Echo chamber: Carlisle Castle dungeon

It was a lot cooler down in the dungeon. Sound took up to 2 seconds to die away. The reverberation was incredible. Sound must repeatedly bounce from wall to wall with little attenuation each time due to the hard surface. We thought that perhaps the arched roof helped to focus sound waves. I estimated height and width of about 6 m. At 330 m/s the sound would take about 0.02 seconds to go from one side the other so there could be up to 100 reflections before it dies away. I tried humming at different frequencies. There was definitely a resonance at a low frequency. The lowest a human can hear is about 20 Hz. The fundamental mode (1st harmonic) for a 6m chamber would have a wavelength of 12 m. So frequency = wave speed/wavelength = 330/12 = 27.5 Hz. Sounds feasible.