TABLE OF CONTENTS Psychoacoustics is the study of the perception of sound. The sound processing performed by the ear and the brain is extremely complex, and its study [ZWI 81] is difficult because it involves subjectivity, as shown by the classification 'hearing, listening, understanding, comprehension' suggested by P. Schaeffer [SCH 66]. In this chapter, we will only be focusing on a few aspects of psychoacoustics: intensity and pitch, which are of direct interest to the musician, and the masking effects, of great use when designing sound compression techniques, such as the famous MP3 [1] format described in Chapter 5.
The sound intensity L I that we defined in Chapter 1, usually expressed in decibels, is a physical measurement of acoustic pressure. However, this measurement does not coincide with our sensation of sound intensity, referred to as loudness, the study of which was developed by H. Fletcher in the 1930's [FLE 29]. First of all, we only hear sounds for a range of frequencies between 20 Hz and 20 kHz. But even inside this interval, for a given decibel level, loudness varies depending on the frequency. In particular, hearing shows a sensitivity maximum between 3,000 and 4,000 Hz (see Figure 4.1), allowing for example the piccolo to effortlessly stand out in a tutti orchestra. If you go back to Figure 1.17, you will also notice that the soprano's harmonics are more intense in that frequency range, so that her voice clearly stands...
