This paper is associated with a video winner of a 2018 APS/DFD Milton van Dyke Award for work presented at the DFD Gallery of Fluid Motion. The original video is available online at the Gallery of Fluid Motion, https://doi.org/10.1103/APS.DFD.2018.GFM.V0018 -- Resumen del vídeo: Propagation of premixed flames in closed vessels is a problem of great importance in science and technological applications. The interaction of the acoustic waves with the reacting front is found widely in power-generation devices, which can compromise their structural integrity. A Hele Shaw cell, of very small thickness, has been used to study the transition of the thermoacoustic oscillations and its dependence on the fuel mixture properties. Two regimes are identified for different values above and below a critical equivalence ratio, which develop strong pulses of the advancing flame and weak oscillations respectively.