Perceptual Coherence : Hearing and Seeing

(termed the amplitude envelopes) are the critical acoustic information.
(Sometimes the term amplitude envelope refers to amplitude of the entire
sound and not the amplitudes of individual frequencies.)
However, before summarizing the evolving spectral envelope, it is worth
considering the usefulness of pitch, loudness, and noisiness as perceptual
cues. All give information about the size and physical construction of the
object. Yet although pitch, loudness, and noisiness may be useful for the
categorization of sounds, they seem restricted in their ability to allow
the identification of specific objects. For example, the fundamental frequency
of the vocal fold vibration is the most important cue for identifying
male, female, and child speakers, and the pitch range and variation in loudness
can provide additional information about the age and size of the
source. But neither pitch nor loudness uniquely specifies an individual.
Helmholtz (1877) describes the seamless transition between noise and
tones (and that was part of the argument in chapter 1). Sound sources can
be understood to fall somewhere on that continuum. At one extreme, the
sound is all noise, for example snare drums, fan noise, gurgling of fluids,
and computer noise; at the other extreme, the sound is the superposition
only of sinusoidal waves. Most sources fall between the ends: Flutes are
characterized by a steady noise due to blowing across the mouthpiece
opening; voices may be characterized by breathiness due to incomplete closure
of the vocal folds; and other sources may be characterized by an initial
noisy sound that evolves into a stronger pitch sound such as the initial
metallic sounds for struck triangles and the initial scratchy bowing sounds
before the bow catches the string and generates a musical note. But, again,
noisiness seems restricted to identifying types of sounds.
Although the amplitude envelope changes smoothly, we can operationally
define three parts: (1) the onset, (2) the steady state, and (3) the decay.
For each part, the goal is to derive acoustic properties that correlate
with the perceived qualities of the sounds.
The Onset of the Sound
The Perception of Quality: Auditory Timbre 349
The rise or attack time from the initiation of excitation until the sound
reaches its maximum amplitude is determined by the manner of excitation
and the damping of the vibration modes. If the damping is similar for all
the modes, then the attack time for all the frequency components is similar
and the amplitudes increase synchronously. If the damping differs, then
the attack times of the components will differ, and the amplitudes of the
components can follow significantly different trajectories. In spite of the
possible differences in onset among the frequency components, the onset
duration usually is measured by the time it takes for the entire sound to
increase from its initiation to its maximum amplitude.