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Acoustic Laser

This is a really neat experiment. Tobin Hahn brought to my attention a picture in an article concerning thermo- acoustic engines. Click HERE to read the article.

The article didn't contain much information concerning the acoustic laser itself, other than to say that it was invented by Reh-Lin Chen at Pennsylvania State University as a simple demonstration of the thermo-acoustic principle. It also stated that catalytic converter material, which is a ceramic honeycomb, was used to construct the element of the acoustic laser. The picture however made it look quite simple. This project sat on the back burner until a student at the Mendocino High School decided to make one for a science project. If you know of any other information about this project in existence, E-MAIL me. I couldn't find any. Which actually made this project even more fun!

(NOTE: I have since spoken with Steven Garrett, the author of the article. See the Update Notes at the end of this page.)

All pictures can be clicked for a larger image.

Materials required for the Acoustic Laser (AL):

Test tube, 25mm by 200mm or larger
Piece of catalytic converter substrate (see below).
6" heating element ribbon from an unloved toaster.
2 one foot long pieces of cloth covered wire (regular wire can be used, see below).
Power supply. This can be in the form of a variac (a variable AC power supply) or a small variable DC power supply. See below for more information on this.

Safety Notes: The element gets hot enough to burn, and so does the tube. It is possible for the glass to shatter, although lab grade test tubes of Pyrex or similar material shouldn't. I haven't personally had one break, but avoid running these devices for more than a couple of minutes. Working the catalytic converter material to the right size is also potentially dangerous. Just use (not so) common sense.

I looked around for catalytic converter material, which I discovered is called "substrate". I wasn't having any luck finding the small quantity needed until I ran across Allen Carroll, the very kind general sales Manager at Applied Ceramics. He asked what size we needed, and offered to send us some free of charge! Which he did very promptly. Thanks, Allen!

This is what the substrate looks like. It measures 5.91" X 5.91" X 1".

This stuff is really cool to look through. Talk about tunnel vision, very weird effect. It is easy enough to cut, but is very brittle, and the slightest mistake will break it. I used a wet tile saw to cut strips out of the material, and then used a knife to cut it to the size needed. Using the tile saw got the substrate wet, which caused a lot of condensation in the tube. I'm lucky it didn't crack the tube. Note that this is a dangerous operation that must be performed by a person well experienced in using tools. A much better method is to cut it with a fine tooth hacksaw and then clean it up with a file. Here is a picture of a finished piece.

Notice that I cut the corners back. This allows a snug fit in the tube. This operation has the greatest chance of ruining your element and making you cry. Take your time! However, unless you really mess it up, like break it in half, it will probably still work.

It is time for the friendless toaster to end it's toasting days. Disassemble and remove the heating ribbon from it. You can also use heater wire from an electric space heater, but it is stiffer and harder to wrap on the element. Save the insulating board that the element was wrapped on, it's handy for other projects. There are a few other parts in there you should hang on to, like a small coil.

Now it is time to wrap the heating element onto the substrate. First, notch the substrate to accommodate the element.

Wrapping the element is tricky and a little time consuming. I use cloth covered wire because the insulation is more heat resistant and doesn't smell as nasty when it burns. You can use regular wire as well, but the plastic will stink as it melts. Strip about two inches of wire from the end of both wires. Poke the wires through the honeycomb so that the bare end sticks out on the end you notched, with the wires diagonal to each other as shown in the picture below. If your wire won't fit through the honeycomb, you can run it along the side, as shown in the last picture. The disadvantage with this is that the wire can create hot spots that will heat the tube up much more quickly, forcing you to shut it down and allow it to cool. Remember you can click for larger images!

Attach one end of the heating wire to one of the wires poking through the element. Wrap it carefully through the notches in the honeycomb, working and wiggling it in with your thumb. When you get there, connect the other end. Clip the ends off fairly short, and bend them gently in so they won't contact the tube when in place.

Now, stuff the completed element into the tube. Poke it in with a pencil or something. If the heating wire comes off the substrate a little when you push it in, it will probably work anyway, try it before you pull it out an re-wrap it unless it is almost all the way off.

As you can see in the picture below, I have tried a number of different sizes of tubes. Note that the smallest size didn't work. I also had trouble with another slightly larger tube. I would recommend a 25mm by 200mm test tube or larger. Columns would probably work as well. The large tube pictured I obtained from a glass blower's shop for US $20. It measures 1-1/2" outside diameter, and is 18" long.

Powering it up requires a power supply, of course. I used a variac, which is a device that allows precise control over household voltages. I run mine at about 5-10 volts depending on the size of the AL. Small DC power supplies can be used too. The main thing is to increase the voltage slowly, or you will blow the heating wire like a fuse and have to re-wrap it.

I had no idea what to expect when I plugged it in. If anything, I thought it would produce white noise. What it does make however, is a tone that matches the fundamental note of the tube (the same tone it makes when you blow across the mouth of the tube). This is why I ended up making several out of different sized tubes. I have hooked them together in series to run them all at once, producing a neat effect and a fair amount of noise. The amount of sound vibration one can feel at the end of the tube is impressive.

If you restrict the airflow into the tube, you will silence the AL. When properly warm, it will operate in any position, including upside down. A wide range of element placements within the tube work, but too near the open end won't. There are odd dead spots in the sound, walk in a circle slowly around the AL while it is operating. Once the substrate gets too warm, it will stop making noise. It seems that this happens quicker on the smaller tubes, but on my large tube I can run it until the glass gets hot enough to force me to turn it off.

I constructed a stand that allows me to point the AL in any direction using the clamp assembly from a clamp style lamp I had laying around. Another victim to science. I flattened the end of the fixture that attaches to the lamp and drilled two holes in each side to put screws through. A picture shows it much better then I can describe it.

The article mentioned above also showed the AL being operated using solar power. In the works is constructing a Fresnel lens and trying this out.

How Does It Work? When heat is applied to one end of the element, the temperature difference between the heated end and the unheated causes a "thermal pump" or "siphon". This draws air through the holes in the element and pushes it out the other end. When the air is heated it wants to expand, but in the confines of the honeycomb it can't, and so it is compressed. This compression raises the air's temperature even more. As the air travels to the cool side of the element, it contracts. This process of expansion and contraction within the confines of the honeycombs in the element cause the acoustic vibrations. The tube acts as an organ pipe or "acoustic resonator", which causes a "standing wave" of sound to build in amplitude as it bounces back and forth inside the tube. This is similar to the cavity in a laser creating the "coherent" light of a laser, hence the term "acoustic laser".

Update Notes: Since I wrote the above page, I have spoken with Steven Garrett, the author of the article that started my experiments. It turns out that Penn State University sells a kit with everything you need to make an AL. I'm glad I didn't know that, I had a lot of fun mucking around with this project. I have a few pieces of substrate that I will send free of charge to any die-hards who want to make their own from scratch.