11.1 Impedance Matching in Sound Production
and Hearing: a Comparative Study
Henry C. Bennet-Clark
Department of Zoology, University of Oxford, South Parks Road, Oxford,
OX13PS, UK
Abstract
Sound production involves stages of impedance matching between the higherdensity body of the animal and lower-density air. Hearing involves further
impedance matching, from air to the higher-density sensory cells. Initial stages of
sound production may include a frequency multiplier that converts slow muscle
contractions into higher frequency mechanical vibrations. The frequency multiplier
may also determine the sound frequency. Larger sound sources allow better
impedance amtching with the air so sound radiation often exploits acoustic
transformers to increase the effective size of the sound source. In many cases the
conversion efficiency of muscle power to sound power is high, giving a large
effective range for the signal. Sound detection often uses acoustic transformers to
concentrate the sound onto relatively dense vibrating sturctures that are coupled to
the sensory cells, providing the inverse of the impedance matching that occurs in
sound production. These transformers may be associated with directional
mechanisms and may drive arrays of receptors that allow frequency analysis.
Sound production and hearing are special cases of the general phenomenon of
impedance matching that occurs throughout biomechanics.
Key words Sound production, impedance matching, efficiency, hearing, frequency
analysis
1 Introduction: the Stages in Sound Production and
Hearing
The chain of processes in acoustic communication in animals involves the
production of a signal which is radiated as sound, the propagation of the sound
through the medium, its reception, and its effect on the receiving animal. Here, in
the context of sensory ecology, I shall confine myself to the physical processes of
the interfaces between the sound-producing animal and its environment or between
the environment and the receiving animal. In sound production the relevant stages
are: (l) conversion of muscle power at low velocity and high force into
mechanical vibration; (2) the transformation of the vibration into acoustic
and Hearing: a Comparative Study
Henry C. Bennet-Clark
Department of Zoology, University of Oxford, South Parks Road, Oxford,
OX13PS, UK
Abstract
Sound production involves stages of impedance matching between the higherdensity body of the animal and lower-density air. Hearing involves further
impedance matching, from air to the higher-density sensory cells. Initial stages of
sound production may include a frequency multiplier that converts slow muscle
contractions into higher frequency mechanical vibrations. The frequency multiplier
may also determine the sound frequency. Larger sound sources allow better
impedance amtching with the air so sound radiation often exploits acoustic
transformers to increase the effective size of the sound source. In many cases the
conversion efficiency of muscle power to sound power is high, giving a large
effective range for the signal. Sound detection often uses acoustic transformers to
concentrate the sound onto relatively dense vibrating sturctures that are coupled to
the sensory cells, providing the inverse of the impedance matching that occurs in
sound production. These transformers may be associated with directional
mechanisms and may drive arrays of receptors that allow frequency analysis.
Sound production and hearing are special cases of the general phenomenon of
impedance matching that occurs throughout biomechanics.
Key words Sound production, impedance matching, efficiency, hearing, frequency
analysis
1 Introduction: the Stages in Sound Production and
Hearing
The chain of processes in acoustic communication in animals involves the
production of a signal which is radiated as sound, the propagation of the sound
through the medium, its reception, and its effect on the receiving animal. Here, in
the context of sensory ecology, I shall confine myself to the physical processes of
the interfaces between the sound-producing animal and its environment or between
the environment and the receiving animal. In sound production the relevant stages
are: (l) conversion of muscle power at low velocity and high force into
mechanical vibration; (2) the transformation of the vibration into acoustic