ABSTRACT
The identification of voices is an everyday activity. It happens when we are greeted by telephone callers who plunge into conversation without bothering to announce their names, or in many of the other cases when we hear a voice and the speaker is for the moment out of sight. It is part of a whole repertoire of methods of identification available to us, from visual identification of a person's features to recognising the characteristic cough or even footstep of a close friend or family member. Some of these kinds of identification are more reliable than others, as common experience confirms. It is considered normal to be able to recognise the face of a person after a single meeting, whereas one normally only recognises the footsteps of people that one hears constantly and knows very well. The face, of course, is dense with information: there are innumerable ways in which faces differ from one another, and they are differences which we are attuned to paying attention to, for everyday life involves dealing with numbers of people and knowing who they are. Footsteps, on the other hand, differ from one another in pulse frequency and in pulse amplitude, but probably in few other ways. The information in a set of footsteps is sparse, and there is no reason to suppose that the human ability to discriminate between them is highly developed, for in the general course of things very little hangs on being able to tell one set of footsteps from another. Like many things, the characteristics of one person's tread can be learnt from sufficient repetition, but the set that anyone can learn to discriminate is probably small. Somewhere in this continuum between the face and the footsteps falls the voice. The sound signal that constitutes the human voice is produced from one or more sources between and including the vocal cords of the speaker and the lips, and its quality is affected by individual habits in sound production and by the configuration and volume of the cavities that make up and are connected with the speaker's vocal tract. So in principle, just as the unique sound of a particular violin depends on the materials it is made of and the structure of its resonating cavities, human voices will derive from the unique apparatus used to produce them, qualities that distinguish them from every other voice.
