IAT 340 notes 6

IAT 340: Sound Design

Introduction

• Course Title: Sound Design

• Topic: Sound Spatialisation

• Institution: Simon Fraser University (SFU)

• Instructor: Philippe Pasquier (Associate Professor)

• Contact: pasquier@sfu.ca

Understanding Waves

Types of Waves

• Transverse Waves:

  • Definition: Displacement of the medium is perpendicular to the direction of wave propagation.

  • Examples:

  • A ripple on a pond

  • A wave on a string

• Longitudinal Waves:

  • Definition: Displacement of the medium is parallel to the direction of wave propagation.

  • Examples:

  • Waves in a "slinky"

  • Sound waves in air

Sound Spatialisation

Waveform Representation

• Longitudinal wave illustrated by:

  • Increased Pressure

  • Decreased Atmospheric Pressure

  • Motion of air molecules associated with sound

Spatial Representation

• Related Variables:

  • $ext{distance}$

  • $ext{wavelength} = ext{λ (meters)}$

Temporal Representation

• Related Variables:

  • $ext{period} = ext{T (seconds)}$

Spatial Sound

Definition of Spatial Sound

• A disturbance of air pressure creates a spherical traveling pressure wave (sound).

• Sound is characterized as spatial and three-dimensional (3D).

Spatial Sound Perception

Cues for Angle Determination

• Temporal Cues:

  • Description: Wavefronts do not arrive at both ears simultaneously.

  • Allows for calculation of angle based on time of arrival.

  • Known as Interaural Time Delay (ITD).

  • Precedence Effect: Perception discards later arrivals; only one location is discerned.

• Amplitude Cues:

  • Description: Extra distance traveled plus head screening effects create differences in amplitude.

  • Known as Interaural Level Difference (ILD).

• Spectral Cues:

  • Description: The head and ears filter sound, especially high frequencies.

  • Known as Head-related Frequency Response (HRFR).

  • Important for distinguishing front-back positioning; helps clarify ambiguity in ITD and ILD.

  • Characteristics: Higher frequencies perceived as “light” (top), lower frequencies as “heavy” (ground).

• Head Movement:

  • Utilized to assess if ITD and ILD cues are increasing or decreasing; aids in discerning front and back location.

Cues for Perception of Distance

• Distance Cues:

  • Loudness diminishes with distance.

  • Ratio of direct to reverberant sound affects perception.

  • Patterns and directions of early delays provide information.

  • Higher frequencies diminish more readily, due to moisture absorption.

  • Knowledge of source spectra and loudness critically informs perception quality.

• Visual Cues:

  • Relation: The voice of a visible character will be spatially associated with the character.

Techniques of Sound Spatialisation

Classification of Techniques

1. Perception Simulation:

   • Objective: Reproduce natural auditory experiences.

   • Examples include:

     • Binaural sound

     • n-channel stereophony

     • Dolby surround

2. Sound-field Simulation:

   • Objective: Reproduce the actual sound field.

   • Examples include:

     • Beam forming

     • Wave field synthesis

     • Ambisonic surround

Types of Sound

• Monophonic Sound:

  • Contexts: Telephone, AM radio.

• Stereophonic Sound:

  • Contexts: CDs, FM radio, and televisions.

  • Introduced Dolby stereo (LR) for films in 1976.

• Multiphonic Sound (also known as stereophonic):

  • Contexts and Formats:

  • 4.1 quadriphony (concerts)

  • 5.1 Home cinema, DVDs

  • 7.1 DVDs

  • 9.1

  • 24.2 for electroacoustic concerts

  • Acousmonium: Speakers as instruments in electroacoustic concerts.

Stereo Techniques

• 2-Stereo Signal (LR):

  • Layout: Covers the sound stage typically in front of the listener.

  • Defines horizontal plane among loudspeakers.

  • Utilizes panning and phase control for sound positioning.

  • Limitations: Limited speaker separation of around 60 degrees to maintain central phantom image.

  • Performance diminishes with listener movement off central axis.

• 3-Stereo (LCR):

  • Enhancement: The central image becomes a stable source instead of a phantom source.

• 5.1 Surround Sound:

  • Central channel locks dialogues to the screen, enhancing clarity for off-centre listeners.

  • Surround speakers provide diffuse ambient sounds and effects, enhancing the immersive experience.

  • Low-frequency effects cater to sound effects requiring substantial bass.

Binaural Recording Techniques

• Definition: Aims to reproduce natural auditory experiences.

• Optimal Setup: Best experienced with headphones; feasible with loudspeakers.

• Recording Methods:

  • Use of a dummy head equipped with microphones in the ears.

  • Playback utilizing Head Related Transfer Functions (HRTF) specific to each ear, potentially averaged or individualized.

  • May include head tracking to counter front-back localization errors.

• Computational demands: High for realistic outcomes, particularly with loudspeakers due to crosstalk issues.

Applications of Binaural Techniques

• Industries utilizing binaural recordings include:

  • Video games

  • Virtual Reality (VR) applications

  • ASMR (Autonomous Sensory Meridian Response) experiences.

• Availability: Examples accessible online for headphone playback.

Ambisonic Surround Sound

• Definition: Sound-field simulation method that operates collaboratively with multiple speakers.

• Typical Configurations: 4 (plan) or 8 (3D) speakers used.

• Encoding/Decoding Process:

  • Consists of encoding sound as sets involving:

  • Overall pressure levels

  • Up-down velocity

  • Front-back velocity

  • Left-right velocity

Beamforming Techniques

• Definition: Involves controlling loudspeaker arrangements to direct sound.

• Application of Delay: Multiple speaker inputs create complex wave interference patterns that shape audio experience.

Wave Field Synthesis (WFS)

• Description: Technique simulating "artificial" wave fronts using large speaker arrays.

• Key Feature: Unlike stereo, sound localization remains consistent regardless of listener position.

Conclusion

• Quote: “Music is the poetry of the air.” – Jean Paul Friedrich Richter