For Derby Maker Faire 2016, we made several very loud Rijke tubes.
What are Rijke tubes?
Most noises you hear every day come from something that moves, whether that is a speaker, your vocal chords, the strings of a guitar, the skin of a drum, or your lungs blowing air into a wind instrument.
Instead, Rijke tubes use heat to cause a self oscillating effect caused by thermoacoustic amplification, which I will try to explain below. The noise generated is a very loud eerie pure tone, as you can hear in the video.
The tubes Pieter Rijke used to demonstrate his newly found effect in 1859 used a metal gauze that would be heated by flame and then make noise once the heat source was removed, until they cooled down. These would make noise for 10 seconds at a time. Instead we use an electric heating element to keep the sound going indefinitely.
How do they work?
A hot filament located a quarter of the way up the tube causes the air to heat up. Hot air rises, maintaining a convection current in the tube. When a sound wave of the right pitch enters the tube, it creates an increase in pressure in the tube. Air rushing in at the bottom slows as it pushes against this pressure. The slowing means this air receives less heat so the average temperature of the air in the tube cools a little. Cold air contracts lowering the pressure in the tube and sucking the air in faster. This causes more heating and a rise in average temperature, causing the air to expand, starting the cycle again.
All this happens at hundreds of times a second, making the eerie sound.
We used nichrome filaments wrapped as coiled coils onto mica supports. The first supports were cut by hand, but soon we found that laser cutting the mica gave really good accurate structures, allowing tighter packing of filament. This meant enabling more power, upping the volume. We got ours creating 92dBA at full power!