Digital Signal Processing Reference
In-Depth Information
ity of the sensors is highest at low frequencies and decreases towards
higher frequencies.
Ossicles (malleus, incus, stapes)
Eardrum
Inner ear
Membrane
Receptors for
low frequencies
Outer
ear
Auditory
nerves
Receptors
for high frequencies
Middle
ear
Eustachian tube
Outer ear
= mechanical impedance converter
high ........middle...............low frequencies
Filter
Filter characteristics
of outer ear and
ear tube and eardrum
(e.g. resonance at ~3 kHz)
Frequency receptors
inside cochlea
Auditory
nerve signals
Fig. 8.4.
Technical model of the human ear
In the following section, we want to investigate those characteristics of
the human ear that are of interest for audio coding. To begin with, the sen-
sitivity of the ear is to a great extent dependent on frequency. Sound sig-
nals below 20 Hz and above 20 kHz are practically not audible. The
maximum sensitivity of the ear is in the range around 3 kHz to 4 kHz; out-
side this range the sensitivity decreases towards higher or lower frequen-
cies. Sounds with a level below a certain threshold (referred to as threshold
of audibility) are not perceived by the human ear. The threshold of audibil-
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