Handbook for Sound Engineers

Psychoacoustics 43

3.1 Psychoacoustics and Subjective Quantities

Unlike other senses, it is surprising how limited our
vocabulary is when talking about hearing.^1 Especially in
the audio industry, we do not often discriminate
between subjective and objective quantities. For
instance, the quantities of frequency, level, spectrum,
etc. are all objective, in a sense that they can be mea-
sured with a meter or an electronic device; whereas the
concepts of pitch, loudness, timbre, etc. are subjective,
and they are auditory perceptions in our heads. Psycho-
acoustics investigates these subjective quantities (i.e.,
our perception of hearing), and their relationship with
the objective quantities in acoustics. Psychoacoustics
got its name from a field within psychology—i.e., rec-
ognition science—which deals with all kinds of human
perceptions, and it is an interdisciplinary field of many
areas, including psychology, acoustics, electronic engi-
neering, physics, biology, physiology, computer sci-
ence, etc.
Although there are clear and strong relationships
between certain subjective and objective quanti-
ties—e.g., pitch versus frequency—other objective
quantities also have influences. For example, changes in
sound level can affect pitch perception. Furthermore,
because no two persons are identical, when dealing with
perceptions as in psychoacoustics, there are large indi-
vidual differences, which can be critical in areas such as
sound localization.^2 In psychoacoustics, researchers
have to consider both average performances among
population and individual variations. Therefore,
psychophysical experiments and statistical methods are
widely used in this field.
Compared with other fields in acoustics, psycho-
acoustics is relatively new, and has been developing
greatly. Although many of the effects have been known
for some time (e.g., Hass effect^3 ), new discoveries have
been found continuously. To account for these effects,
models have been proposed. New experimental findings
might invalidate or modify old models or make certain
models more or less popular. This process is just one
representation of how we develop our knowledge. For
the purpose of this handbook, we will focus on summa-
rizing the known psychoacoustic effects rather than
discussing the developing models.

3.2 Ear Anatomy and Function

Before discussing various psychoacoustic effects, it is
necessary to introduce the physiological bases of those
effects, namely the structure and function of our
auditory system. The human ear is commonly consid-

ered in three parts: the outer ear, the middle ear, and the

inner ear. The sound is gathered (and as we shall see

later, modified) by the external ear called the pinna and

directed down the ear canal (auditory meatus). This

canal is terminated by the tympanic membrane (ear-

drum). These parts constitute the outer ear, as shown in

Figs. 3-1 and 3-2. The other side of the eardrum faces

the middle ear. The middle ear is air filled, and pressure

equalization takes place through the eustachian tube

opening into the pharynx so normal atmospheric pres-

sure is maintained on both sides of the eardrum. Fas-

tened to the eardrum is one of the three ossicles, the

malleus which, in turn, is connected to the incus and

stapes. Through the rocking action of these three tiny

bones the vibrations of the eardrum are transmitted to

the oval window of the cochlea with admirable effi-

ciency. The sound pressure in the liquid of the cochlea

is increased some 30–40 dB over the air pressure acting

on the eardrum through the mechanical action of this

remarkable middle ear system. The clear liquid filling

the cochlea is incompressible, like water. The round

window is a relatively flexible pressure release allow-

ing sound energy to be transmitted to the fluid of the

cochlea via the oval window. In the inner ear the travel-

ing waves set up on the basilar membrane by vibrations

of the oval window stimulate hair cells that send nerve

impulses to the brain.

3.2.1 Pinna

The pinna, or the human auricle, is the most lateral (i.e.,

outside) portion of our auditory system. The beauty of

these flaps on either side of our head may be ques-

tioned, but not the importance of the acoustical func-

tion they serve. Fig. 3-3 shows an illustration of various

parts of the human pinna. The entrance to the ear canal,

or concha, is most important acoustically for filtering

because it contains the largest air volume in a pinna. Let

Figure 3-1. A cross-section of the human ear showing the

relationship of the various parts.

Pinna Ossicles Cochlea

Auditory

nerve

Eustachian

tube

Round

window

Oval

window

Eardrum