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Deep Equalization Technique: The Mouth-fill


Mouth-fill is an equalization method that you use when going to depths where simply using Frenzel 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 actually quite 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 really happening.  And a little anatomy always helps!


The Frenzel technique

In true Frenzel, the tongue compresses the trapped air under the roof of the mouth. 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 (not the epiglottis) is closed. Air cannot escape out the mouth because the tongue applies an airtight seal around the upper teeth. Nowhere else to go, the air enters the eustachian tubes and into the middle ear.

There are essentially 4 components to performing Frenzel.

The first one 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!

What is the 2nd component? Soft palate in neutral position. Soft palate is a soft fleshy part toward the back of the roof of the mouth. If you draw a line with your finger from the back of your upper teeth all the way back until you start to gag, that’s where the soft palate is. Now inhale big and with an open mouth, exhale very slowly out the nose, then mouth, then nose again and keep switching. Every time you switch, the part that you feel you’re raising and lowering is the soft palate. How do we determine if the soft palate is in neutral position? It is when you can exhale out your nose and your mouth at the same time.

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 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. Now, play with raising and lowering (relaxing) the soft palate. Remember what neutral soft palate feels like – it actually feels the most relaxed and natural so that makes it a bit easier.

The 3rd component 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 under the roof of the mouth. Doesn’t get any easier than that!

The last being you need to be able to move the tongue up and down (while keeping the tongue block) to pressurize and force the air up the eustachian tubes.


It is also possible to perform Frenzel using the “k” block instead of 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 relaxed position.  Always remember to trap enough air in the beginning.  As you begin to master these blocks, you should be able to perform Frenzel with less and less air.

Putting it all together

While performing a tongue block, with some air trapped under the roof of the mouth, pinch the nose and then pump the tongue. Did you feel your ears pop? If pumping doesn’t work, your glottis is likely to be open. Another possibility is that you’re not making a good seal with your tongue that you’re just moving your tongue in the air space inside 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, 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, with the nose pinched, extend your chin forward and with the use of your diaphragm, move as much air into your mouth. By now, your cheeks should be fat.

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. This should help make it easier keeping your glottis closed.

4. Keep your soft palate in the 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.

6. When you can no longer equalize with your jaw and cheeks because of the reduced air volume, start equalizing using the tongue (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 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 are able to 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 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. Really, 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 moving more air in the mouth than it can contain and forming an airtight seal with the lips.

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 you glottis closed. The success and effectiveness of mouth-fill depends on many factors, but with proper technique, relaxation, and practice, mouth-fill will take you to new depths.


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.


This article is geared towards providing beginner to intermediate 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.

December 1, 2014 | Emil Lars