The eardrum is a thin, translucent membrane that is about half a pinky finger deep inside the ear. The ear canal has an S-shaped curve that makes the eardrum outwardly invisible and more difficult to damage (except with cotton swabs). The eardrum is designed to pick up sound waves and transmit them to the bones for amplification.
But the ear canal is not the only way to the eardrum. From the pharynx, which is an extension of the mouth, there is a passage upward to the eardrum, the Eustachian tube. It is needed to equalize the pressure between the outside of the eardrum and the inside of the eardrum – this is necessary for proper wave transmission.
If the mouth is closed, during an explosion, the sound wave coming through the external ear canal (ear side) dramatically increases the pressure from outside the eardrum, causing it to bend in the direction of less pressure – inside. And if the blast wave is strong enough, the eardrum may not be able to withstand the pressure and burst.
If the mouth is open, the blast wave travels to the eardrum not only through the external ear canal (ear side), but also through the mouth-pharynx-Eustachian tube. Both the ear and pharyngeal sides of the eardrum will be subjected to the same pressure, balancing each other out.
For the same reason you should not plug your ears with your mouth open at the same time during an explosion – in this case the pressure from the mouth side will be higher than from the ears side, and the eardrum will bend and rupture, only in the other direction.
So during an explosion, you either plug your ears and shut your mouth. Or don’t plug your ears and open your mouth.
When listening to loud music, it is also better to keep your mouth open in order to reduce harm.
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