Course overview

  • We’ll begin with acoustics and psychoacoustics because binaural audio, ambisonics, and Dolby Atmos all depend on how sound behaves and how we perceive it.
  • Over the semester, we’ll work with stereo, binaural audio, ambisonics, 5.1, and Dolby Atmos.
  • Today’s class ends with a listening session that compares these formats.

Fundamentals of sound

Diagram showing how sound travels through the outer, middle, and inner ear
  • We can study sound in three ways: as physical vibrations and waves, as activity in the ear, and as a perceptual experience.
  • Sound, noise, and music are contextual labels. The same signal may fit any of them, depending on its purpose and setting.

How you locate a sound

  • According to Rayleigh’s duplex theory from 1907, interaural time differences help us locate low frequencies, while interaural level differences help us locate high frequencies.
  • The maximum time difference across a human head is about 650 microseconds.
  • The head shadow depends on frequency. For a sound 90 degrees to one side, the shadow measures about 10 dB at 3 kHz, 20 dB at 6 kHz, and 35 dB at 10 kHz. Below roughly 2 kHz, sound waves bend around the head and weaken this cue. That is why hearing relies on both time and level differences.
Diagrams showing cone-of-confusion positions that produce identical localization cues

The cone of confusion

  • Sounds arriving from several directions can produce identical time and level differences. Those two cues alone cannot distinguish front from back or up from down.
  • The pinna resolves elevation with a spectral notch that slides from about 5 kHz for sounds straight ahead to about 10 kHz for sounds overhead.
  • Head movement provides more information. When you turn your head, both cues change and help reveal the sound’s direction.

How you judge distance

  • In a free field, the sound level falls by 6 dB each time the distance doubles. To make a sound seem half as far away, however, its level must increase by about 10 dB.
  • Indoors, the strongest cue is the ratio of reverberant to direct sound.
  • Familiar sounds can override these cues. A shout tends to sound far away, while a whisper tends to sound close, regardless of playback level.

Immersive audio technologies

Dolby Atmos

Atmos is an object-based format. Sounds carry position metadata, and a renderer maps them to the available speakers.

Ambisonics

Ambisonics is a scene-based format. It uses spherical harmonics to encode a full sphere of sound, which can then be decoded for different speaker layouts.

Listening session

  • We’ll listen to some 5.1 mixes with the time we have left.

Source

The localization and distance material comes from Elizabeth Wenzel, Durand Begault, and Martine Godfroy-Cooper, “Perception of Spatial Sound,” ch. 1 of Agnieszka Roginska and Paul Geluso, eds., Immersive Sound (Routledge, 2017).