I Interesting sound-making hydrogen-burning device

[Spinnor]

There is an interesting video on the Wiki page for Hydrogen. A metal enclosure made of two funnels with a smaller hole at the top and a larger hole at the bottom is initially is filled with hydrogen gas. The covered holes are opened and the hydrogen escaping the top hole is lit. After a few seconds sound is produced. The pitch of the sound decreases and then there is a bang. Some interesting physics you might enjoy and puzzle over.

https://en.wikipedia.org/wiki/File:19._Експлозија_на_смеса_од_водород_и_воздух.webm

 

[Baluncore]

My guess.
The flame at the top is unstable as it burns above the resonant chamber top hole.
The speed of sound in H (1310 m/s) is about four times that in air, (343 m/s), (so MW H₂ ≈ 1/4² that of air ?).
Each flame pulse, draws more air in from below, reducing H concentration until below 75% H. Sound frequency falls as air enters.
When the flash-point is reached, flame can enter the resonator, and consume the remaining 2H₂ + O₂ from the internal air.

 

[Lnewqban]

Copied from that page and linked ones:

"Explosion of a hydrogen–air mixture. The bi-conical vessel is filled with hydrogen gas. The hydrogen is ignited and starts to burn, while air enters from the bottom, making an air–hydrogen mixture that is slowly air-enriched. When the critical composition is approached, the burning becomes unstable and produces a sound wave with a decreasing frequency (due to the oscillations of the air-enriching mixture). Once the critical composition is reached, the bang is inevitable."

"Explosive combustion of hydrogen. Escaping hydrogen is ignited, while the removal of the bottom cap allows air to enter. Eventually, the air mixes with the hydrogen inside the container, causing an explosion. A similar process occurs during a backdraft, with the introduction of oxygen and mixing with unburnt gases causing abrupt or even explosive combustion."
https://en.wikipedia.org/wiki/Backdraft

"At normal atmospheric pressure [the flammability limit] is 4% to 75%, based on the volume percent of hydrogen in oxygen it is 4% to 94%, while the limits of detonability of hydrogen in air are 18.3% to 59% by volume."
https://en.wikipedia.org/wiki/Hydrogen_safety