Sound, decibels and hearing 23 The human auditory system is truly remarkable. In addition to the charac- teristics described in earlier sections, it is equipped with protection systems which limit ear drum (tympanic membrane) movement below 100 Hz. These are to prevent damage, yet still allow the brain to interpret, via the help of body vibration conduction, what is really there. The ear detects pressure half- cycles, which the brain re-constructs into full cycles, and can provide up to 15 dB of compression at high levels. By means of time, phase and amplitude differences between the arrivals of sound at the two ears, we can detect direc- tion with great accuracy, in some cases to as little as a few degrees. Human pinnae (outer ears) have evolved with complex resonant cavities and reflectors to enhance this ability, and to augment the level-sensitivity of the system. Our pinnae are actually as individual to each one of us as are our finger- prints, which is the first step in ensuring that our overall audio perception sys- tems are also individual to each one of us. When we add all of this together, and consider the vagaries introduced by what was discussed in Section 2.3.1, we should be able to easily realise how the study of acoustics, and audio in general, can often be seen as a black art. Hearing perception is a world where everything seems to be on an individually sliding scale. Of course, that is also one reason why it can be so fascinating. However, until it is sufficiently understood, frustration can often greatly outweigh fascination. Psychoacoustics is the branch of the science which deals with the human perception of sound, and it has made great leaps forward in the past 25 years, not least because of the huge amount of money pumped into research by the computer world in their search for `virtual' everything. Psychoacoustics is an