Various researchers over the years have noticed that the adolescent male voice does not "break" overnight, but mutates downwards in discrete steps that correspond to growth patterns during puberty.  The most well-known worker in this field was John Cooksey, who died in 2012.  His most important work was published in 1977, revised during the 1990s and issued in its final form in 2000.  Ron Morris once said to me in conversation "Cooksey had it nailed".  My own work has proved this to be so.  The boys I have studied have all, without exception, done exactly what Cooksey said they would do.  That is a strong recommendation for any scientist!