Digital Signal Processing Reference
In-Depth Information
Auricle
Ohrmuschel
Gehörknöchelchen
Hammer, anvil, stirrup
Semicircular canals
Labyrinth
Vestibular nerve
Cochlea
Schnecke
Auditory
nerve
External
auditory
canal
Gehörgang
Eardrum
(Tympanic
membrane )
Trommelfell
Eustachische Röhre
Eustachian tube
Illustration 79:
Structure of the hearing organs
Acoustic fluctuations in pressure are transported to the eardrum. This is effected by the mechanics of the
ossicles between the cochlea and the eardrum. The ossicles produce an ingenious travelling wave in the
cochlea which is filled with liquid.
This wave should be seen in principle as a sweep signal - see Chapter 6 “System Analysis". The whole
spectrum of the frequencies present in the fluctuation of pressure is spread over the length of the cochlea
canal, i.e. certain areas of sensory hair cells are responsible for specific frequencies.
What is the role of hearing in this process? It can be shown that the actual pattern recog-
nition is carried out here or at least is prepared here. Illustration 79 shows the structure of
the hearing organs.
The acoustic fluctuations in pressure of the sound wave reach the eardrum from the auricle
(funnel effect) or from the outer auditory canal. These fluctuations in pressure are con-
veyed by a mechanical system of ossicles - hammer and stirrup - to the most important
"sub-system", the cochlea.
This is intended as a simplified description of these processes. The cochlea has the overall
shape of a rolled-up tapering funnel and is filled with liquid. Along this funnel "sensory
hairs" are distributed which are connected to nerve cells - neurones. These are the actual
signal sensors.
The fluctuations in pressure create a travelling wave which subsides towards the end of
the cochlea. This takes place in a particular way which is called frequency dispersion.
High-frequency components in the pressure fluctuation align themselves at a different
part of the wave than low-frequency elements. Thus well-defined parts of the cochlea with
their sensory hairs are responsible for low, medium and high frequencies.
