Dumagat Freedive International Freediving School

Learn and experience freediving in Dauin with the country’s most experienced and longest-tenured professional freediving instructor.

Deep Equalization Technique: The Mouth-fill


Mouth-fill is an equalization method that is performed when freediving to depths where the standard Frenzel equalization method will fail. What happens is that there will be a point in your dive when the volume of air in your mouth will be so compressed that you will no longer be able to use it to equalize. At the same time, the volume of your lungs will be just as compressed that it’s nearly impossible to move air from the lungs to the mouth for equalizing. To remedy the problem, you need to move a big bubble of air from the lungs to the mouth while you still can.  This is usually around 20-25 meters, just before your lungs reach residual volume.

While it takes time to master, mouth-fill is simple in theory and the process itself is just as simple if you break it down in steps. Mouth-fill is a more advanced version of Frenzel so we need to first understand the idea behind Frenzel. Some people do Frenzel without even thinking about it so it’s a good idea to discuss it so everyone understands what’s happening.  And a little anatomy always helps!

The Frenzel Technique

In Frenzel, the movement of the larynx compresses the trapped air inside the mouth and the nasal cavity.  The pressure forces the air to enter the nasal cavity and tries to escape out the nose, but the nostrils are pinched shut.  Air cannot escape back into the lungs because the glottis is closed.  Air cannot escape out the mouth because the tongue applies an airtight seal either against the upper teeth or in the back of the mouth.  Nowhere else to go, the air enters the eustachian tubes and into the middle ear equalizing the pressure.

The 4 elements to performing Frenzel

The first element is to have a closed glottis.  Where and what is glottis?  Glottis is the opening between the vocal cords in the throat.  You can make the “aahhh” sound and by stopping the sound, with your mouth open, you are closing your glottis.  You can also try inhaling fully through the mouth and then while keeping the mouth open, hold the air in the lungs by closing the glottis.  Super easy!

The 2nd element?  Soft palate in the neutral position.  The soft palate is the soft fleshy part toward the back of the roof of the mouth.  If you drew a line with your finger from the back of your upper teeth to the back until you start to gag, that’s where the soft palate is.  Inhale big and with an open mouth, exhale very slowly out the nose, then mouth, then nose again and keep switching until you run out of air to exhale.  Every time you switch between the nose and the mouth, the part that you feel you’re raising and lowering is the soft palate.

How do we determine if the soft palate is in the neutral position?  It is when you can exhale out the nose and the mouth at the same time. We can do it very easily.  It is normally in a neutral position. If the back of the throat feels relaxed, it is in the neutral. When you tense the throat, the tendency is to raise the soft palate.

As you’ve noticed, controlling the glottis and the soft palate is not at all difficult.  We do it all the time whenever we talk – whenever we make oral and nasal sounds!  What’s difficult is controlling them together.  When you close your glottis, you will likely raise your soft palate.  But we need to have the soft palate in the neutral position.  Try this: inhale fully through the mouth and then, with the mouth open, hold the air in the lungs by closing the glottis.  While holding the air with closed glottis, play with raising and lowering (relaxing) the soft palate.  Remember how soft palate in the neutral position feels like – it feels the most relaxed and natural so that makes it a bit easier.

The 3rd element is you need to be able to trap some air between the tongue and the roof of the mouth by performing a tongue block.  Try to say the letter “t” without making the sound.  Remember: there must be some air trapped between the tongue and under the roof of the mouth.  Doesn’t get any easier than that!

The last being you need to be able to pump the larynx up and down (while keeping the throat and tongue blocks) to pressurize and force the air up the eustachian tubes. It will feel very much like you are pumping the back of your tongue because perhaps it is that action that pulls the muscles in the larynx as opposed to the muscles surrounding the larynx doing the pumping.  The experts have yet to agree about this.


It is also possible to perform Frenzel using the “k” block instead of the tongue or “t” block.  Try to say “ka” without making the sound.  By sealing the opening between the tongue and the soft palate, you can lock the air in.  And then by pumping the back of the tongue, you can pressurize the trapped air to equalize.  Practice with the mouth open and with the tip of the tongue in a relaxed position.

You can also say the letter “n” to make a tight seal between the tongue and the roof of the mouth.  And like in the “t” or “k” blocks, make sure to always trap a bubble of air.  As you begin to master these blocks, you should be able to consistently move air from the lungs to the mouth, between the tongue and the roof of the mouth.

Now that we’ve reviewed the fundamentals of Frenzel, let’s discuss mouth-fill further so we can dive deeper.

For a more detailed explanation of the Frenzel maneuver along with learning tips, exercises, and troubleshooting, refer to our Frenzel article.

Working out the math

Remember Boyle’s law from your first freediving course?  If you dive from the surface to 10 meters, the pressure will double in a matter of 10 meters. Makes sense? 1 Bar to 2 Bar. But… if you begin your dive from 30 meters, you will reach 70 meters before the pressure doubles. 4 Bar at 30 meters, 5 at 40m, 6 at 50m, 7 at 60m, 8 at 70 m. It took 40 meters before the pressure doubled!

This goes to show that the deeper you dive, the slower the relative effect of pressure will be. This is why mouth-fill is so effective on deeper dives. You do a mouth-fill at roughly 20-25 meters and it allows you to equalize to great depths. Simply because the relative effect of pressure is much slower at depth than from the surface.

You know by now that it becomes very difficult, if not impossible, to equalize beyond 30 meters because the air in the mouth gets too compressed and the volume significantly reduced. At the same time, because the air in the lungs is also too compressed, it’s very difficult, if not impossible, to move air from the lungs to the mouth. So why not fill your mouth with air before 30 meters? Say, 20 meters? This way you will have plenty of air to equalize with. If you think about it, you’ll get to 50 meters before the volume of air in your mouth gets reduced to half its size when the pressure will have doubled (3 Bar at 20m, 4 Bar at 30m, 5 Bar at 40m, 6 Bar at 50m). You know you can equalize without difficulty from the surface to 10 meters, the point where the volume of air in the mouth is reduced to 50%, right?

Breaking down mouth-fill step-by-step

1. Before you reach residual volume, say 20 meters, shift as much air into your mouth either by contracting your abdominal muscle or by pulling air using your jaw and cheeks. By now, your cheeks should be fat. Some may find that extending the chin forward, as it opens the glottis, makes it easier to shift air to the mouth.

2. Immediately close your glottis so the air doesn’t escape back into the lungs. You need to be mindful to keep the glottis closed throughout the entire mouth-fill process.

3. Tilt your chin back towards your chest if you extended it. This should help make it easier to keep your glottis closed.

4. Keep your soft palate in a neutral position.

5. As the air in the mouth slowly get compressed, start compressing the air in your mouth using your jaw and cheeks to equalize. With a mouthful of air, some may find it easier to keep a constant pressure instead of doing sequential pumps.

6. When you can no longer equalize with your jaw and cheeks because of the reduced air volume, start doing standard Frenzel. Reach new depths!

Simple right? A good way to practice mouth-fill dry is to let a little bit of air escape out your mouth to emulate the increasing pressure at depth. By doing this, you’ll get to experience equalizing with your jaw and cheeks and then the back of the tongue (creating movement in the larynx) eventually.

Exhale / FRC dives

An exhale dive or Functional Residual Capacity (FRC) dive is when the air in your lungs is neutral. It’s a passive exhale meaning you have the same volume of air as when you do a normal exhale.

So why are we talking about FRC? Exhale dives are a very effective way to train mouth-fill. When you do an exhale dive, you can simulate depth without going deep. Exhale or FRC dives are generally used as an effective warm-up technique before a big dive. However, it is also an effective way to train mouth-fill! On exhale dives, you could theoretically dive to 10 meters and be able to simulate a dive to 30 meters. That means we can train mouth-fill without having to go past 30 meters each time! How cool is that?! What an easy, convenient and safe way to practice the mouth-fill technique!

Exhale dives expose you more to the possibility of a lung squeeze. On an exhale dive, not only are great depths simulated but are reached at a greater speed which may not give you enough time for blood-shift to happen in full degree. Be very conservative with depth and have a regular thoracic stretching program in place! Remember that the more flexible your chest is, the less likely you are to experience a lung squeeze. Also, use a line you can pull down on (free immersion) so you have total control of your descent.

To train mouth-fill using FRC, perform a mouth-fill on/near the surface and descend head-first very slowly concentrating on not losing the air in the mouth as you equalize. This will take some time to master.

Common problems with Mouth-fill and how to fix them

1. Losing the mouth-fill by swallowing. This happens when the glottis is opened. Practice closing the glottis on dry land. This is perhaps the most common problem when starting to learn mouth-fill. I had this problem myself. Most everyone I’ve trained with had this problem too. But if I can learn it, so can you! So don’t be discouraged in the beginning. The only difference between you and me is that I have spent more time practicing mouth-fill. Practice makes perfect!

2. Air escapes through the lips. We don’t want to waste air. On land, practice not shifting more air in the mouth than it can contain and always have an airtight seal with the lips.

3. An important matter that is often neglected in understanding equalization is relaxation. When we tense up, equalization becomes difficult and even impossible. We tend to tense up when we aren’t comfortable with a dive. When you’re not ready or prepared for the dive, unable to relax through the descent, having early contractions, maybe feeling tired, anxious, or stressed, equalization suffers greatly. Tension causes us to raise the soft palate. The blockage makes it impossible to pump air into the eustachian tubes. It takes a certain level of awareness to realize this on a dive, but it is important to understand that a lot of equalization issues are due to tension. Eliminate the potential causes of stress and tension on a dive.

One key thing to remember when doing a mouth-fill is to go big and don’t swallow (if you will forgive the pun) – inflate your cheeks fully and keep your glottis closed. Concentrate on the technique rather than depth. It will come.  As I always say to students, with proper technique comes depth. The success and effectiveness of mouth-fill depend on many factors, but with proper technique, relaxation, and practice, mouth-fill will take you to new depths.

Hope you enjoyed reading this and found it to be helpful. Please follow our Instagram @dumagatfreedive or like our Facebook page. Much appreciated. Thank you.


Freediving Team of Finland. (2008). The Frenzel Technique. Retrieved July 2013, from http://www.freedivingfinland.net.

Eric Fattah. (2006). Frenzel-Fattah Equalizing Workshop: Step-by-step Guide to Optimal Equalizing for Freedivers and Scuba Divers. Retrieved July 2013, from http://liquivision.com.

Special thanks to Andrea Zuccari for debunking a widely accepted equalization myth and for pointing out an earlier error, and Linda Paganelli for driving the discussion leading up to its correction.

Also thanks to Adam Stern for his recent MRI equalization video and for his explanations.


This article is geared towards providing intermediate to advanced freedivers an easy, user-friendly and practical way to learn and understand the Frenzel and Mouthfill equalization techniques. The information herein is offered solely as informational and not intended as a substitute for instruction from equalization experts or from attending a formal freediving course. Freediving and breath-holding can be dangerous if practiced without proper knowledge and education. They should be practiced with a trained buddy or a qualified instructor. Attend a formal freediving course.

By: Emil Lars

Author: Dumagat Freedive International Freediving School

Learn and experience freediving in Dauin with the country’s most experienced and longest-tenured professional freediving instructor. Dumagat Freedive is a locally operated professional freediving center in the Philippines. The center offers courses from beginner to instructor levels. Formerly situated on the island of Romblon, it has since moved to Dauin in Negros Oriental. The center delivers personalized freediving education using SSI’s world-class training methods, materials, and standards. Come and see us for your next freediving adventure!

Comments are closed.