Page 89 - The Master Handbook Of Acoustics

64 CHAPTER THREE

Timbre is another subjective term. The analogous physical term is
spectrum. A musical instrument produces a fundamental and a set of
partials (or harmonics) that can be analyzed with a wave analyzer and
plotted as in Fig. 1-15. Suppose the fundamental is 200 Hz, the second
harmonic 400 Hz, the third harmonic 600 Hz, etc. The subjective pitch
that the ear associates with our measured 200 Hz, for example, varies
slightly with the level of the sound. The ear also has its own subjective
interpretation of the harmonics. Thus, the ear’s perception of the overall
timbre of the instrument’s note might be considerably different from the
measured spectrum in a very complex way.
In listening to an orchestra in a music hall, the timbre you hear is
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different for different locations in the seating area. The music is com-
posed of a wide range of frequencies, and the amplitude and phase of
the various components are affected by reflections from the various
surfaces of the room. The only way to get one’s analytical hands on
studying such differences is to study the sound spectra at different
locations. However, these are physical measurements, and the subjec-
tive timbre still tends to slip away from us. The important point of this
section is to realize that a difference exists between timbre and
spectrum.

Localization of Sound Sources

The perception of a direction to a source of a sound is, at least par-
tially, the result of the amazing encoding function of the external ear,
the pinna. Sound reflected from the various ridges, convolutions, and
surfaces of the pinna combines with the unreflected (direct) sound at
the entrance to the auditory canal. This combination, now encoded
with directional information, passes down the auditory canal to the
eardrum and thence to the middle and inner ear and on to the brain for
interpretation.
This directional encoding process of the sound signal is indicated
in Fig. 3-14. The sound wavefront can be considered as a multiplicity
of sound rays coming from a specific source at a specific horizontal
and vertical angle. As these rays strike the pinna they are reflected
from the various surfaces, some of the reflections going toward the
entrance to the auditory canal. At that point these reflected compo-
nents combine with the unreflected (direct) component.