Any elementary textbook of physics will give the basic facts of sound as a longitudinal wave motion transmitted through a medium and emanating from a source of mechanical vibration. The problem is that, when it comes to describing and analysing insect sounds and their use in communication, it is almost impossible to avoid problems of anthropomorphism. This is, of course, a difficulty in all studies on animal behaviour. Sound is defined in dictionaries as a "sensation caused in the ear by vibrations of the surrounding air" (The Concise Oxford Dictionary of Current English, 6th edition, 1976). Following this definition, the term sound can only be used of vibrations which are heard by the human ear and transmitted through the surrounding air. If this were followed, sound would be a useless concept for describing animal behaviour. The human ear is sensitive to a range of vibrations with frequencies from about 30 Hz to 15 kHz. The terms "ultrasound" and "ultrasonic", though useful, are anthropomorphic and refer to vibrations having frequencies above those detectable by the unaided human ear. A wide variety of animals, including many insects (e.g. many bushcrickets, cicadas, etc.), have ranges of acoustic sensitivity which extend well above that of the human ear and ultrasound is critical in the lives of many animals (Sales and Pye, 1974). Clearly such a limited definition of sound is not useful in the scientific study of animal behaviour.
Insect sounds have been known and documented since the writings of the classical Greek philosophers, most notably Aristotle. It is only in the past 50 to 60 years that the subject of insect acoustic signals and associated behaviour became available as a subject of intense scientific investigation. The invention of ever-newer techniques for the recording and analysis of sounds has rapidly expanded during this period and continues to do so today in the digital age (Eliopoulos,
Chapter 2). The dictionary-based definition, with its anthropomorphic slant, is inadequate and no longer useful to us. One of the earliest English-speaking workers to confront this problem was Pumphrey (1940) in a classical review on insect hearing. He defined sound as "any mechanical disturbance whatever which is potentially referable by the insect to an external and localised source". This then involves vibrations transmitted through any medium, fluid or solid, and is certainly broader than some workers, including many authors in this book, might wish. Good discussions on these problems are given in the excellent volumes of Haskell (1961) and Ewing (1989). A strict definition of sound, as vibration transmitted through fluid media, such as air and water, clearly excludes vibrations through solid substrates which are now known to be very widely used in insect communication and have considerably contributed to this book.
All sounds derive from vibrations which in turn set particles into motion in the surrounding media, whether these be fluid (air, water, etc.) or solid. Even the same animal by means of appropriate receptors may detect external vibrations as airborne and substrate transmitted emissions. We know that vibrations passing through different media may have very different physical properties (Michelsen et al., 1982; Ewing, 1989). Animals may perceive such vibrations in similar ways and, more important, may respond in similar ways. It is trivial to argue about the strict uses of the words sound and vibration. In this book we, as editors, have taken a liberal view and allowed the individual authors their own usage, so long as it is clear and consistent. Some authors attempt clearly to separate sound and vibration as two different phenomena, others take a wider view. Many problems in biology are not real ones, but result from the human desire to make strict and exclusive definitions for parts of effectively continuous phenomena.
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