Perception of sound  (Page 20/3)

Introduction

A basic understanding of human perception of sound is vital if you wish to design music synthesis algorithms to achieve your goals. Human hearing and other senses operate quite well in a relative sense. That is, people perceive properties of sound such as pitch and intensity and make relative comparisons. Moreover, people make these comparisons over an enormous dynamic range: they can listen to two people whispering in a quiet auditorium and determine which person is whispering the loudest. During a rock concert in the same auditorium, attendees can determine which vocalist is singing the loudest. However, once the rock concert is in progress, they can no longer hear someone whispering! Senses can adapt to a wide range of conditions, but can make relative comparisons only over a fairly narrow range.

In this module you will learn about pitch and frequency , intensity and amplitude , harmonics and overtones , and tuning systems . The treatment of these concepts is oriented to creating music synthesis algorithms. Connexions offers many excellent modules authored by Catherine Schmidt-Jones that treat these concepts in a music theory context, and some of these documents are referenced in the discussion below. For example, Acoustics for Music Theory describes acoustics in a musical setting, and is a good refresher on audio signals.

Pitch and frequency

Pitch is the human perception of frequency . Often the terms are used interchangeably, but they are actually distinct concepts. Musicians normally refer to the pitch of a signal rather than its frequency; see Pitch: Sharp, Flat, and Natural Notes and The Circle of Fifths .

Perception of frequency is logarithmic in nature. For example, a change in frequency from 400 Hz to 600 Hz will not sound the same as a change from 200 Hz to 400 Hz, even though the difference between each of these frequency pairs is 200 Hz. Instead, you perceive changes in pitch based on the ratio of the two frequencies; in the previous example, the ratios are 1.5 and 2.0, respectively, and the latter pitch pair would sound like a greater change in frequency. Musical Intervals, Frequency, and Ratio offers additional insights.

Often it is desirable to synthesize an audio signal so that its perceived pitch follows a specific trajectory . For example, suppose that the pitch should begin at a low frequency, gradually increase to a high frequency, and then gradually decrease back to the original. Furthermore, suppose that you should perceive a uniform rate of change in the frequency.