Why You Equalize More Frequently at 5m Than at 25m

In a recent theory session of my Level 2 freediving course, we discussed how equalization frequency drops dramatically with depth — as often as possible near the surface, barely once every few meters past 20 m. We talked about it extensively and I understood the math, but I could not shake the feeling that I was missing the intuition. Naturally, this kept me up at night — so here is a blog post with particle simulations about it.

What equalization is

The middle ear is an air-filled cavity behind the eardrum, connected to the throat via the Eustachian tube — normally closed. The eardrum is a membrane: it transmits pressure, so the air inside the middle ear is always at the same pressure as the water outside. The way it achieves this is by deflecting inward, reducing the volume of air in the middle ear until the pressures match. This deflection is what hurts. To equalize, you open the Eustachian tube and push air into the middle ear, restoring the eardrum to its neutral position.

Why the first meters are the worst

Water pressure increases by 1 atm for every 10 m of depth. The pressure increase per meter is constant — it does not care how deep you already are. So why does the eardrum deflect more near the surface?

The answer comes from how gas works. Pressure is molecules hitting walls. A molecule bouncing in a box hits the walls at a rate that depends on how far it has to travel between bounces. Shrink the box and it has less distance to cover, so it hits more often. Half the volume → double the hit rate → double the pressure. This is Boyle’s Law.

Now here is the key. Imagine three sealed boxes, each containing the same molecules, but at different starting pressures — 1, 2, and 3 atm. The higher the pressure, the smaller the box (the gas is already compressed). Now add the same pressure to all three:

The volumes in the bottom row are already reduced by exactly the amount Boyle’s Law predicts — which is what we set out to understand. But watch the hit rates: adding the same pressure to each box raises the average hit rate by roughly the same amount in all three cases, even though the volume changes are dramatically different. The 1 atm box loses half its volume. The 3 atm box loses just 8%. Yet both gain the same number of wall collisions per second.

This is the geometry driving the inverse relationship between pressure and volume. In a tight box, molecules are already bouncing fast — a small reduction in distance yields many more collisions. In a spacious box, the same number of extra collisions requires a far larger squeeze. Boyle’s Law is not an abstract formula — it falls straight out of particles and walls.

The numbers

From Boyle’s Law ($P_1 V_1 = P_2 V_2$), the volume at depth $d$ without equalizing is:

$V(d) = \frac{V_0}{1 + d/10}$

$\frac{dV}{dd} = -\frac{V_0 / 10}{(1 + d/10)^2}$

The rate at which volume shrinks per meter of descent scales as $1/(1 + d/10)^2$. The equalization interval — how far you can descend before your eardrum deflects past the pain threshold — scales as the inverse:

If you equalize every meter at the surface, by 30 m you could go 16 meters between equalizations. The interactive below shows this as eardrum deflection across three depth intervals:

What to take away

Equalize early and often — the physics are harshest in the first 10 meters. Waiting until it hurts is far more costly at 3 m than at 25 m. Once you are past 15 m, the gas has already stiffened and the calm begins.