Book of the Force
Chapter 27, page 392
Causes for Changes in Vocal Ranges
There are many causes for changes in vocal ranges. Some changes are temporary [see page 399]. Other changes are permanent [see page 410].
Chapter 27, page 410
Permanent changes in vocal ranges are changes that cannot be reversed. Some changes are caused by damage to the vocal cords through unhealthy habits [drinking, smoking cigarettes, etc., see page 416].
Another permanent cause of change in vocal range is childbearing.