If you have ever freedived, you have probably noticed something counterintuitive: you need to equalize constantly in the first few meters, and then progressively less often as you go deeper. By 20 meters, a single equalization can last you several meters of descent. Near the surface, you are popping your ears as often as possible.

This is not just a feeling. There is clean physics behind it. We recently discussed this in a theory session of my Level 2 Freediving course and I could not fully wrap my head around it. Now that I spent some more time thinking about it, it all got clear and I thought it would make an interesting blog entry.

Equalization

The middle ear is a small air-filled cavity sealed on one side by the eardrum and connected to the back of the throat via the Eustachian tube — which is normally closed. As you descend, rising water pressure pushes on the eardrum from the ear canal side, deflecting it inward. This deflection is what causes pain, and eventually barotrauma if ignored.

To equalize, you actively open the Eustachian tube — via Valsalva, Frenzel, or similar techniques — and let air flow from your throat into the middle ear until pressures are balanced.

Other air spaces handle this differently. The lungs compress passively as the chest wall yields, and the sinuses are normally connected to the nasal passage and equalize on their own. The middle ear is the odd one out: its connection is closed by default and requires conscious effort to open.

Pressure and depth

Water pressure increases by 1 atm per 10 m:

$P(d) = 1 + \frac{d}{10} \quad [\text{atm}]$

From the ideal gas law to Boyle’s Law

The ideal gas law is $PV = nRT$. Temperature here can be assumed constant: the middle ear cavity sits inside the skull, surrounded by tissue at body temperature, and descents are slow enough for heat exchange to keep up. With $T$ and $n$ fixed:

$PV = \text{const} \quad \Longrightarrow \quad P_1 V_1 = P_2 V_2$

This is Boyle’s Law. But what does $PV = \text{const}$ actually mean?

Why pressure and volume are linked

Pressure is just molecules hitting walls. The more often they hit per unit area, the higher the pressure. Now imagine a sealed chamber: same molecules, but you shrink the space. Each molecule has less distance to travel before it reaches a wall, so it hits more often. Half the volume → double the hit rate → double the pressure.

The simulation below makes this visible. Increase the external pressure and watch the chamber shrink — the same particles now collide with the walls far more frequently.

Boyle's Law — particle view

External pressure1.0 atm

This is why volume change is not linear with pressure. At 1 atm the chamber is spacious and compresses easily. At 3 atm the particles are already packed tightly — the same additional 1 atm of external pressure barely moves the membrane further, because the crowded gas is already pushing back hard.

What this means for your eardrum

The eardrum is a membrane — it transmits pressure. As outside pressure rises during descent, the middle ear pressure must follow. By Boyle’s Law, that means the air volume behind the eardrum shrinks. The eardrum deflects inward to accommodate the lost volume — and this deflection is what causes pain.

A membrane displaced by $\Delta x$ over area $A$ sweeps out volume $\Delta V = A \cdot \Delta x$, so $\Delta x \propto \Delta V$. To equalize is to push air back through the Eustachian tube, restoring the volume and returning the eardrum to its neutral position.

How fast does $\Delta V$ grow with depth? Say we start at the surface with pressure $P_1 = 1$ atm and middle ear volume $V_0$, then descend to depth $d$ without equalizing. The outside pressure rises to $P_2 = 1 + d/10$, and Boyle’s Law tells us:

$P_2 \cdot V_2 = P_1 \cdot V_1 = 1 \cdot V_0$

Solving for $V_2$:

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

Take the derivative — the volume lost per meter of descent:

$\left|\frac{dV}{dd}\right| = \frac{V_0}{10} \cdot \frac{1}{\left(1 + d/10\right)^2}$

The $(1 + d/10)^{-2}$ term is the whole story. The equalization interval scales as its inverse:

Depth	Pressure	Interval relative to surface
0 m	1 atm	1×
10 m	2 atm	4×
20 m	3 atm	9×
30 m	4 atm	16×

Try it

The interactive below shows the eardrum deflection simultaneously for three 10-meter windows. Scrub or animate to see how much more the membrane moves near the surface.

Relative air volume vs. depth

0m → 10m

Δvol = 0.0%

10m → 20m

Δvol = 0.0%

20m → 30m

Δvol = 0.0%

Descent within interval0.0 m

Animate 10m descent

The chart shows the compression rate as a continuous curve, normalised to 1 at the surface. More than half of all equalization work in a 30 m dive happens in the first 10 meters.

Why this matters

This is why instructors hammer early equalization — often before you even submerge. The pressure gradient is steepest at the top, not the bottom. Waiting until it hurts is far more costly at 3 m than at 25 m: shallow, the window closes fast. Deep, the physics have already relaxed.

Equalize early, equalize often, and enjoy the calm below 15 m.