1.1 The human auditory system

The human auditory system serves several important purposes in daily life. One of the most prominent features is to understand spoken words, which allows people to communicate in an efficient and interactive manner. In case of potential danger, the auditory system may provide means to detect dangerous events, such as an approaching car, at an early stage and react accordingly. In such cases, the great advantage of the auditory system compared with the visual system is that it allows us to monitor all directions simultaneously, including positions behind, above and below. In fact, besides a 360-degree ‘view’ in terms of both elevation and azimuth, the auditory system also provides an estimate of the distance of sound sources. This capability is remarkable, given the fact that humans have only two ears and yet are capable of analyzing an auditory scene in multiple dimensions: elevation, azimuth, and distance, while recognition of a sound source might be considered as a fourth dimension.

But besides being a necessary means for communication and to provide warning signals, the human hearing system also provides a lot of excitement and fun. Listening to music is a very common activity for relaxation and entertainment. Movies rely on a dedicated sound track to be exciting and thrilling. Computer games become more lifelike with the inclusion of dedicated sound tracks and effects.

In order to enjoy music or other audio material, a sound scene has to be recorded, processed, stored, transmitted, and reproduced by dedicated equipment and algorithms. During the last decade, the field of processing, storing, and transmitting audio has shifted from the traditional analog domain to the digital domain, where all information, such as audio and video material, is represented by series of bits. This shift in representation method has several advantages. It provides new methods and algorithms to process audio. Furthermore, for many applications, it can provide higher quality than traditional analog systems. Moreover, the quality of the material does not degrade over time, nor does making copies have any negative influence on the quality. And finally, it allows for a more compact representation in terms of information quantity, which makes transmission and storage more efficient and cheaper, and allows devices for storage and reception to be of very small form factor, such as CDs, mobile phones and portable music players (e.g. MP3 music players).