52
Hom-like exlcmal pinna
up to 30 dB gain in
sound pressure due
to the external pinna,
dependem on irs size
Henry C. Bennet-Clark
Sound collection and impedance matching
in a mammalian ear
Thin C.'U'drum
membrane
""'- f
further gain due to~
ca. 3 : I area ratio
within the external
ca. 30 : I
Liquid-filled
U-lube
mealus
bas1lar membrane
Force-lrallsf erring
3-levcr syslcm of ossiclcs
ca. 10 : I lever ratio
Pressure-equal iscd
air-filled middle car cavily
Fig. 4. Diagram of the ear of a therian mammal to show the sound-collecting roles of the
external pinna and external auditory meatus with the impedance matching roles (in italics)
of the chain of ear ossicles and the relative areas of the eardrum and oval window in the
cochlea (in italics). (From data in Shaw 1974; M0ller 1974; Lewis 1983)
In the ears of mammals, the large-area pirma collects sound and leads into the
tapering external auditory meatus (Fig.4); these act together to increase the sound
pressure at the tympanum. The middle ear ossicles of the mammalian ear act as a
lever system that transfers the movements of the tympanum to the oval window of
the cochlea (Fig. 4). The lever ratio of the ossicular chain is about 1.3 to I and the
area ratio between the tympanum and the oval window is about 14 to I, so the
product of these values gives an overall transformer ratio between the tympanum
and the oval window of about 18 : I (M0ller 1974); this gives to an impedance
ratio of ( 18)2 (M0ller 1974; Fletcher 1992) or 330 to I which is about one tenth of
the optimum for ideal impedance matching (cf. Table 1). The threshold sound
sensitivity of the ear (20 J!Pa) corresponds to a vibration amplitude in air of about
I o-Il m, which, after transduction via the eaxtemal auditory meatus, tympanum
and ossicular chain, leads to a movement of the basilar membrane and hence of the
hair cells of I o- 11 m (Fletcher 1992). Despite the considerable impedance
Hom-like exlcmal pinna
up to 30 dB gain in
sound pressure due
to the external pinna,
dependem on irs size
Henry C. Bennet-Clark
Sound collection and impedance matching
in a mammalian ear
Thin C.'U'drum
membrane
""'- f
further gain due to~
ca. 3 : I area ratio
within the external
ca. 30 : I
Liquid-filled
U-lube
mealus
bas1lar membrane
Force-lrallsf erring
3-levcr syslcm of ossiclcs
ca. 10 : I lever ratio
Pressure-equal iscd
air-filled middle car cavily
Fig. 4. Diagram of the ear of a therian mammal to show the sound-collecting roles of the
external pinna and external auditory meatus with the impedance matching roles (in italics)
of the chain of ear ossicles and the relative areas of the eardrum and oval window in the
cochlea (in italics). (From data in Shaw 1974; M0ller 1974; Lewis 1983)
In the ears of mammals, the large-area pirma collects sound and leads into the
tapering external auditory meatus (Fig.4); these act together to increase the sound
pressure at the tympanum. The middle ear ossicles of the mammalian ear act as a
lever system that transfers the movements of the tympanum to the oval window of
the cochlea (Fig. 4). The lever ratio of the ossicular chain is about 1.3 to I and the
area ratio between the tympanum and the oval window is about 14 to I, so the
product of these values gives an overall transformer ratio between the tympanum
and the oval window of about 18 : I (M0ller 1974); this gives to an impedance
ratio of ( 18)2 (M0ller 1974; Fletcher 1992) or 330 to I which is about one tenth of
the optimum for ideal impedance matching (cf. Table 1). The threshold sound
sensitivity of the ear (20 J!Pa) corresponds to a vibration amplitude in air of about
I o-Il m, which, after transduction via the eaxtemal auditory meatus, tympanum
and ossicular chain, leads to a movement of the basilar membrane and hence of the
hair cells of I o- 11 m (Fletcher 1992). Despite the considerable impedance