Digital Sound & Music: Concepts, Applications, & Science, Chapter 4, last updated 6/25/2013
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4 Chapter 4 Sound Perception and Acoustics
4.1 Concepts
4.1.1 Acoustics
The word acoustics has multiple definitions, all of them interrelated. In the most general sense,
acoustics is the scientific study of sound, covering how sound is generated, transmitted, and
received. Acoustics can also refer more specifically to the properties of a room that cause it to
reflect, refract, and absorb sound. We can also use the term acoustics as the study of particular
recordings or particular instances of sound and the analysis of their sonic characteristics. We'll
touch on all these meanings in this chapter.
4.1.2 Psychoacoustics
Human hearing is a wondrous creation that in some ways we understand very well, and in other
ways we don't understand at all. We can look at anatomy of the human ear and analyze down
to the level of tiny little hairs in the basilar membrane how vibrations are received and
transmitted through the nervous system. But how this communication is translated by the brain
into the subjective experience of sound and music remains a mystery. (See (Levitin, 2007).)
We'll probably never know how vibrations of air pressure are transformed into our
marvelous experience of music and speech. Still, a great deal has been learned from an analysis
of the interplay among physics, the human anatomy, and perception. This interplay is the realm
of psychoacoustics, the scientific study of sound perception. Any number of sources can give
you the details of the anatomy of the human ear and how it receives and processes sound waves.
(Pohlman 2005), (Rossing, Moore, and Wheeler 2002), and (Everest and Pohlmann) are good
sources, for example. In this chapter, we want to focus on the elements that shed light on best
practices in recording, encoding, processing, compressing, and playing digital sound. Most
important for our purposes is an examination of how humans subjectively perceive the
frequencies, amplitude, and direction of sound. A concept that appears repeatedly in this context
is the non-linear nature of human sound perception. Understanding this concept leads to a
mathematical representation of sound that is modeled after the way we humans experience it, a
representation well-suited for digital analysis and processing of sound, as we'll see in what
follows. First, we need to be clear about the language we use in describing sound.
4.1.3 Objective and Subjective Measures of Sound
In speaking of sound perception, it's important to distinguish between words which describe
objective measurements and those that describe subjective experience.
The terms intensity and pressure denote objective measurements that relate to our
subjective experience of the loudness of sound. Intensity, as it relates to sound, is defined as the
power carried by a sound wave per unit of area, expressed in watts per square meter
(W/m2).
Power is defined as energy per unit time, measured in watts (W). Power can also be defined as
the rate at which work is performed or energy converted. Watts are used to measure the output
of power amplifiers and the power handling levels of loudspeakers. Pressure is defined as force
divided by the area over which it is distributed, measured in newtons per square meter
(N/m2)
or
more simply, pascals (Pa). In relation to sound, we speak specifically of air pressure
amplitude and measure it in pascals. Air pressure amplitude caused by sound waves is
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