See pressure waves created by sound waves with your phone in real time!

The purpose of this instructable is to share a unique, low cost and sensitive Schlieren optical setup that does not require the use of a razor or edge in front of the camera lens.

This setup was created with the primary intent to visualize pressure differentials in the air caused by sound waves. The acoustic field is created using ultrasonic transducers set up in a manner to achieve acoustic levitation, such as those created by TinyLev or by resonant standing waves (Harvard Natural Sciences example). This setup can of course also be used to visualize most other forms of Schlieren as well.

Step 1: The Basic Physics Behind Schlieren Photography

There are many explanations, examples, and ways to accomplish visualization of Schlieren. I won't go into great depth re-explaining the details but rather touch lightly on the key points. If you are here chances are you already have a sufficient understanding of the physics.

Schlieren (from German meaning "streaks") are optical inhomogeneities in transparent material not necessarily visible to the human eye. Such as those caused by the refraction of light from heat waves or acoustic pressure waves in air, neither of which we can see under "normal" conditions.

The basic phenomena you are seeing when looking at a schlieren image or video is light being refracted by the difference in air density caused by a variety of means. With a dash of science, we can amplify the effect of this refraction. The small change in the path of the light causes it to not fall on the camera, resulting in a dark area, or vice versa. Three common physical processes we encounter every day, usually without realizing it, that modify air density relative to neighboring air are:

heating or cooling it
sound traveling through it
introducing other gasses to the "air"

Above are a couple drawings roughly demonstrating what we are doing to accomplish the visualization of these phenomena. I encourage you to read more.

More info:

http://www.google.com/search?q=schlieren

https://en.wikipedia.org/wiki/Schlieren

https://en.wikipedia.org/wiki/Schlieren_photograph...

https://en.wikipedia.org/wiki/Schlieren_imaging

Step 2: Mirror and Mount

I am using a 160MM Cassini telescope primary mirror which can be found on eBay for about $70. This mirror is great for this purpose, it's handmade and well tested. It also has a long focal length of 1300mm which is great. The longer the focal length the greater the divergence from the focal point will be when the light is refracted by the subject being observed. This means that a small change in refraction can easily move the light away from our tiny focal point resulting in an image that is modified by that refraction.

I created a simple mount to cradle the mirror that can be found here: Spherical Primary Mirror Mount - 160mm by thingengineer https://www.thingiverse.com/thing:2586322

The mirror can be glued to this base or sit freely for easy storage or other use later.

160mm Spherical Primary Mirror Mount.stl
Download View in 3D

Step 3: Full Linear Stage Option

I am currently using a full traditional liner stage to allow fine adjustment in the X and Z plane (or up/down, left/right from the perspective of the camera). There is a nice one on Thingiverse developed by user oet found here:

Linear Stage 95x95mm M6 by oet https://www.thingi verse.com/thing:1351132

Here is a list of parts I purchased on Amazon to complete the linear stage. This is everything you'll need other than printing the plastic parts themselves.

Some of these are kits so obviously it's more than you need but you can always use this kind of stuff on other projects! You would be a bit better off with actual 8mm linear motion rod vs the lathe bar. However, the lathe bar came in a size that was close enough I could just grind it down to fit rather than bother with cutting stainless steel. The lathe bar worked just fine.

You will also need the following items to complete the tripod mounted linear stage with phone holder:

My GoPro Mount and Hinge for Linear Stage by thingengineer https://www.thingiver se.com/thing:2586302
The parts for the universal phone holder in the Modular Mounting System by yyh1002 https://www.thingiverse.com /thing:2194278
The GoPro Tripod mount by tmorris9 https://www.thingi verse.com/thing:2194278

I printed everything in PETG but PLA would work just as well. All of the STL files required have been uploaded to this Instructable but make sure to check out the links to the Thingiverse files to like the cool things and credit the creators!

Step 4: Specialized Micro Linear Stage Option

The traditional linear stage in the previous step works well but its range of motion is overkill for what we need in this scenario. Realistically about 1mm of movement in the XY plane is all that is needed to position the focused point source of light onto our lens as long as you are close on your initial course adjustment.

I am working on a specialized linear stage (prototype is shown above) that can be printed in one part (minus knobs if you want them). This new linear stage:

has enough range of motion to make fine adjustments to the location of the focused light source.
is much lighter thus reducing the requirement for a heavy/robust mounting platform.
is much cheaper to make, the only parts that aren't printed are the M4 hex bolts and nuts.
uses built-in printed plastic springs that can be adjusted in size to hold heavier phones or cameras.
looks cool because it is cool.
has a tab for mounting a razor or color film if you want to experiment with higher contrasts of refraction.
has a tab for drilling a pinhole so you do not have to use a piece of tape or foil on the phone's LED.

As of the date this Instructable was published I just have a couple more features to tweak before testing and publishing it on Thingiverse.

Step 5: Tripod Alternative

Another mounting option if you don't have a tripod is this:

Bottle cap mount for Go Pro by 3delivery https://www.thingiverse.com/thing:503338

Or the "Flat Stand" that is included in the Modular Mounting System above. Although neither were sturdy or heavy enough for the full linear stage with the phone mounted in my opinion. You're better off with a heavier tripod or vice such as the PanaVice shown above.

Step 6: Point Light Source

The LED on the iPhone can be used as is with some test subjects that create lots of refraction such as the flame from a candle. To see weaker levels of refraction we need a very fine point of light. This is easy to accomplish by just covering the LED with something that blocks light and has a small hole in it. With a smaller light source, a smaller amount of refraction reduces the total light going into the camera by a greater percentage than if there was a larger light source. (This is the same reason shadows from smaller light sources are sharper than those from larger ones.)

I've shown a few different methods to make a pinhole cover for the LED.

Black electrical tape punched with pencil lead or something similar.
Foil surrounded by electrical tape with a window cut out. The foil creates a sharper and smaller pin hole.
Foil tape. Easiest method.

When using foil color the front of it with a black marker after punching the hole. This ensures a sharper, high contrast light source especially if there is light is reflecting off a source in front of the phone.

You can also experiment with a colored film over the LED to create different looks and in some cases even highlight specific features.

Another interesting thing to experiment with is the placement of the pinhole filter over the LED. There are multiple LED's in the iPhone's "flash", each has a different color temperature. Depending on where you place your pinhole you will see a different colored light source. This can be advantageous or deconstructive to your viewing experience depending on the subject.

Step 7: Setup

In this example I am using an iPhone 7, I have not tested other phones or cameras.

You want to set the distance between the mirror and your tripod at 2 x the focal length of your mirror. The mirror I am using has a focal length of 1300mm so 1300mm x 2 = 2600mm = about 102 inches.

The face of the mirror and phone need to be as parallel as possible to reduce lensing artifacts. Adjustments should be made in any direction except for throwing off these parallel planes. If you know your mirror is level you can use the compass/level app to level the phone.

Open the Camera App or whatever photography app you are using. Some apps give better control over focus, exposure, LED light level, etc and can be a big help but the default camera app will work fine.

Turn on the flash (LED). If you have your tripod close to the correct spot you may already see the focused point of light on or near the front of the phone. If not it helps to get a sheet of white paper and turn the lights down to find it. Here you are just trying to rough in the location of the spot by moving the tripod in all directions, keeping in mind the parallel plane we are trying to preserve.

Once you have a sharply focused point of light on the face of the phone on or very near the camera you can switch to the fine adjustment if you are using an XY translation table or something similar. Dial in the spot so it is centered on the camera lens.

You should now be able to see an overexposed whitish light in the mirror. If there are lobes protruding from the edges of the mirror do your best to make adjustments to center the light within the mirror. If you have lens flare effects off to the side of the mirror your phone and mirror planes are not parallel. It will still work if you don't do this but I have experienced better end results with near perfect alignment.

Once your point light source, mirror and camera are aligned you can begin to adjust the focus and exposure to start seeing Schlieren. Rising heat from a small tea light candle is a good subject to use when trying this for the first time. It is generally easy to see the Schlieren effect from this type of subject even if almost everything is going wrong.

Use focus lock - holding your finger down on a location to lock the focus and exposure level. Then scroll up or down next to the focus box to change the exposure level. It may take a few tries to get the focus to lock where you want it and the exposure level at the same time.

Once you are comfortable using focus lock and are able to get nice clear images of the heat rising off the candle by adjusting the exposure level and tweaking camera position you can move on to subjects that are more difficult to see such as the acoustic fields generated by TinyLev.

The subject being observed should be as close to the mirror as possible, be careful with flame sources such as the candle obviously. This will increase the amount of divergence from refraction that the light undergoes before reaching the camera thus increasing the visible Schlieren effect.

Step 8: Conclusion

Conditions at the time and location of testing were approximately:

50 feet above sea level
72 degrees Fahrenheit
25% relative humidity

I have experimented with adding a razor to cut out additional light at the focal point in front of the camera lens as well as using add-on telephoto lenses to provide greater zoom. Each of these individually or together reduced the ability to see weak refraction such as that from the TinyLev acoustic field. If anyone as different results I would be interested in hearing them.

Step 9: Video Results

In video #:

Around 30 seconds, I move the TinyLev away from the mirror. The acoustic field is no longer visible at about two feet away.
I am adjusting the exposure level.
The pinhole is over the warmer colored LED versus the cooler colored one.
I am moving a reflector through the acoustic field. You can see the formation of standing waves at every multiple of 1/2 wavelength.
I am moving a suspended object up and down using the phase shift built into TinyLev.
A 25KHz transducer with a reflector moving up creating standing waves at every multiple of 1/2 wavelength. Also shown is the application of alcohol to accentuate the pressure waves with the fumes. The focus wasn't great in this one.
A 25KHz Transducer pair is running at 2 x wavelength. There is water on the bottom transducer. When the amplitude is turned up the water turns to mist and forms droplets near the anti-nodes. This isn't using the Schlieren setup but it's cool!