However, as knowledge of human physiology has increased over the past two hundred years, so has the understanding of the physical process of singing and vocal production. As a result, many vocal pedagogists have redefined or even abandoned the use of the term head voice. In particular, the use of the term head register has become controversial since vocal registration is more commonly seen today as a product of laryngeal function. For this reason, many vocal pedagogists argue that it is meaningless to speak of registers being produced in the head. The vibratory sensations that are felt in the head are resonance phenomena and should be described in terms related to vocal resonance, not to registers. These vocal pedagogists prefer the term "head voice" over the term "head register." These vocal pedagogists also hold that many of the problems which people identify as register problems are really problems of resonance adjustment. This helps to explain the controversy over this terminology. Also, the term head register is not used within speech pathology and is not one of the four main vocal registers identified by speech pathologists.