Nearby explosions and open mouth

[bunyip]

I recall an old war film many years ago where the Germans are going to blow a bridge and they tell the civilians hiding in the nearby tunnel to cover their ears and open their mouths as protection against the blast wave.
Why?Does an open mouth help to prevent lung damage by equalising the pressure in and out?
I mentioned this to someone who had used explosives in the Royal Engineers but he had never heard of doing this.
Is this just a myth?

[Ringo]

From here:

Q: In some movies scenes involving explosives actors are asked to keep their mouth open and warned that if they don’t their eardrums could implode. Why would a blast from explosives require the actors to keep their mouths open?

A: When the explosion goes off there is a pressure wave caused by the rapid burring of materials and thus the rapid change in volume of the explosive. If the actor’s mouth is closed, air at the pre-explosion pressure is trapped in the actor’s sinus area. The shock wave causes an area of high pressure to form around the actor. When the actor’s mouth is closed, the difference of pressure between the sinus area and the outside air can cause the ear drums to burst inward. However, if the actor’s mouth is open, the pressure wave will be the same both inside and outside the actor’s mouth and the actor should be safe. This same effect happens (although with less severity) when you go diving or when you go from high to low altitudes.

[Mangetout]

The channel that links your middle ear (the cavity behind your eardrum) and the outside world (via your throat)is called the eustachian tube; it’s this that makes a popping sound when you experience a difference in air pressure; when you swallow or yawn*, it opens up, making an audible click and equalising the pressure.

*I can open my eustachian tubes voluntarily.

[Wikkit]

*Originally posted by Mangetout *
***I can open my eustachian tubes voluntarily. **

Can’t most people. I can sit here for minutes popping my ears. It’s a requirement of diving to any depth, isn’t it? I know SCUBA divers can be put out of commission by cold when the tubes are plugged, but I’d think most people would be able to do it voluntarily if they knew what they were doing.

I’ve been pretty close to ground bombs, which are a couple pounds of flash powder that is detonated hanging from a frame. It can make your ears ring a bit, but I’ve never blown an eardrum before by having my mouth closed. Larger scale detonations may be a different matter, though.

[Mangetout]

Not everybody. can do it voluntarily (at least not without swallowing or opening their mouth.