In reduced density, you often see a different vibratory pattern in the vocal cords. We often see that the vocal cords stretch vertically, and are thereby not able to produce the mucosal wave that we mostly see in fuller density. The vibratory pattern in reduced density becomes more of a ‘rocking’ movement in and out.
Different vibratory patterns create different sounds. The vibratory pattern of the ‘rocking’ movement in and out, creates the thinner, hollow, restrained or more contained sound we recognise as reduced density.