Over millions of years, changes to our vocal organs have allowed us to produce a rich mix of sounds. One such change was the loss of the air sac - a balloon-like organ that helps primates to produce booming noises.
All primates have an air sac except humans, in whom it has shrunk to a vestigial organ. ...To find out how this changed the sounds produced, Bart de Boer of the University of Amsterdam in the Netherlands created artificial vocal tracts from shaped plastic tubes. Air forced down them produced different vowel sounds, and half of the models had an extra chamber to mimic an air sac.De Boer played the sounds to 22 people and asked them to identify the vowel. If they got it right, they were asked to try again, only this time noise was added to make it harder to identify the sound. If they got it wrong, noise was reduced.He found that those listening to tubes without air sacs could tolerate much more noise before the vowels became unintelligible.The air sacs acted like bass drums, resonating at low frequencies, and causing vowel sounds to merge...Observations of soldiers from the first world war corroborate de Boer's findings. Poison gas enlarged the vestigial air sacs of some soldiers, who are said to have had speech problems that made them hard to comprehend.
From New Scientist.