by Anatoly RODIONOV, Dr. Sc. (Tech.), Chairman of Research Council for Problems of Applied Hydrophysics of St. Petersburg Scientific Center of the Russian Academy of Sciences (RAS)
World ocean development by a human being is impossible without development of hydrogeology, hydrobiology, other sciences, including hydroacoustics studying the peculiarities of acoustic wave transmission, reflection, fading in real aquatic environment for the purposes of underwater location, communication, search for sunken objects, etc. When developing such equipment, designers try to take into consideration wildlife experience, including echolocation systems which dolphins have.
Initial steps in this area were taken more than four decades ago. In 1964, the first bioacoustics laboratory in our country was set up on the initiative of Higher Nervous Activity Department Head Ervand Airapetyants, Doctor of Biological Sciences, and his student Alexei Konstantinov (later on Doctor of Biological Sciences) at Leningrad (now St. Petersburg) University. The opportunities of scientists sharply increased after the Navy oceanarium in the Black Sea city of Sevastopol (now it is under the jurisdiction of Ukraine) was set up in 1965. Together with the experiments on using aquatic mammals for search and rescue purposes, a wide range of scientific problems was studied here, including the capabilities of animals to orient themselves in space with the help of a natural sonar (asdic). A new direction was supported by Academician Aksel Berg (1893 - 1979), specialist in the area of radiotechnology and electronics, physicist Leonid Brekhovskikh (1917 - 2005), physiologist Pavel Simonov (1926 - 2002), zoologist Vladimir Sokolov (1928 - 1998), corresponding member of the USSR Academy of Sciences, author of the writings on development of automatic and telemechanic equipment Boris Sotskov (1908 - 1972), and others. In total, more than fifty organizations joined studying the Cetacea. Among them there are academic institutes (the Institute of Evolutionary Physiology and Biochemistry named after I. M. Sechenov (Leningrad), the Institute of Ecology and Evolution Problems named after A. N. Severtsov, and Institute of Oceanology named after P. P. Shirshov (Moscow), Murmansk Sea Biological Institute, Pacific Oceanological Institute named after V. I. Ilyichev; institutions of higher education (M. V. Lomonosov Moscow State University, Leningrad State University), branch institutes (Central Scientific and Research Institute "Morphyspribor" and Aircraft Instrument-Making Institute (Leningrad), Acoustic Institute named after Academician N. N. Andreyev (Moscow), Polar Institute of Sea Fishery and Oceanography (Murmansk). Researches forecast good prospects for using animals not only in marine operations but also for civil purposes*.
* See: "Sea Animals Guard Their Habitats". Science in Russia, No. 1, 2006. - Ed.
Close attention was paid to the sensor peculiarities of dolphins. There proved to be quite a lot of them. These mammals' sight is well adapted for underwater and above-water seeing. Also, they have a sense of smell. And, they can pave the way from one hemisphere to the other by magnetic field lines of the Earth. In the course of evolution a perfect system of acoustical signals was formed with the Cetacea. With its help they orient themselves in space, get food, and convey the information to each other.
At the time of joint hunt dolphins easily define their proper spatial attitude with respect to sea surface and congeners. Certainly, they have a visual system. However, it's not efficient at night. Nevertheless, the animals moving in the water even at great speed do not run into each other. Each radiates echolocation impulses, otherwise they can't fish, by sending signals so as not to bother each other. A school can be very large, up to a hundred species, but they act in coordination.
Thus, in the course of evolution nature took care of the fact that several systems operating in the sound channel at the same time were formed with dolphins. These are navigation, echolocation and communication systems. They are systems because they radiate signals with the help of special organs playing a part of transmitting antennas and receive "messages" or reflected proper impulses by relevant receivers. In so doing, the sequence "radiation-receipt" in their navigation and echolocation device must have proper synchronization, while the communication system, like that of a human being, does not require timing of signals. In order not to disturb congeners, echolocation signals are coded. This is the only way to differentiate a sound reflected by a target from proper impulses, i. e. each individual animal has its decoder. If the mammals living ashore have ears acting as receivers of acoustic energy and earflaps helping define its direction, the sea animals do not have such a device, but they have organs playing a lacking function.
Dolphins' radiating system includes all means of forming an acoustic field, which are known today. Nowadays, there was built its reasonably plausible scheme consisting of a "generator" of short impulses (an "airgun"), a reflector and refractor. The operating principle of acoustic gun was designed in the 1980s by Director of the Acoustic Institute Nikolai Dubrovsky, Dr. Sc. (Phys. & Math.), and his student Lyudmila Ghiro, Cand. Sc. (Phys. & Math.). An elastic tube imitating a nasal passage of an animal is overlapped in the middle by another elastic tube acting as a sphincter (a muscular stopper). The latter, with high pressure on the one side of the tube, is open for a short time and pressure in its both areas is leveled. As a result, an impulse of acoustic pressure is formed (it is not improbable that there can be several such "generators" in the radiating device of aquatic mammals).
A skull plays the role of a complicated reflector. Finally, the signal is formed in a melon (acoustic
Sonograms of long impulses (1), their harmonic components (2, 3, 4), communication signals (5, 6), echolocation impulses (7).
A, B - impulses with frequency modulation, C - with frequency manipulation, D - with frequency manipulation and modulation.
Non-steady spatial pattern of dolphin's echolocation impulse: time (1) and frequency-response (2) characteristics of impulse along an acoustic axis of spatial field of radiation; 1', 2', 1", 2" - the same characteristics in case of deviation from the acoustic axis.
lens) - it focuses sound vibrations and agrees relevant characteristics of an antenna and water. The biophysical characteristics of this lens were measured in the laboratory of Institute of Physiology named after A. A. Ukhtomsky of St. Petersburg State University, headed by senior research officer Yuri Ivanenko. It was shown that its parameters smoothly change from the center to periphery. Also, temperature gradients influence the process of forming of an acoustic field. Having chosen polymers, which are similar to a melon by their physical and chemical characteristics, including the opportunity of temperature control from the center to periphery, the scientist has built a bionic model which is like dolphin's acoustic lens by characteristics. It is supposed that four pairs of "bags" within the skull can play a double role: to change geometrical dimensions of a melon and generate ultrasound vibrations. In the first case a radiation field width is varied.
The experiments on recognition of underwater objects by the sea animals with the help of echolocation have revealed that a bottlenose dolphin finds the target at the distance of up to 700 m, while a white whale-up to 2 km. The information capacity of a signal is so high that they accurately define steel targets which are different in quantitative content of carbon! Also, in the course of experiments the animals' capability to classify objects was revealed: practically, they always find the features required for such a complicated operation.
The characteristics of dolphins' hearing have been studied in detail. What range do they hear in? How does the response characteristic of receiving system change depending on frequency? In order to answer these and other questions, it was required to create new engineering equipment. For example, at Leningrad State University there were designed generators to receive signals with any front of amplitude rise/fall, automatic control of duration, fill frequency and prescribed characteristic of phase.
According to the data of the experiments, Vladimir Chilinghiris, Cand. Sc. (Biol.) from the Institute of Physiology named after A. A. Ukhtomsky set up a hypothesis: an ultrasound passes to animal's inner ear along the lower jaw bone. Later on, other specialists confirmed this assumption.
When studying dolphin's echolocation system, it was discovered that it radiates signals and receives them within a large frequency range - from 20 to 200,000 Hz (to compare: the capabilities of human hearing cover 20 - 20,000 Hz). In the middle of the 1970s the appearance of the equipment recording the animals' signals without errors must have helped decode the language which is so far from the human one. However, soon it was found out that information reception and delivery mechanism built on narrow-band signals wasn't suitable to solve such a complicated problem. What is the reason?
As it is known, radio communication is carried out with the help of electromagnetic oscillations. However, waves of low (sound) frequencies are not suitable for it per se because they change comparatively slowly and practically are not radiated. Thus, a sort of means of transportation-oscillations of high frequency (in this
Echolocation with the help of super broadband signals with a zero carrier frequency: 1, 2 - integral signals with interval and time coding of a sequence of impulses; 3 - spectral and time coding with the help of elementary components; 4 - resulting echolocation impulse.
case it is called a carrier frequency) - is used as a carrier of telegraphic messages, speech, music, etc. As a result of impact of low frequency oscillations, the nature of high frequency oscillations, i. e. one of the parameters-amplitude, frequency or phase-changes, too, which is called modulation. Till recently, narrow-band signals with a pre-assigned carrier modulated according to an amplitude have been transmitted through a radio or acoustic channel more often than any other signals. Say, by analogy with a piano, they played one key varying only force of pressure on it. However, with the development of radiotechnical and hydroacoustic systems it was noted that the wider frequency band of a signal (by the same analogy-quick running over the sequence of keys), the higher is interference immunity of communication. With this modulation frequency changes in time. Dolphin's sound "messages" are similar to the piano chord-all frequencies are available in it at the same time, i. e. the signal is broadband and concentrated in time.
While forming an electromagnetic or acoustic impulse in space, its building-up period stands out (increase in amplitude), then a stable oscillating process follows (carrier frequency does not change), and later on the signal decreases. The narrower a pass band of transmitting antenna, the longer are rise and decrease periods, and the longer and more stable is an oscillating process which is characterized by carrier frequency. There is no such steady process in dolphin's impulses. We can say that it uses signals with a zero "carrier frequency." It is worth adding that the system is more resistant to different types of interferences the more modulations it uses. And dolphins use so many of them that we can neither count, nor study them for the time being...
Animals' signals obtained in the experiments reminded of a solitary wave with a half-wave of compression and discharge. The spectral characteristic on an impulse along the acoustic axis has no definite maximum, but it depends on azimuth. The first experimental data have shown that spatial characteristic of dolphins' sound field is close to the forecast one for broadband signals-there are no sidetones of radiation zero, which reduces ambiguity of defining an aim by angular coordinates. While making a task more complicated or worsening the correlation of signal/interference, animals sharply increase them (they radiate pulse packets). From the viewpoint of the theory of signals, they use interval and time coding.
The experiments relating to dolphins locating mines have shown phenomenal results. Out of 100 percent of
Complete sequence of dolphin's echolocation impulses when solving the problem of target detection, consisting of six pulse packets; amplitude - by axis of ordinates, time (sec) - by axis of abscissas.
secretly installed mines they found... 105 percent ("additional" mines were the ones silted at the depth of not less than 0.5 m, which had been left since the Great Patriotic War of 1941 - 1945). This is a serious factor of sonar operation. For the time being we can only dream about similar factors for technical systems. However, this unique result, first of all, demonstrates interference immunity of broadband impulses radiated by animals.
It should be noted that today's requirements for radio equipment operation are so high that it is impossible to solve problems only by using current narrow-band systems. The above-mentioned experiment shows that the future is with broadband devices, because it is on their basis alone that appearance of hydroacoustic and radio systems with higher interference immunity, secrecy of coding and speed of information transmission, super broadband antennas, devices to diagnose different media, medical tomographs with improved definition quality and reduced radiation energy, new technologies of satellite navigation, radars for non-contacting location of mine fields, etc. are possible.
When carrying out the underwater works of military or civil designation, there emerges a problem of building a navigation positioning system similar to the satellite GPS*. However, devices in use won't fly along the earth orbit but will float on the water surface. They will automatically define their own co-ordinates by GPS space system and then transfer them to the user's device being under water or even on the sea floor. Receiving information from three or four such "satellites", people working under water will be able to accurately determine their location. In so doing, the system of signals usual for them, in fact, won't change. Only frequency at which these devices will operate and coding will be renewed.
Most of modern systems of radio- and hydrolocation are of resonant type. They use a generator of signals and so-called high-Q filter, which includes a radiating antenna as a radiating element. A receiver is built on the same filter but with a receiving antenna. All further gating of useful signal from noises takes place in two devices-those of data processing and decision-making. It is clear that the narrower the filter band, the less information can be transmitted. On the other hand, just such a filter is required to get high amplitude of a signal: and the more it is, the farther a signal is transmitted. That is a paradox. If one wants the signal to be transmitted farther, a less modulation range should be set. However, in this case it is possible to transmit less information. Information capacity of signal is not increased by using preassigned carrier but by the one which changes smoothly (frequency modulation) or sharply, by leaps and bounds (frequency manipulation). By the way, we have already said that a range of these changes is limited by high-Q filters' breadth.
As to the dolphin's echolocation signal, it is realized, at the least, with the help of two systems of modulation: an impulse one as signal duration changes depending on a current task, and a frequency one as a turndown of this parameter lies in the whole "field" of the system of hearing. Impulses can be long and very short up to one period, while a speed of varying frequency as high as possible. It is impossible to single out "carrier frequency" with them, it is spread in the spectral field like in frequency-modulated signals. Their duration is marginally possible.
At the beginning of the 1960s, an American scientist Henning Harmut put forward an idea of information transmission system based on signals without "carrier frequency," which caused ironic comment from many specialists who thought impossible technical implementation of such impulses in space. Certain time has passed, and now broadband and super broadband sig-
* GPS is the Global Positioning System created in the USA. It's main component consists of several dozens of space satellites permanently transmitting radio signals and creating an information field around the earth. The GPS receiver locates them and defines coordinates by measuring distances up to several satellites. - Ed.
Dolphin's echolocation system: 1 - acoustic lens; 2 - nasal bags; 3 - respiration valve, 4 - skull.
nals, their processing systems are the most popular area of radio technology in great demand.
Many scientists tried to design a radiating system to form signals in space without "carrier frequency." For example, in the 1980s in the department of hydro-acoustics of the Sevastopol oceanarium they tried to do this with the help of an electric spark antenna (Vyacheslav Saprykin, Dr. Sc. (Tech.)). There are structural concepts of building broadband radiating elements in the range of frequencies of more than 1 MHz. With small levels of acoustic pressure their analogues of up to 1 MHz have already been created. However, comparing the signals formed with the help of technical systems with their dolphins' analogues shows how much biological technologies exceed engineering ideas. Nevertheless, designers do not give up their efforts. The specialists of the chair of acoustic impulse of the Institute of Electrical Engineering named after V. I. Ulyanov (Lenin) in St. Petersburg have achieved the best results in creating a broadband antenna to form an acoustic impulse similar to dolphin's signal in parameters.
The problem of locating mines, first of all, bottom ones, often silted mines or even submerged in the ground still remains topical. In so doing, body frames of modern products can have anti-hydrolocation coating as per the recommendations of "stealth" technology. The complexity of the problem of locating mines is aggravated by the need to reliably classify a revealed hydroacoustic contact. Now dolphins cope with this better than the most advanced man-made systems, which is confirmed by using these animals in the military exercises and local wars by the United States Navy. Since 2002 the Office of Naval Research, Space and Naval Warfare Systems Command, Prospective Military Research and Development Committee of US Department of Defense have financed projects on the systems of search and classification of silted bottom mines based on biosonar. There are planned developments of autonomous mini-systems of a "dolphin robot" type and devices based on new technologies with antennas using dolphin-like signals. They suppose that basing on the latter they will manage to create 3D equipment.
It will take great efforts to master hydrospace. To have an efficient assistant in the environment, which is alien for us, is not a whim but a need. Research into aquatic mammals in whose physiology and behavior there is so much obscure, must be actively continued in our country, too.
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