 At 169 m / s , the speed of sound in xenon gas is lower than that in air due to the lower average speed of the heavy xenon atoms compared to nitrogen and oxygen molecules . Hence , xenon lowers the rate of vibration in the vocal tract when exhaled . This produces a characteristic lowered voice <unk> , an effect opposite to the high @-@ <unk> voice caused by inhalation of helium . Like helium , xenon does not satisfy the body 's need for oxygen . Xenon is both a simple <unk> and an anesthetic more powerful than nitrous oxide ; consequently , many universities no longer allow the voice stunt as a general chemistry demonstration . As xenon is expensive , the gas sulfur hexafluoride , which is similar to xenon in molecular weight ( 146 versus 131 ) , is generally used in this stunt , and is an <unk> without being anesthetic . 
