The short answer
Breathe helium and you sound like a cartoon, so the obvious explanation is that helium raised your pitch. It did not. Your pitch is set by how fast your vocal folds slap together, and that rate is decided by their tension, their mass and their shape. Helium does not reach any of those. The physics department at UNSW, whose page on helium speech is the clearest public account of it, puts the pitch squarely with the folds and says outright that the note does not move.
What helium reaches is the space above the folds. Sound travels through helium at roughly 965 meters per second against about 343 in air, close to three times faster, and the resonances of a tube depend on how fast sound moves inside it. Fill your throat and mouth with a faster gas and every one of those resonances climbs. Your voice is the same buzz, played through a tube that now rings much higher.
A voice is a buzz and a tube
Two separate things make the sound anyone hears when you talk, and each one can move while the other holds still.
The first is the buzz. Air from your lungs drives your vocal folds open and shut a hundred or so times a second, and that rate is the note. Say a long ah and slide it up: you are changing how fast the folds cycle, and nothing else about your anatomy has to move.
The second is the tube. That buzz then travels through your throat, mouth and nose, which together behave like a pipe with a few strong resonances. Those resonances let some frequencies out loudly and hold others back, and the loud ones are called formants. A longer tube resonates lower, a shorter one higher, which is why your ear reads formants as the size of the person talking.
The buzz carries the note. The tube carries the vowel and the apparent size. A party balloon moves the second one and leaves the first alone.
What helium changes, and what it leaves alone
A tube resonates at frequencies set by its length and by the speed of sound in whatever fills it. Keep the tube and change the gas and the resonances scale by the speed ratio, which for helium against air is close to three. The first formant of a relaxed ah sits near 700 Hz in air; with helium in the way it lands nearer 2 kHz. The second and third climb by the same factor, and the higher ones run past the range where speech carries much energy.
Meanwhile the folds do what they were doing. They are flesh under tension, not a column of air, so the gas around them does not change how fast they cycle. That is the asymmetry worth holding onto: helium moves the filter and never touches the source.
So why does it sound higher?
Because your ear does not measure pitch in isolation. It listens to the whole sound and takes the bright, small, thin quality of lifted formants as a smaller person, and a smaller person is someone you expect to have a higher voice. The UNSW page calls this an illusion of pitch, which is the right word for it: the note is where it was, and the formants above it moved by a factor of three.
You can prove it on yourself without any helium. Sing a steady note and slide only your tongue and lips, as though mouthing different vowels, and the note stays while the sound changes character. That is the tube moving under a fixed buzz, which is what helium does to you in one step and much further. Do the reverse, hold the vowel and slide the note, and you hear how different a real pitch change sounds.
The part that actually breaks: vowels
Vowels are told apart by where the first two formants sit relative to each other. Lift every formant by the same factor and those positions no longer land where a listener expects them, so the vowels stop reading cleanly. Whole sentences often still get through, because context fills the gaps, but single words get harder and a list of unrelated words gets very hard.
This stops being a party trick at depth. Divers working deep breathe helium in place of nitrogen, and their speech arrives so distorted that crews cannot reliably understand it. Engineers have built machines to undo it since the late 1960s, documented in the IEEE literature of the day and still studied now, and what those machines do is slide the formants back down while leaving the pitch where it is. A helium unscrambler is a formant shifter with a job to do.
Doing the same thing on purpose
Once you can state what helium does in one sentence, the audio tool that does it deliberately is easy to recognize. A formant shifter estimates the shape of your tube, slides it up or down, and puts your original buzz back through it. Up by a lot is the helium sound. Down is the bigger chest that deep voice effects are built from.
In Patchd the Formant Shifter is one of the Studio voice-fx nodes. Its main dial runs -12 to +12 semitones with 0 as untouched, and because it works in the time domain rather than on analysis blocks it adds no latency of its own, so you can hear the change while you talk. The neighboring Pitch Shifter carries both dials at once, which is how you move the note and the size independently, or move one and watch the other stay put. Which dial does what is laid out in formant shift vs pitch shift.
The takeaway
Helium does not raise your pitch. It raises the speed of sound inside your throat, which lifts that throat's resonances by nearly a factor of three while your vocal folds keep the rate they had. Your ear reads the lifted formants as a smaller talker and reports that as a higher voice. The reason vowels get hard to follow is the same one, and it is why divers on a helium mix need a machine that slides the formants back down.
You can hear the move for yourself rather than reading about it. Patchd is a Windows audio mixer with a full effects rack on every channel, and the Formant Shifter sits in the Studio voice-fx rack with its dial on a -12 to +12 range. It is in open beta now, and if you clear the terms your invitation arrives straight away. The Studio tier is $5.99 a month, $39.99 a year, or $119.99 once for a lifetime license, and the real-time engine is identical on both tiers.