Audio refers to sound, especially as it is recorded, transmitted, processed, or reproduced using mechanical or electronic systems. The term is used across multiple fields—from physics and engineering to music production and consumer electronics—and covers both the physical phenomena of sound waves and the technologies that handle them.

Nature of sound

In physical terms, audio consists of pressure variations that move through a medium such as air, water, or solids. These pressure fluctuations carry information about amplitude (loudness) and frequency (pitch). Frequency is measured in hertz (Hz) and human hearing typically spans roughly 20 Hz to 20,000 Hz, though the exact range varies with age and individual differences.

Human perception

Perception of audio depends on the ear and brain interpreting vibrations. Loudness is perceived on a roughly logarithmic scale; objective measurements use units such as pascals for pressure and decibels (dB) for relative levels. Other perceptual attributes include timbre, which distinguishes different sound sources, and spatial cues that allow listeners to localize sound.

Audio technologies

  • Recording — converting acoustic sound into electrical or digital signals using microphones and transducers.
  • Processing — manipulating audio signals with equalization, compression, reverb, filtering, and other effects.
  • Transmission — sending audio over wires, radio waves, or networks; includes analog broadcast and digital streaming.
  • Reproduction — converting electrical or digital signals back to sound using loudspeakers, headphones, or implants.

Analog and digital audio

Analog audio represents sound as a continuous electrical waveform, while digital audio encodes that waveform as discrete numbers sampled at regular intervals. Common digital concepts include sampling rate (how often samples are taken) and bit depth (the resolution of each sample). Higher sampling rates and bit depths can capture more detail, though perceptual benefits depend on the application and listening conditions. Compact Disc audio, for example, uses a sampling rate of 44.1 kHz and a 16-bit depth.

File formats and codecs

Digital audio is stored and exchanged in a variety of file formats and under different compression schemes. Lossless formats retain the original signal without quality loss, while lossy codecs reduce file size by removing information deemed less perceptible. Choices of format and codec affect compatibility, storage requirements, and perceived fidelity.

Applications

  • Music — creation, recording, and playback.
  • Film and television — dialogue, sound effects, and musical scoring.
  • Broadcasting and streaming — delivering audio to mass and niche audiences.
  • Telecommunications — voice transmission for phone and VoIP services.
  • Interactive media and gaming — dynamic audio that responds to user actions.
  • Assistive technologies — hearing aids and other devices that improve accessibility.

Production stages

  1. Capture — selecting microphones and placement to record sound sources.
  2. Edit — arranging, trimming, and cleaning audio material.
  3. Mix — balancing levels, panning, and applying processing to create a coherent soundscape.
  4. Master — final adjustments for loudness, tonal balance, and consistency across playback systems.

Measurement and standards

Audio engineers and scientists use standardized measurements for consistency and quality control. Frequency is measured in hertz, sound pressure in pascals, and levels often expressed in decibels (dB). Various standards address sample rates, file formats, loudness normalization, and broadcast requirements; adherence to appropriate standards helps ensure interoperability and predictable playback.

Considerations for quality and accessibility

Perceived audio quality depends on recording technique, signal chain, processing choices, and playback environment. Noise, distortion, and improper equalization can degrade intelligibility and fidelity. Accessibility considerations—such as clear speech levels, captions, and assistive listening systems—improve access for listeners with hearing loss and other needs.

In everyday usage, "audio" can refer to the content (a recording or track), the equipment involved (an audio system), or the field of technology and study that enables sound capture and reproduction.