COURSE NOTES
Introduction to Computer Music, Principles of Sound

Course Goals

• Study the use of computers to create and analyze musical sounds

• Study the use of computers to combine musical sounds into compositions

• Study methods for computer storage and post-processing of music

Computer Sound Synthesis

• Analysis of musical sound

  • Fourier analysis and transforms

  • Filtering and windowing

  • The phase vocoder algorithm

• Synthesis of musical sound

  • Additive synthesis

  • Subtractive synthesis

  • Nonlinear synthesis

Computer Music Synthesis

• Generation of music using computers

  • Direct transcription

  • Stochastic (random) algorithms

  • Artificial intelligence algorithms

• Post-processing and storage of music using computers

  • Reverberation and localization

  • MIDI

  • Popular sound file formats

Interdisciplinary Nature of Computer Music

• Music (composition and performance)

• Computer Science (programming and algorithms)

• Electrical Engineering (digital signal processing)

• Physics (acoustics)

• Psychology (cognition theory)

Principles of Sound

• Sound is the perception we sense when the components of a medium (e.g. air) are subject to a vibration that is detected by the inner components of the ear.

• Sound waves are the vibrations that travel through the medium.

• The frequency f of a (periodic) sound is the number of cycles that pass the point per unit time. (Hertz = cycles/second)

• The period T of a (periodic) sound is the time it takes for one cycle to pass a given point. Thus, T = 1/f.

• The amplitude A of a sound wave in air is the amount the pressure changes (maximally) from atmospheric pressure.

• The intensity I of a sound wave is a measure of its power, measured in decibels.

• Physical sound intensity is usually measured in decibels above the "threshold of hearing" at 1000 Hz (I_ref = 10^-12 watt/m²)

• IL = 10 log₁₀(I/I_ref)

• Intesity level can be measured from any reference.

• Examples:

  • Rock band (10 watt/m²). Find IL.
    Answer: IL = 130 dB.

  • Quiet home (30 dB). Find I.
    Answer: I = 10^-9 watt/m²

  • Doubling the intensity of a sound corresponds to an increase of how many decibels?
    Answer: +3 dB

Fletcher-Munson Curves [HANDOUT]

Types of Musical Sounds

• Strings (violin, guitar, banjo)

• Wind (flute, tuba, trumpet)

• Percussion (drums, piano, xylophone)

• Voice

Other Sound Properties

• Sound intensity is affected by the type of medium and the distance travelled.

• The velocity of a sound wave is affected by the temperature of the medium.

• The direction of a sound wave is affected by the properties of surfaces in the path of the sound wave.

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