EXAMPLE: Radio engineering in biology
Experimenting with light and prisms in the mid-seventeenth century, Isaac Newton demonstrated that what we perceive to be colored light (e.g., red, orange, yellow, green, blue, and violet), if mixed in proper proportions, would yield light that appeared white. He also could decompose light that appeared white into components spread across the visible spectrum; and the human brain could discriminate the components of that spectrum with fairly high (spectral) resolution. We now know that human color discrimination is accomplished by neural computations based on the relative amplitudes of neural signals from three types of cones (S cones, M cones and L cones). Each type of cone contains a photopigment with maximum sensitivity to light of a specific wavelength (Short, Medium and Long).
Imagine that you wanted to communicate with friends by signalling with light. With unfiltered white light you could send one message at a time. If you filtered the light (e.g., with colored glass or a prism), you could send a red message, an orange message, a yellow message, a green message, etc., all simultaneously. With appropriate filters (e.g., again, colored glass or a prism) a friend could sort these messages, selecting the one to follow, rejecting the others.
The radio-frequency energy first sent by Marconi and his colleagues was generated by electric sparks across a small gap between electrical conductors. That energy was spread over a very broad electromagnetic spectrum. With it, a person could send only one message at a time. The trick would be to design and build spectral filters (tuned structures) that could divide the radio spectrum into spectral bands. At the sending end, such a structure could be used to limit the bandwith of the broadcast signal (often by using the tuned structure in a feedback loop in the signal generator). At the receiving end it could be used to limit the spectral band being attended (again, often by using the tuned structure in a feedback loop). Each transmitting station then could have its own part of the radio spectrum and broadcast messages that would not interfere with those from other stations. Each receiving station could select the transmitting station to be attended. By the second decade of the 20th century, major parts of the business of radio engineering were design of filters that would selectively pass part of the radio-frequency spectrum and reject the rest of it, and design of transmitters and receivers that employed those filters either directly or in feedback loops.
Radio signals usually were translated into audible signals (e.g., a series of short and long tones) for perception at the receiving end. In the case of voice communication, radio transmissions incorporated audible voice signals at the sending end. To limit the bandwidths of the voice signals and prepare them for transmission, spectral filters were designed and built for the audible acoustic spectrum. It was in the form of modulation that the broadcast radio-frequency wave carried a replica of the voice signal. This required translation (transduction) of the audible voice from an acoustic signal to an electric signal, which would be used to modulate the radio-frequency wave. Translation back to audible signals at the receiver required recovery of the replicas by the process of demodulation and transduction in the other direction-- from electric to acoustic. In this transduction process at the receiver, it was important to maximize the acoustic power in the audible message. Transducer design and use, modulator and demodulator design and use, and power maximization also became major parts of early 20th-century radio engineering. For power maximization, transformers were required; and transformer design and use became a major part of early radio engineering. As engineers were designing the various components of radios, they began to distinguish between processes that could be carried out without the aid of supplemental energy (i.e., passive processes) and processes that were carried out more effectivly with the aid of supplemental energy (i.e., active processes).
Hearing the choruses of insect, frog, and bird voices in typical spring or early summer woodland or meadow settings, one should wonder how the listening animal can pick out the sounds that are important to it, how it even can separate animal sounds from the sounds of the breeze or the creek. The ability to sort sounds must be quintessential to hearing, just as much as the ability to sort radio signals was to early radio engineers (footnote). It seems no wonder, then, that at the very cutting edge of the physiology of hearing in animals, both vertebrates and invertebrates, one finds that the research centers around spectral filters in the ear, what physical processes might they involve and how they might involve feedback, transduction in the ear, and how it's achieved, the use of modulation and demodulation in the process of translating sound signals in the ear into spike patterns in afferent auditory axons, and how those two things are achieved (SEE NOTE), transformers that maximize sound energy transfer from air or water to the sensory structures of the ear, and how they are realized, and which of these biophysical phenomena involve passive processes, which involve active processes.
Design, synthesis, and analysis of filters, and of systems employing filters, feedback loops, transducers, transformers, modulators and demodulators, power maximization, passive processes and active processes all were consolidated into one grand theory-- network (or circuit) theory, which by the early 1930s had become the core of electrical engineering. It was network theorists who generalized Onsager's celebrated reciprocity theorem to include passive transducers with rotation-- bringing the concepts of antireciprocity and gyrators into network theory. Network theory became a metamodel for nonequilibrium thermodynamics. It gave us a recipe for modeling and understanding any system based on classical physical phenomena. And that covers almost all of current biophysics, not just the biophysics of ears or of sensory organs.
With electrical network analysis presently being done largely with digital computers, and with electronic filtering (as well as filter design) largely being done digitally, network theory is no longer a core subject in electrical-engineering education. Perhaps it will migrate to the field of biophysics and survive there.
(Network models in nonequilibrium thermodynamics)
(Tuning in the vertabrate ear)
(Further theoretical discussion about filters and tuning in vertebrate ears))
(Examples of circuit-theory applications to biophysics of the ear, from the Appendix to Evolution of Hearing, edited by Manley, Popper and Fay; Springer Verlag, 2004)