Inhale a sip of any of these two gasses and your voice will change its pitch instantaneously!
The speed of sound in these gasses is much higher (helium) or lower (Sulfur Hexafluoride) causing the frequency to change. The wavelength remains unchanged as the size of your mouth (resonance box) is also not changed. The wave equations (speed = wavelength x frequency ) tells us that if speed goes up and wavelength is not changed then frequency changes with speed and will also go up. Mr Savage explains.
If you want to try it do not overdo it! If you fill your lungs completely there will be no more oxygen in your system and you’ll pass out or worse….