duckbill exhaust = quieter bubbles?

[reefrat]

I wonder if the air conditioner filter mesh would be fine enough to diffuse the flow? Home centers and appliance stores have that by the package.

Pete

Whatever you put over an exhaust outlet you need to be sure that it does not impede the flow of bubbles! I am not well versed in these matters but I have read that modern second stage designs have small wide open exhaust tees to vent exhaust bubbles quickly and efficiently so that there is no back pressure on exhalation- apparently the performance of many balanced downstream valves is now so good now that resistance to exhalation is a limiting factor in overall work of breathing (WOB).
Once again, I don't want to upset anyone who wants to keep this forum strictly pre-80's but I have to mention here (as an example) that I tried a new Aqualung Titan reg recently and was pleasantly surprised how quiet the exhaust bubbles were due to to the baffled design of the exhaust outlets.

When I say "stealth" I am envisioning a scuba unit that is quieter in every respect. Perhaps operates at a lower IP and uses a larger diameter low pressure seat to reduce the velocity of gas release on inhalation and therefore sound more like inhaling on a snorkel than a scuba demand valve. The exhaust valve would designed to form bubbles with minimal noise and divert them away from the face/ ears toward the rear of the divers head. I have done a little research on what actually causes the noise when bubbles are formed is an interesting and surprisingly complex, as a example:
"Gas bubbles, when entrained in water or other liquid, can generate high sound pressures in the liquid. Significant sound pressures are associated only with volume pulsations of the bubble, whereas oscillations in the shape of the bubble do not result in appreciable sound. Calculations have been made of the sound pressures resulting from excitation of volume pulsations by the following mechanisms: by bubble formation, coalescence, or division; by the motion of a free stream of liquid containing entrained bubbles past an obstacle; and by the flow of liquid containing entrained bubbles through a pipe past a constriction. The calculation of the sound pressure generated by bubble formation has been verified by measurements with bubbles formed at a nozzle."

© 1956 Acoustical Society of America

I am sure there has been enough research in the military, as well as industrial sectors, to design an efficient exhaust valve that is much quieter than conventional types.

A scuba unit that works like a "normal" one, but is relatively quiet to dive would probably generate a lot of interest!
Cheers

 

[spectrum]

I'm recognize the need to freely exhaust, that's part of why I suggested such a coarse medium.

A lot of the sound I believe actually comes form the mushroom valve or duckbill opening and closing as it goes in and out of balance. This is not unlike what your cheeks do when you pass gas or toot. The rest is the bubble action as it transitions/shears from the confinement of the body or can in this case to open water.

Adding a diffuser with a comparable escape area would be analogous to a bubbler stone in an aquarium compared to the airline alone.

 

[John C. Ratliff]

Luis and all,

I too have conducted some studies on exhaust resistance. My setup was slightly different, but the results are similar. This was done on March 14, 2013 but due to a lot of things I have yet to write it up.

I used a Dyer Mk II, Model 25 Inclined Plane Monometer, calibrated to zero inches of water and balanced to level. The regulator is sitting in one inch of water, so the valve is submerged for exhalation resistance. The setup was to determine whether exhalation resistance varies with different exhaust designs. The designs tested are the duckbill valves (2 designs), Healthways original exhaust design, and the Nemrod medified mushroom exhaust. The breaking water pressure and the "hard exhalation," which was a max value, were recorded. I will present this in a different thread, but here are some of the results:

Healthways Scuba Gold Label (mushroom valve) USD Curved mouthpiece/hose: Break, 0.8-1.4 inches; hard exhalation, 3.0 inches.
USD Aqualung-Broxton (duckbill, old-style horn), straight USD hose/mouthpiece : Beak, 0.8-1.2 inches; hard exhalation, 1.8-2.0 inches.
USD Mistral--Straight Mouthpiece: break, 0.8-1.2 inches; hard exhalation, 2.3 inches.

Because of the different locations of the actual exhaust inside the can, no attempt was made to adjust the reading for the water head. I would say that this has an error of plus or minus .25 inches of water.

Bubble Noise

This is both a perception, and a real physical phenomena. The perception is that the noise of a double hose regulator is less than a single hose. Some of this has to do with the bubbles going over the ears in a single hose regulator. Bubbles not only generate noise, but also vibration, and I think the vibration is greater for a single hose regulator due to its proximity to any fish. I have experimented with longer hoses on a double hose regulator, and see that with longer hoses, and the placement of the regulator in the center of the back, bubble noise is lessened by quite a bit (perceptional). Any vibrations would have to transmit through the diver to be perceived by the fish too. Concerning a bubble difuser, this would in my opinion vastly increase exhalation resistance.

SeaRat