Handbook for Sound Engineers

44 Chapter 3

us assume for the moment that we have no pinnae, just
holes in the head, which is actually a simplest model for
human hearing, called the spherical head model. Cup-
ping our hands around the holes would make sounds
louder as more sound energy is directed into the open-
ing. How much does the pinna help in directing sound
energy into the ear canal? We can get some idea of this
by measuring the sound pressure at the opening of the
ear canal with and without the hand behind the ear. Wie-
ner and Ross^4 did this and found a gain of 3 to 5 dB at
most frequencies, but a peak of about 20 dB in the
vicinity of 1500 Hz. Fig. 3-4 shows the transfer function
measured by Shaw,^5 and the curves numbered 3 and 4
are for concha and pinna flange, respectively. The
irregular and asymmetric shape of a pinna is not just for
aesthetic reasons. In Section 3.11, we will see that it is
actually important for our ability to localize sounds and
to aid in spatial-filtering of unwanted conversations.

3.2.2 Temporal Bones

On each of the left and right sides of our skull, behind
the pinna, there is a thin, fanlike bone—namely, the
temporal bone—covering the entire human ear, except
for the pinna. This bone can be further divided into four
portions—i.e., the squamous, mastoid, tympanic and
petrous portions. The obvious function for the temporal
bone is to protect our auditory system. Other than
cochlear implant patients, whose temporal bone has to
be partly removed during a surgery, people might not
pay much attention to it, especially regarding acoustics.
However the sound energy that propagates through the
bone into our inner ear, as opposed to through the ear
canal and middle ear, is actually fairly significant. For

patients with conductive hearing loss—e.g., damage of

middle ear—there are currently commercially available

devices, which look something like headphones and are

placed on the temporal bone. People with normal hear-

ing can test it by plugging their ears while wearing the

device. Although the timbres sound quite different from

normal hearing, the filtered speech is clear enough to

understand. Also because of this bone conduction, along

with other effects such as acoustic reflex, which will be

discussed in Section 3.2.4.1, one hears his or her own

Figure 3-2. Highly idealized portrayal of the outer ear, mid-
dle ear, and inner ear.

Ossicles Cochlea Basilarmembrane

Ear

drum

Ear

canal

Oval

window

Round

window

Outer

ear

Middle

ear

Eustachian

tube

Inner

ear

Figure 3-3. The human outer ear, the pinna, with identifica-

tion of some of the folds, cavities, and ridges that have sig-

nificant acoustical effect.

Figure 3-4. The average pressure gain contributed by the

different components of the outer ear in humans. The

sound source is in the horizontal plane, 45q from straight

ahead. (After Shaw, Reference 5.)

Scapha

Helix

Fossa

Concha

Lobule

Intertragic

notch

T Total: 45o

1 Spherical head

2 Torso and neck, etc.

3 Concha

4 Pinna flange

5 Ear canal and eardrum

Frequency–Hz

Acoustic gain components–dB

T 5 3 1 4 2

20

15

10

5

0



5



10

0.2 0.5 1.0 2 5 10