Today I got my set of cheap borosilicate test tubes, 25 x 150 mm, for
my acoustic laser experiments. My best "stack" remains a 20 mm tall sample
from the PSU.EDU group. Its heated by 3 coil-segments of nichrome from a
hair dryer and a PC power supply. Using less than 3 coils worked but the coils
burnt out quickly. With the 3-coil heater I could run for a minute before the
cool side of the stack heated. I wrapped the hot end in aluminum foil and pointed
the PC power supply fan output at the cool end, and could run for minutes and minutes.
(Until the wife told me to stop because the kid was going to sleep.)

I love the point where the gain is just enough to make it "ring" ---you blow across
the tube and the tone continues for a while, decaying slowly, exponentially. Until
the gain is enough to lase, and the tone continues indefinately. A spectrum shows
a narrow line when this happens, about 500 hz for my setup.

I also found that a radiometer some distance away was vibrating (not spinning, but
rocking) which was an interesting resonance.

I need to get either a schottky diode to rectify a loudspeaker-as-acoustic-to-electric
converter output OR get a multimeter that can read AC. To see if I can harvest power.

I also find that you need an absorber to the *side* of the thing for best performance. My body
worked, also a spherical ornament decorated with pointy lights. Its all about the
reflection at the open end.

I will have to see how far it carries outside, tomorrow.