Overview of How Digital Audio Works

Understanding Digital Sound

How Sound Works

  • Definition of Sound: A series of vibrations traveling through a medium, typically air.

  • Compression and Rarefaction:

    • Compression: Increased air pressure as drum head vibrations move outward.

    • Rarefaction: Decreased air pressure during inward movements.

  • Sound Waves: Longitudinal waves; outward-moving variations in air pressure.

  • Acoustics: Study of sound wave behavior in different environments.

Dimensions of a Sound Wave

  • Amplitude: Change in air pressure; determines loudness.

    • Graph Representation: Peaks (crests) = compression; troughs = rarefaction.

  • Frequency: Number of oscillations per second (measured in Hertz, Hz).

    • Higher frequency = higher pitch; lower frequency = lower pitch.

  • Wavelength: Distance between crests; inversely proportional to frequency.

Period and The Doppler Effect

  • Period: Time for one complete oscillation of a wave.

  • Doppler Effect: Change in frequency perceived when sound source moves relative to listener.

  • Sound Speed: Approximately 1,125 feet (343 meters) per second in air.

Sine Waves and Overtones

  • Sine Wave: Basic waveform used to represent sound; ideal for pure sounds.

  • Overtones:

    • Fundamental Frequency: The main frequency heard.

    • Harmonics: Whole-number multiples of the fundamental frequency.

    • Inharmonic Partials: Frequencies not related to the fundamental.

Phase and Wave Interference

  • Phase: Relative position in a cycle of a waveform.

  • Wave Interference: Results in:

    • Constructive Interference: Waves amplify each other.

    • Destructive Interference: Waves reduce each other's amplitude.

Sound and Perception

  • Psychoacoustics: Study of how humans perceive sound frequencies and amplitudes.

  • Musical Pitch: Dependent on frequency; training helps identify notes.

  • Loudness: Subjective perception related to amplitude.

Digital Audio Basics

  • Analog vs. Digital:

    • Analog: Continuous signal representing sound (more susceptible to noise).

    • Digital: Discrete samples representing sound values (requires A/D and D/A converters).

  • Sampling Rate: Frequency of samples taken; determines frequency fidelity.

  • Bit Depth: Number of discrete levels for signal resolution; impacts dynamic range.

Common Sampling Frequencies and File Formats

  • Standard Formats:

    • 16-bit/44.1 kHz: CD audio standard.

    • 24-bit/96 kHz: Higher resolution.

  • File Types:

    • Lossless Formats: WAV, AIFF.

    • Lossy Formats: MP3, AAC.

Midi Overview

  • MIDI (Musical Instrument Digital Interface): Protocol for recording musical gestures and events.