Question

Deep-sea divers often breathe a mixture of helium and oxygen to avoid the complications of breathing high-pressure nitrogen. At great depths the mix is almost entirely helium, which has the side effect of making the divers' voices sound very odd. Breathing helium doesn't affect the frequency at which the vocal cords vibrate, but it does affect the frequencies of the formants. The text gives the frequencies of the first two formants for an "ee" vowel sound as 270 and 2300 Hz.

What will these frequencies be for a helium-oxygen mixture in which the speed of sound at body temperature is 720m/s ? (Use 350 m/s for the speed of sound in the air.)

Answer 1

Nope. Pure oxygen is deadly to a diver. It causes what's called oxygen toxicity that can cause a diver to suffer seizures at the very least.
Oxygen content in a breathing mix is depth dependent. The deeper you go, the less oxygen you want in your mix or the less exposure you want to that mix. Over the years from trial and error, dive tables have been developed for the different gas mixes. Basically these tables help a diver figure out mathamatically how to conduct a dive to a certain depth using a certain gas mix for a certain amount of time. They tell you what you can and cannot do. Oxygen isn't the only concern in a breathing mix, since we can't breathe 100% O2. There has to be another gas in that tank(s). It will be nitrogen and maybe even helium, depending on the dive.
The most common gas mix scuba divers use is exactly what you're breathing right now. 21% oxygen, a little over 78% nitrogen and less than 1% other trace gasses like argon. Recreational divers are trained to limit their depth to 130 feet on this mix, but not because of the oxygen content, rather it's because of the narcotic effect that the nitrogen content being inhaled under pressure creates. Well trained and experienced divers can still use this mix to go deeper. 250 feet is possible on it. To go any deeper though, you need to start reducing the oxygen % in that mix ( nitrogen too since it's narcotic and it's effects just get worse) and replace it with a gas that you can breathe and won't cause any narcotic effect or be toxic at depth. The gas of choice is helium since humans can inhale it and it doesn't get narcotic until you're VERY deep. That's why you'll hear some divers on communications gear that sound like they just sucked on a party balloon. This gas blend is commonly called tri mix. It has Oxygen, helium and a little bit of nitrogen. Gas mixes can go down to as low as 6% oxygen content, a little less than 1/3 of what you're used to on the surface but because of the pressures involved, both on the diver and in the diver ( the inhaled gas), you don't suffocate or pass out. If you were in a sealed room at the surface, at 1 atmosphere of pressure ( what's acting on your body right now), you'd be on the floor in a minute breathing this. Why it works this way has to do with a little physics and what's called partial pressures of a gas. Not sure I need to go into that and Dalton's law of partial pressures, but you can easily google that for examples.
There is one other gas mix that divers use commonly called Nitrox. It's kinda like super charged air. Instead of less oxygen as a percentage of volume, it has more. Any diving gas that has more than 21% oxygen by volume can be considered Nitrox or enriched air. You're probably thinking " Wait a minute, isn't oxygen toxic to a diver at depth?" Yep, it is. That's why this mix is used at shallow depths and the higher the oxygen content, the shallower your maximum depth can be. Typically an enriched air mix (nitrox) can be anywhere from 32% to a maximum of 50% oxygen by volume. The 50% mix is used only for things like shallow water decompression stops to help rid the body of absorbed nitrogen a little quicker than normal. The other mixes are actually used for the dive. The advantage using enriched air is that there is a reduced nitrogen content without the expense of helium ( it's costly) which in turn helps limit the amount of nitrogen your body takes on. This gives you a little longer bottom time before you need to either head up or start a decompression schedule.

The exhaled bubbles are carbon dioxide, some oxygen and some nitrogen ( not all of the nitrogen you inhaled though, some is trapped in your body and the reason why divers need to be careful how much nitrogen they absorb) and if you dived trimix, helium. Your exhalations bubble up through the water column until they break surface and mix with the atmosphere.