Squeal vibrations, glass sounds and the stick-slip effect

Abstract

The origin of the squeal acoustic emissions when a chalk is rubbed on a blackboard or better on a ceramic plate, and those when a wet finger is rubbed on a smooth surface, such as a glass surface, is sought in the stick-slip effect between the rubbing surfaces. The elastic agency is sought in a shear band between the two surfaces characterized by very low shear modulus. In the case of the squealing chalk, it can be argued that the shear band is a layer of chalk powder, about 0.3 mm thick, forced to slide over the ceramic plate surface. In the case of the wet finger on a glass surface, it can be argued that the shear band is the layer of soft tissue between the epidermis and the finger bone, and that the water layer simply facilitates the stick-slip effect.