2. Cetacean Ears
71
and the debate will continue, primarily because so little physiological data
are available and the results are equivocal. To provide a framework for
assessing this debate, the relevant concepts of middle ear function will first
be reviewed.
5.2.1 Middle Ear Functions
Because the basic land mammal ear comprises an air-filled middle ear and
a fluid-filled inner ear, there is an air-to-fluid impedance mismatch that must
be overcome for efficient transfer of sound energy. Consequently, middle
ears are commonly thought of as impedance-matching devices or transformers that counteract the approximately 36dB loss expected from the
impedance differences between air and the fluid-filled inner ear. The impedance transformation is achieved primarily by mechanical advantages
derived from the difference in the area of the middle ear membranes (large
tympanic versus small oval window) and from the lever ratio of the bony
ossicular chain linking the membranes. Acting in concert, these create a
pressure gain and concomitant particle velocity reduction at the inner ear.
For marine mammals this function may not only be unnecessary but
disadvantageous.
Improved power transfer is not necessarily the only function of the
middle ear. Recent studies propose a complementary middle ear function,
called the peripheral filter-isopower function, in which the middle ear has
a tuning role (for comprehensive discussions see Rosowski 1994; Yost 1994).
In land mammals, the middle ear is an air-filled cavity with significant differences among species in volume (V), stiffness (K), and mass (M). Middle
ears in each species are differentially tuned to a specific middle ear resonance based on the combined chain of middle ear impedances, which, in
turn, depend upon the mechanical properties of the middle ear components.
For any animal, the sum of impedances is lowest, middle ear admittance is
greatest, and energy transmission is most efficient at the middle ear's resonant frequency (f). As might be expected, this frequency also tends to be at
or near the frequency with the lowest threshold (best sensitivity) for that
species (Fay 1992). Impedance (Z) can be thought of as the sum of resistance and reactance. Because friction is minimal in the middle ear, resistance can be discounted. Reactance (X) is determined by mass and stiffness.
Stiffness and mass act inversely in a frequency-dependent system:
fresonance = (1I21t) (KlM)ll2. Calculating middle ear impedances is far more
complex than simply describing the middle ear elements, but, in general,
increasing stiffness in the middle ear system improves the transmission of
high frequencies; adding mass to the system favors low frequencies. Consequently, in addition to impedance matching, middle ears may also be evolutionarily tuned by having frequency selective elements with different
mass and stiffness in each species. Ultrasonic species have relatively stiff
ossicular chains with small, stiff-walled cavities. Low-frequency species, like
heteromyid desert rodents, mole rats, and elephants, have large middle ear
71
and the debate will continue, primarily because so little physiological data
are available and the results are equivocal. To provide a framework for
assessing this debate, the relevant concepts of middle ear function will first
be reviewed.
5.2.1 Middle Ear Functions
Because the basic land mammal ear comprises an air-filled middle ear and
a fluid-filled inner ear, there is an air-to-fluid impedance mismatch that must
be overcome for efficient transfer of sound energy. Consequently, middle
ears are commonly thought of as impedance-matching devices or transformers that counteract the approximately 36dB loss expected from the
impedance differences between air and the fluid-filled inner ear. The impedance transformation is achieved primarily by mechanical advantages
derived from the difference in the area of the middle ear membranes (large
tympanic versus small oval window) and from the lever ratio of the bony
ossicular chain linking the membranes. Acting in concert, these create a
pressure gain and concomitant particle velocity reduction at the inner ear.
For marine mammals this function may not only be unnecessary but
disadvantageous.
Improved power transfer is not necessarily the only function of the
middle ear. Recent studies propose a complementary middle ear function,
called the peripheral filter-isopower function, in which the middle ear has
a tuning role (for comprehensive discussions see Rosowski 1994; Yost 1994).
In land mammals, the middle ear is an air-filled cavity with significant differences among species in volume (V), stiffness (K), and mass (M). Middle
ears in each species are differentially tuned to a specific middle ear resonance based on the combined chain of middle ear impedances, which, in
turn, depend upon the mechanical properties of the middle ear components.
For any animal, the sum of impedances is lowest, middle ear admittance is
greatest, and energy transmission is most efficient at the middle ear's resonant frequency (f). As might be expected, this frequency also tends to be at
or near the frequency with the lowest threshold (best sensitivity) for that
species (Fay 1992). Impedance (Z) can be thought of as the sum of resistance and reactance. Because friction is minimal in the middle ear, resistance can be discounted. Reactance (X) is determined by mass and stiffness.
Stiffness and mass act inversely in a frequency-dependent system:
fresonance = (1I21t) (KlM)ll2. Calculating middle ear impedances is far more
complex than simply describing the middle ear elements, but, in general,
increasing stiffness in the middle ear system improves the transmission of
high frequencies; adding mass to the system favors low frequencies. Consequently, in addition to impedance matching, middle ears may also be evolutionarily tuned by having frequency selective elements with different
mass and stiffness in each species. Ultrasonic species have relatively stiff
ossicular chains with small, stiff-walled cavities. Low-frequency species, like
heteromyid desert rodents, mole rats, and elephants, have large middle ear