Auditory Content

Auditory Content
An Introduction

Prof. David Bernstein
James Madison University

Computer Science Department

bernstdh@jmu.edu

Sound

Defined:
- A series of vibrations moving as waves through air or other gases, liquids, or solids
Our Primary Concern:
- Air waves (i.e.,successive compressions and rarefactions of air )

Sound Waves

Longitudinal:
- Whereas light waves are transverse (i.e., the wave is perpendicular to the direction of travel), sound waves are longitudinal (i.e., the wave us in the direction of travel)
Traveling:
- The air molecules disturb neighboring molecules, transferring their energy to them

Sound Waves (cont.)

A Periodic Pressure Wave

Sound Waves (cont.)

A Periodic Pressure Wave

Hearing

Perception of Soundwaves (An Approximation)

Volume:
- Difference in amplitude (measured in decibels)
- Audible Sounds: 0dB
- Painful Sounds: 120dB
Pitch:
- Differences in frequency (mesaured in Hz)
- Acoustic Signals: 15Hz to 18kHz
Timbre:
- Pronounced tam-bur
- Includes the waveform's distribution of energy at different frequencis (i.e., spectra) and its envelopes/transients (i.e., the attack, sustain and decay)

Perception of Sound Waves (cont.)

Perception of Sound Waves (cont.)

Localization Defined:
- Our ability to locate the origin of a sound
Localization Mechanisms:
- The interaural time difference (i.e., the difference in the time it takes for a sound to reach both ears). Most people can detect a difference of about 20 microseconds (millionths of a second).
- The interaural density difference (i.e., the difference in amplitude caused by our head interfering with the sound wave).
- Frequency filtering performed by the outer ear.

Perception of Sound Waves (cont.)

Complex Wave Forms:
- Created when two or more soundwaves travel through the same medium at the same time
Our Perception:
- We can (often) perceive the "original" waves (i.e., we perform Fourier analysis)
Biological Basis:
- Different locations of the basilir membrane are "tuned" to different frequencies (except when the tones are close)

Auditory Output Devices

Conventional Loudspeakers:
- One or more drivers, each of which consiste of a diaphragm (often a semi-flexible cloth or paper with a ... attached) mounted in a basket
- The electrical signal turns an electromagnet on and off, causing the diaphragm to vibrate, creating pressure waves
Electrostatic Speakers:
- Use a very thin electrically conductive diaphragm mounted in between two charged panels
Planar Magnetic Speakers:
- Use a thin magnetically conductive diaphragm mounted between two magnetic panels

Rendering

Using a Soundbank:
- The synthesizer has a database of sounds
Using Continuous Waveforms:
- The synthesizer uses pure waveforms to create complex waveforms