Acoustics and Digital Audio Basics

Acoustics and Sound

• Acoustics is the branch of physics that studies sound
• Sound pressure levels (loudness or volume) are measured in decibels (dB)
• Sound waves vary in sound pressure level (amplitude) and in frequency or pitch

Digital Audio

• Digital audio is created by representing the characteristics of a sound wave using numbers
• Digitizing involves converting analog signals to digital numbers
• Digitized sound is sampled sound, with samples taken every nth fraction of a second
• The quality of digital recording depends on the sampling rate and bit depth

Sampling Rates and Bit Depth

• Common sampling rates include 44.1 kHz (CD-quality), 22.05 kHz, and 11.025 kHz
• Sample sizes are typically 8 bits or 16 bits
• Higher sampling rates and bit depths result in higher quality audio

Mono and Stereo

• Stereo recordings use two microphones (left and right) and require twice as much storage space as mono files
• Stereo recordings are more lifelike and realistic than mono recordings

Archiving Digital Audio

• Archiving audio at a higher specification than needed allows for potential future upgrades in playback technology

Editing Digital Recordings

• Basic sound-editing operations include:
  • Trimming: removing dead air or unnecessary extra time from a recording
  • Splicing and assembly: removing extraneous noises and assembling longer recordings
  • Volume adjustments: adjusting the overall volume of a recording
  • Format conversion: converting between different audio formats
  • Resampling or downsampling: converting to a lower sampling rate or bit depth
  • Fade-ins and fade-outs: smoothing out the beginning and end of a sound file
  • Equalization: modifying the frequency content of a recording
  • Time stretching: altering the length of a sound file without changing its pitch
  • Digital signal processing (DSP): processing a signal with special effects
  • Reversing sounds: playing a sound in reverse
  • Multiple tracks: editing and combining multiple tracks

MIDI Audio

• MIDI (Musical Instrument Digital Interface) is a communications standard for electronic musical instruments and computers
• MIDI provides a protocol for passing detailed descriptions of a musical score
• MIDI data is not digitized sound, but a shorthand representation of music stored in numeric form
• Composing original scores can be done using MIDI

MIDI vs. Digital Audio

• MIDI advantages:
  • Smaller file size
  • Flexibility in editing and playback
  • Device-independent playback quality
• MIDI disadvantages:
  • Playback quality depends on the end user's MIDI device
  • Limited use for spoken dialog
• Digital audio advantages:
  • Wider selection of application software and system support
  • No music theory knowledge required
• Digital audio disadvantages:
  • Larger file size
  • Limited editing flexibility

Audio File Formats

• Common audio file formats include:
  • AIFF (Audio Interchange File Format)
  • WAV (Waveform Audio File Format)
  • MP3 (MPEG Audio Layer 3)
  • AAC (Advanced Audio Coding)
  • MP4 (MPEG-4 Part 14)
• Codecs (compressor-decompressor) software compress and decompress audio data for storage and transmission

Managing Audio Files and Integrating into Multimedia Projects

• Considerations include:
  • File format compatibility
  • Sound playback capabilities
  • Sound type and placement in the project
  • Acquiring and editing source material
  • Testing sound-and-image synchronization
  • Copyright and licensing issues