Why do our ears pop? Have you ever been on a train going through a tunnel or plane and your ears pop?I was wondering why this happens and I know it relates to pressure but don't know  exactly the reason
 A: Part of your question is about human physiology; that's true.  And that part is simple.  Air pressure outside your eardrums needs to equalize with air pressure inside your eardrums, and this happens when the Eustachian tubes open up a little to let some air into the cavity behind your eardrums.
The physics parts of your question are about trains and airplanes.  And there are two parts to that.  First, the airplane goes up really high, where there is less air between you and outer space, so there's less air pushing down, so the air pressure is lower.  If your ears were adjusted to the pressure at ground level, they'll need to adjust when the plane moves to higher altitude -- and vice versa.
The second (and more interesting) phenomenon is due to the speed of either the airplane or the train through the air.  Bernoulli's principle says that an increase in the speed of the air around the train or plane implies a decrease in the pressure of that air.
Now, when the train enters a tunnel, suddenly all that air in the tunnel has to start moving out of the way -- or just gets pushed ahead of the train.  Either way, the pressure is doing funny things.  Ultimately, this changes the pressure outside of your ear, which requires your Eustachian tubes to adjust that pressure, which it does with a little pop.
Bernoulli's principle is also used to measure the speed of a plane relative to the air in something called a pitot tube, which uses the difference between the pressure at the front of a forward-facing tube and the pressure on the side of the plane (at a static port) to figure out how fast the air is moving.
A: When a train rushes through a tunnel, it tries to push the air out of the way, but the narrow confines of the tunnel force the air to be compressed in front of the train, as though the train were a piston in an air compressor.
The compressed air tries to find extra volume wherever it can, and since the train does not make as tight a fit to the tunnel walls as a piston, some air escapes compression by flowing between the train and the tunnel walls, eventually filling the void behind the train.
Air rushing past the train is forced into the narrow area between train and tunnel, just as though it were entering the venturi of a carburetor.  So it accelerates and can flow at greater velocity than even the train's forward motion.  The Bernoulli effect (http://hyperphysics.phy-astr.gsu.edu/hbase/pber.html) causes the fast-flowing air to lower the pressure between the railcars and the tunnel walls.  This creates suction that draws air out of the train, effectively lowering the pressure inside the railcars.
There are two kinds of energy between the railcars and the tunnel walls:  Kinetic energy of fast flowing air molecules, and kinetic energy of chaotically compressed air molecules.  The laminar flow of the air is more powerful and more organized than the compression of air in the imperfectly sealed space, so the organized kinetic energy of laminar flow increases as the chaotic kinetic energy of compressed gas decreases.  Energy per unit volume remains constant and conserved as air velocity increases and air pressure decreases.
Your ears pop because your eustacian tubes are trying to equalize your internal pressure fast enough to compensate for the air being sucked out of the railcars.  The popping sound is air bubbles from your eustacian tube entering the middle ear.
A: Measure first, think later. The best tunnel for experiencing ear discomfort around here is a train tunnel under a canal, the Drontermeertunnel. The graph shows the air pressure recordings I made in the train. A and B are the gates of the tunnel. Orange indicates when I experienced ear discomfort (although it wasn't easy to decide when discomfort started and stopped exactly). The graph shows that the ear discomfort corresponded to peaks of the oscillation. 
The pressure increases before A because the train descends on a slope before the canal; it briefly increases when entering the gate; the average pressure decreases inside the tunnel due to the Venturi effect. The damped oscillation is the fundamental frequency of the tunnel. The tunnel is an open-end air column. 

A: I think that it appends because our body is very adaptable. At normal pressure the eardeum vibrate at frequencies related to the pressure, when the pressure change, our body must adapt so that we can feel the same frequencies 
