Page 67 - Master Handbook of Acoustics

CHAPTER 4

The Perception of Sound

T he study of the physical structure of the ear is a study in physiology. The study of human

perception of sound is one of psychology and psychoacoustics. Psychoacoustics is an

inclusive science embracing the physical structure of the ear, the sound pathways and their function,
the human perception of sound, and the interrelationships of these. In many ways, psychoacoustics is
the human basis for the entire field of audio engineering. Its role in the design of perceptual codecs
such as MP3 and AAC is obvious, but psychoacoustics is also vital in architectural acoustics, telling
us, for example, how a room’s sound field is interpreted by the listener.
A stimulus wave striking the ear drum sets in motion mechanical movements that result in

electrical discharges that are sent to the brain. The brain recognizes and interprets those discharges,
creating the sensation that we call sound. That process is far from simple. Even after decades of
vigorous research, our knowledge of human hearing is still incomplete.
Listening to a symphony orchestra, concentrate first on the violins, then the cellos and the double

basses. Now focus your attention on the clarinets, then the oboes, bassoons, and flutes. The ability of
the ear-brain to separate individual parts of a complex sound wave is only one of the remarkable
powers of the human auditory system. Moreover, a keen observer can listen to the sound of a violin
and pick out the various overtones apart from the fundamental. The human ear is, by far, the most
sophisticated and complex device in all of audio engineering.

Sensitivity of the Ear

The sensitive nature of our hearing can be underscored by a thought experiment. The bulky door of an
anechoic chamber is opened, revealing extremely thick walls, and 3-ft wedges of glass fiber, pointing
inward, lining all walls, ceiling, and what could be called the floor, except that you walk on an open

steel grillwork.
You sit on a chair. This experiment takes time, and you lean back, patiently waiting. It is very eerie
in here. The sea of sound and noises of life and activity in which we are normally immersed and of
which we are ordinarily scarcely conscious are now conspicuous by their absence.
The silence presses down on you in the tomblike silence as the minutes pass. New sounds are

discovered, sounds that come from within your body. The pounding of your heart and the blood
coursing through the vessels become audible. If your ears are keen, your patience is rewarded by a
hissing sound between the thumps of the heart and the slushing of blood—it is the sound of air
particles striking your eardrums.
The human ear cannot detect sounds softer than the motion of air particles on the eardrum. This is

the threshold of hearing. There would be no reason to have ears more sensitive, because any lower-
level sound would be drowned by the air-particle noise. This means that the ultimate sensitivity of