Mouthfill Equalization: How Freedivers Equalize at Depth

Why is mouthfill needed?

In Frenzel, the tongue acts like a piston, pushing the air in the mouth and nose towards the ears. That air comes from the lungs. As you descend, the lungs shrink with pressure, and at some point it becomes hard to bring air from the lungs into the mouth for Frenzel. This depth varies from person to person; for many divers it is somewhere around 20–30 metres. Mouthfill means filling the mouth like a “tank” while the lungs still have enough air, then continuing to equalize deeper with that air.

How does it work?

1. Charge: during the descent, before the lungs are compressed further, fill the mouth and cheeks with air taken from the lungs.
2. Lock: close the glottis so the air in the mouth cannot escape back to the lungs; equalization is now independent of the lungs.
3. Equalize: with the nose closed, the cheeks, tongue and jaw gently send the air in the mouth to the ears, in small portions for the rest of the descent.

There is no forceful blowing from the lungs and no pushing with the chest muscles here. That would be Valsalva, which at depth is both ineffective and dangerous.

Prerequisites

• Frenzel that is automatic and relaxed in the head-down position
• Conscious control of the glottis and soft palate
• A relaxed, controlled depth progression

How to learn it

Mouthfill is first learned on land with dry exercises: filling the mouth, closing the glottis and sending air to the ears in small portions. Then come shallow-water exercises that simulate depth, such as dives at FRC (lung volume after a relaxed exhale). These must always be done with an instructor and a buddy. We teach mouthfill at Performance Freediver level and in coached depth training.

Stop and ascend

If you feel ear pain, dizziness or a reverse block, end the dive. Equalization at depth is never forced. For the basics, see our guide Mastering Equalization Techniques.