Little tunnel with echoes; big tunnel without on the way to Threlkeld

 

We walked the old railway line from Keswick to Threlkeld. The longer tunnel has been reopened. A child was trying to make echoes inside but it was acoustically dead. Later we went into the much shorter tunnel. This one had very clear echoes.

So question of the week is about why this happens. We noticed that the long tunnel was not at its original depth. You can tell because the recesses for the workers to hide in when the train came past would only accommodate very small children now! The short tunnel is much deeper. Is that the reason? Or is it to do with the length? I also need to think about possible changes in acoustic impedance as the air goes from the enclosed tunnel out into the air. Is that enough to cause a significant reflection? We were in the middle of the big tunnel when we concluded that there were no echoes. Might there be echoes closer to the end. I have found no answers yet but I have found this brilliant webpage about a tunnel in Sheffield that might need a visit!