Re: Why are drivers cautioned to not blow horn in a tunnel

The answer to your question is resonance cavities. 

What is a resonance cavity? 
A cavity or hollow space in a certain area always has specific dimensions. 
These dimensions will give rise to what is called standing waves. A 
standing wave is formed when a wave is reflected back from a surface in 
such a way that it reflects back onto it self. Thus the wave is amplified 
and a sound wave will become much louder. When a horn is blown in a tunnel 
it is always a possibility that the specific frequency of the horn might 
be the same as the resonance frequency of the tunnel cavity. This might 
amplify the horn’s sound, which possibly could have effects like breaking 
a driver’s attention and causing an accident. I am not certain to how much 
the sound can be amplified – but if it was a very load horn there is the 
possibility that damage to the ears can be done – This is not likely but 
always a possibility. The amplification depends on how much of the wave is 
reflected onto itself, the frequency of the wave and the density of the 
air in the tunnel,and the dimentions of the tunnelalso plays a role.