An old mining tunnel disappears into a hillside. You would like to know how long the tunnel is, but it’s too dangerous to go inside. However, you figure out a way: using your subsonic loudspeaker and amplifier, you find resonances at 4.5 Hz and 6.3Hz, and at no frequencies in among these. It’s rather chilly inside the tunnel, so you estimate the sound speed to be 335m/s. What’s the result of your measurement?