Many acoustic sensory organs (e.g., mammalian cochleae, reptile and bird basilar papillae, frog amphibian papillae) have a quintessential feature in common, each of them comprises an array of (band-pass) filters, each filter covering a relatively narrow frequency band and each communicating to the central nervous system through its own private channel. The incoming acoustic signal thus is decomposed into spectro-temporal components, which are translated individually into neural signals and sent over the filter's private channel. Each component (each output from an individual filter) in the lower-frequency bands is converted (by transduction) directly into a generator potential in a sensory receptor cell; and this, in turn, modulates the instantaneous firing rate of spikes being sent along one or more primary afferent axons (the private channel) to the central nervous system. Owing to low-pass filtering inherent in the structures of nerve cells, generator potentials and spike rates are incapable of being modulated directly by components (filter outputs) in higher-frequency bands (e.g., above about 500 Hz in frogs and reptiles, 5000 Hz in mammals, 10,000 Hz in birds). In those higher-frequency bands, each filter output is demodulated during the transduction process, and each generator potential and spike train follows the lower-frequency components (i.e., a smoothed version) of the amplitude envelope of the band-pass filter output rather than the filter output itself. {In the frogs and gerbils that we have studied, demodulation is accomplished by a square-law nonlinearity, and each generator potential and spike rate follows a smoothed (low-pass filtered) version of the square of the amplitude envelope of the band-pass filter output (see footnote), (see also ref 1, p. 100) and (ref2, p. 18)}. In frequency bands near the transition from low- to high-frequencies, each generator potential and spike train exhibits a component that follows the filter output itself, and a component that follows a smoothed version of the square of its amplitude envelope.