Rijka: Singing Heat

Optics, Waves & Sound arc, page 116 of 193. Back to The Pixelated Physics Universe

Who they are

Rijka breathes fire into a glowing pipe, and it roars back a single pure tone.

Rijka is a character of the Optics, Waves & Sound arc: drawn in pixels, ruled by one piece of real physics, and never allowed to break it.

The physics

In 1859 P. L. Rijke put a hot metal gauze inside a vertical tube open at both ends and heard it sing loudly. The sound stops when the tube is laid horizontal and the upward air flow stops. Rayleigh gave the reason in 1878: an acoustic oscillation grows if heat is added where and when the pressure is high. In an open–open tube the fundamental has a pressure node at each end. The drive is strongest with the gauze about a quarter of the way up from the bottom; a gauze in the upper half damps the sound instead. The same feedback causes dangerous combustion instability in rocket engines and gas turbines.

$$ \begin{gathered} f \approx \frac{c}{2L} \end{gathered} $$

Honest caveat: c is the warm-gas value, so the pitch depends on how hot the air is. Rijke used a hot gauze, not a flame; flame versions are variants.

Sources

• Rijke (1859) Notiz über eine neue Art, die in einer an beiden Enden offenen Röhre enthaltene Luft in Schwingungen zu versetzen, Ann. Phys. 183, 339
• Rayleigh (1878) The explanation of certain acoustical phenomena, Nature 18, 319

Crossovers

Rijka meets Oscillayne (Harmonic Pulse / Waveform Resonance). Rijka's pipe sings because the heated gauze adds heat in step with the sound, feeding energy into the acoustic mode. It is Oscillayne's resonance driven by heat instead of a push.

Rijka meets Ringward (Q Factor Resonance). Rijka's tube rings at its natural acoustic mode. Ringward points out that heat input must outrun the losses that set the Q, or the tone dies away.

Wear the lore

• Rijka – Singing Heat Physics T-Shirt

Concept keywords: Singing Heat, Rijka, Optics, Waves & Sound, physics lore, Pixelated Physics