The human ear is divided into three parts: outer ear, middle ear and inner ear. There is a membrane less than110 mm between the outer ear and the middle ear, which is called tympanic membrane. In fact, the auditory nerve in the inner ear should really feel the sound. When sound waves arrive, the eardrum is first vibrated, and then the vibration will spread from the eardrum to the middle ear and finally to the inner ear. When the plane takes off or lands, due to the vertical acceleration, the gas in the cabin will move downward or upward relative to the plane due to inertia. When the plane takes off, the gas in the engine room can drop to a lower position relative to the plane. At this time, the pressure at the bottom of the engine room will increase and the pressure at the upper part of the engine room will decrease. Similarly, when the plane lands, the gas relative to the plane is concentrated to the upper part. At this time, the pressure at the top of the engine room will increase, but the pressure at the lower part of the engine room will decrease. In the process of accelerating takeoff and landing, the surface of the eardrum is subjected to external air pressure, but this pressure is constantly changing.

If a person keeps his mouth closed, a closed system will be formed from the closed throat and glottis to the oral cavity and tympanic membrane, and the pressure cannot change rapidly with the external air pressure. When taking off, the external pressure is greater than the internal pressure, and the eardrum will inevitably sag inward; On the contrary, when landing, the external pressure is less than the internal pressure, and the eardrum is bound to protrude outward. As soon as I go in and out, my ears will feel bloated. However, when the tension of the eardrum is far greater than the maximum value that the eardrum can bear, the eardrum may rupture. In addition, the current supersonic aircraft, when flying near the ground, will rush through the tight air, which will produce very large sound wave vibration, and it is also very likely to break the eardrum.

It is worth mentioning that there is a tube in the middle ear that goes straight to the throat. This tube is medically called "eustachian tube". The eustachian tube is usually closed, but when we open our mouth, especially when swallowing, the eustachian tube can be opened and closed by itself. Keep the pressure inside and outside the eardrum basically unchanged, so that the eardrum no longer protrudes or sinks to one side. In addition, when the sound from the outside is very loud, open your mouth quickly, and sound waves will reach the eardrum from two different directions, namely the external ear and the eustachian tube, so that the pressure inside and outside the eardrum will cancel each other out and the eardrum will not break.

Chewing gum is a swallowing action, so this action can avoid the pressure difference inside and outside the eardrum. In addition, opening your mouth can also avoid eardrum rupture. By the same token, when being shelled on the battlefield, those experienced soldiers will also use the method of opening their mouths to avoid the great damage caused by the strong change of local pressure on the eardrum caused by the huge energy sound waves generated by the explosion.