The Acoustic Laser

John W. Dooley, Physics Department
Millersville University

Earlier this winter I took two physics students to Pennsylvania State University for a work shop on building an acoustic laser. Both students came home with a working acoustic laser. The devices are contained in a 1-cm diameter "test tube" about 20 cm in length. When active, they produce a nearly pure tone at about 400 Hz.

The Acoustic Refrigeration Group at Penn State has information and even a kit that you can buy to make your own acoustic laser. The web address to start with is:
http://www.acs.psu.edu/thermoacoustics/refrigeration/laser-
demo.htm

Figure 1 is a photograph of the Penn State device. (The device runs very hot. Note the burn mark on the table.)

The laser is so powerful that you can feel the acoustic wind coming from the mouth of the tube.

To understand how it works, we can start with an ordinary standing wave in a tube, as in an earlier article. The tube is closed at one end and open at the other. Air is free to move in and out of the open end, but is trapped when it moves toward the closed end.

At the open end, the air pressure is held at atmospheric pressure by the air in the room. At the closed end, the pressure rises above atmospheric pressure when air rushes into the tube. The pressure at the closed end falls below atmospheric pressure when air rushes out of the tube.

Figure 2 shows a sketch of the tube and two graphs. The graph below is a plot of air velocity versus position in the tube. The graph above is a plot of air pressure versus position in the tube. The graphs represent four different instants in time.

The solid squares for the velocity graph represent the instant when air rushes into the mouth of the tube at the greatest rate. The open squares for velocity represent the air rushing out of the tube mouth at the greatest rate. Throughout the process, the air velocity at the closed end is fixed at zero. This is the air velocity that you can feel with your hand at the mouth of the tube.

The solid square pressure graph represents the instant when air has stopped moving into the tube, making the pressure at the closed end the largest possible pressure. The open squares represent the instant when air has stopped rushing out of the tube, pulling the pressure below atmospheric pressure at the closed end.

In the previous article, we pumped sound energy into the tube the easy way, by putting a small loudspeaker at the mouth of the tube. In the acoustic laser, the energy is delivered at the closed end of the pipe. To see how, we take several steps.

First, we place a membrane near the closed end as illustrated in Fig. 3. It's purpose is to focus attention on that region. The membrane is like a slack sail, bulging left or right, depending on which way air is flowing in the tube.

Second, we build a piston into the closed end of the tube. The piston moves in out, and is synchronized with the sound pressure. The piston moves in when the sound pressure is high and out when it is low. The piston raises the high pressure gas to an even higher pressure, and pulls the pressure lower at the low pressure part of the cycle.

The effect is to increase the pressure swing in the tube, and increase the amplitude of the sound oscillation. The piston does work on the gas and increases the sound energy.

In the acoustic laser, the pressure at the closed end is not raised by a piston. Now pressure is raised by adding heat to the region at the closed end (Fig. 4). The heat is kept near the closed end by a ceramic plug, as in the sketch. The plug material was designed for use in catalytic converters for cars. It is full of tubes that allow air to move back and forth through it. Like the membrane, the plug allows the sound oscillation to continue.

As the sound wave continues through a cycle, air flows into the hot region, raising the pressure as usual. The heater warms this air, and cause the pressure to rise above the pressure from the sound vibration alone.

Continuing the cycle, air flows out of the hot region, down the tube, where it is cooled by its surroundings. (Remember that the ceramic plug causes a temperature gradient from hot on the left to cool on the right.) As the gas flows out, the pressure ordinarily drops.

Because the moving gas is also being cooled, the pressure drops even more (Fig. 5). Without this cooling, the effect of the heater would simply be to raise the ambient pressure. Combining the cooling with the heating cause the amplitude of the sound wave to rise. The heater pumps energy into the sound wave.

The action is similar to a Stirling "displacer" engine in which the working gas is moved from one place to another to alternately heat and cool it.

The device is similar to a laser in that it starts with a non-equilibrium situation (high local temperature instead of overpopulated high-energy quantum states). The wave itself stimulates change toward equilibrium, carrying heat past the plug and dumping it at the cool end of the tube. In the process the amplitude of the wave is increased.

I thank Steve Garrett and Matt Poese of the thermoacoustics group at Penn State University for offering this device and for helping me to understand it. Their primary interest is thermoacoustic refrigeration. (They recently installed one of their environmentally non-destructive refrigerators at Ben and Jerry's in New York.) Their primary interest includes education and outreach. For this reason they offer the acoustic laser as a doorway into this developing field.