Introduction

The world we live in is full of different kinds of sounds; it is obviously hard to imagine a world without any sound sources. Some sounds may originate from far-away sources but they die away before reaching our ears so we do not hear them. Some others, however, reach our ears, but their energies are so weak that we do not perceive them clearly. There are sounds that we can clearly hear. Among those, some may convey very important information while others may not carry anything really useful, but purely interfere with the sounds that do contain useful information. The sound that carries the information that we want or need, is normally labelled as “desired.” The sound that does not contain any useful information but interferes with the desired sound, is usually referred to as “noise.”