength multiples:
0.75, 1.25, 1.75, 2.25, 2.75, 3.25, 3.75 ...
Lets try calculating what the frequencies of the 2.75 multiple would be.
frequency = Speed of Wave / Wavelength
Wavelength = 0.914 m / 2.75
Wavelength = 0.33236 m
Frequency = (320 m/s) / (0.33236 m )
Frequency ~= 963 Hz
Similarly 1.75 would result in ~612 Hz.
Now that you are ready to see the results of the standing wave in the sealed tube I have some bad news. If you have ever tried turning a bottle upside down to drain water out and you see the water struggling to get out of the bottle because the air and the water are having a hard time replacing each other. We have the same issue here where the air in the sealed tube has no where to go but through the same hole that the fog is trying to come in through so the fog is unable to displace the air which means that the fog struggles to enter the tube. It is for this reason that I moved forward and I started to drill the holes. Sorry for the small disappointment but I am hoping that we will see that desired effected in the next experiment.
There however is a consolation in the next step because we get to test this idea by filling the perforated tube with fog and shutting the source off allowing the fog to settle in the bottom of the tube.
Step 7: Drilling Holes in the Tube
Ok. Some of you might be thinking enough of this science smoke and mirrors lets get to the modulate fog curtain. Well to make that happen I need to drill a few holes... strike that a lot of holes.
I started small thinking that I could always go bigger so my first attempt was with a 3/64" bit but that turned out to be too small to allow the fog to move in. Next I went wit ha 1/16" bit which was better but still not satisfying enough. I settled on using a 3/32" bit to make the holes large enough but not so large that I would start to have a problem with my 1/4" spacing.
I decided to go with a 1/4" hole spacing with 3" gap on both ends. This gives me a 30" section with a hole every 1/4" which means I drilled 120 holes into this plastic tube.
I marked out the line I intended on drilling with some painters tape and used the tape to hold my markings as well so that I would not need to draw directly on the tube reducing clean up and hopefully eliminating walking and scratches.
The result was a nice hole riddled tube. Now if only my back would quit complaining after being bent over for so long drilling all of those holes three times.
Step 8: Testing the Finalized Tube
Now it is time to pull back the curtain to see if we can make a fog curtain that we can control.
Priming the fog tube is as easy as flipping a few switches on the fog machine. The machine's pump starts to hum and there is a rhythmic sound sort of like an old steam powered train as the fog starts to flow out and fill the collection tube. It then rolls out of the collection tube into the Fog Ruben's tube like a low lying storm front. Quickly the tube fills completely with fog as it gets pushed out of the holes on the top of the tube into 120 jets of fog.
Testing to see if there is a variation in the flow of the fog when a standing wave is achieved is hard to see. The flow in some sections becomes more turbulent and in others it is calmer. I am thinking that the turbulent sections are the points of compression becasue the high pressure pushes the fog out but the acoustic wave pressure is so high that when it inverts to a low pressure it tries to pull the fog back into the tube which cause the turbulent appearance. The smooth flow sections I am thinking are the nodes where a more constant pressure exists allowing for the smoother flow of the fog. All that said I could not see much change inside the tube with a standing wave until I turned the fog machine off.
Once the fog machine is turned off there is a considerable amount of fog that settles in the tube because I am using a low lying fog mixture. This fog sits at the bottom spread evenly along the tube until a standing wave is hit. When a resonant standing wave is struck there are clear points of turbulence and calm in the tube. The nodes look just like what they did before the sound was turned on smooth pocket of fog in the tube. The anti-nodes however are mixing the air turbulently stirring the fog off of the bottom of the tube and pushing it out of the vent holes.
I will turning the sound up to 11 to see if there is a way to modulate the fog coming from the tube when the fog machine is in full production mode.
Step 9: Summary Thoughts
Wow. What an interesting project. Like I said in the stage where I built the fog collector I will definitely be looking into making a fun and portable fog machine.
Also like I said early if anyone knows anyone that turns in the same circles as Explosions in the Sky send me a message because it would be great to connect to see if we could make some beautiful music together.
Ok, summary thoughts....
The picture above is what happens when there is insufficient air for the fog to expand into. There is considerable amount of condensation and that quality of the fog is super weak.
I was really hoping to be able to see the test with the sealed tube work but after trying it it makes perfect sense why the fog would not fill the tube.
Now once I had enough and big enough holes drilled into the tube the fog would creep into the clear tube.This allowed me to effective test the seal tube hypothesis with the fog being able to flow in but since it was heavier than air it settled into the bottom of the tube away from the venting holes until the acoustic exciter was turned on.
Now looking forward if this were to find a practical purpose as a modulated fog curtain it should be much big so i would likely consider designing a solution to match 4" plastic drainage tube.
I did really enjoy seeing how after I had shut off the fog machine and the fog was settling in the tube that at some frequencies it looked undisturbed and at others there was clear turbulence which I think is attributed to anti-nodes in the tube or where compression would be occurring. While I was never able to acheive a strong enough standing wave to stop the flow of the fog from the tube I am still curious if it is possible if there was just a less overwhelming amount of fog.
All told I really enjoyed this project.
Leave me a comment on the YouTube video or the instructable if you have other projects ideas or feedback on this project!
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