Sonics

Type of physical science: Classical physics

Field of study: Acoustics

Sonics involves the technical applications of mechanical waves in the engineering, medical, and industrial fields. Sonar, a subfield of sonics, involves the use of electromechanical devices to send and receive underwater sound pulses.

Overview

The branch of physics that concerns itself with the study of mechanical waveforms—waves that propagate within physical media such as liquids, solids, and gases—is called acoustics. The subbranch of acoustics that deals with the technical applications of mechanical waves in various fields such as engineering, medicine, and industry is termed sonics. Sonics is frequently, though not exclusively, concerned with the study of waves that are inaudible to humans. Such waves exist both below the threshold of human hearing (subsonic) and above it (ultrasonic).

No matter what medium they propagate through, sound waves always possess certain physical characteristics that are used to group wave types into useful categories for descriptive and analytical purposes. Thus, sound waves are described as possessing amplitude, wavelength, and frequency. Waves, which can be simply thought of as a cyclic disturbance of some medium, always travel in series or groups, sometimes called wave trains. Each individual wave within the train has a maximum height from top to bottom, measured from the low point of its trough to the high point of its crest. This measurement is the wave amplitude. The distance from a particular point on one wave to a like point on an adjacent wave is the wavelength of that particular wave series. These like points can be the wave crests, the wave troughs, or any other convenient or easily measurable point in the wave cycle. A complete wave cycle can be conceived of as two like points within any two consecutive waves in the series. The time it takes for these two points to pass by some arbitrary fixed point in space is called the wave frequency.

For sound waves, frequency is expressed by a unit of measurement called a hertz. One hertz is defined as one wave cycle per second. As many sound waves travel at frequencies far in excess of this rate, the hertz value of sound is typically expressed in orders of thousands of hertz. Thus, units such as kilohertz, megahertz, and even gigahertz are frequently employed to express these high sound frequencies.

As a conceptual yardstick, it is useful to realize the relative limitations of human hearing, as the average person's audial range extends only from roughly fifteen hertz to twenty kilohertz. Many animals have evolved sound organs that can perceive and transmit frequencies radically lower or higher than the human range. Among these more sonically capable animals are marine mammals such as dolphins. Human-engineered equipment that is analogous to a dolphin's organic sound apparatus is generically termed sonar.

Sonar is a subfield of the greater realm of sonics. It is an electromechanical technology that allows the location, and sometimes the identification and range determination, of underwater objects both moving and static. Sonar technology is predicated on the same fundamental theoretical framework and mathematics as other branches of sonics. The development of such an underwater sensing technology was perfected originally for military purposes, using equipment that either received or sent sound pulses in water. From its advent, basic sonar could be subdivided into two distinctly different system types: passive sonar and active sonar. Passive sonar equipment is designed for nonactive listening, meaning that passive sonar is strictly limited to the reception and analysis of sounds propagated outward from an underwater sound source. These sources can take the form of any of one or more diverse sound generators, from biological broadcasters such as dolphins, whales, and some fishes to both surface and subsurface military and civilian vessels generating either intentional or unintentional sound waves. Unintentional subsurface sound or noise can be generated by propulsion equipment such as propellers and jets, onboard power generators, and life-support systems. Intentional sound is typically the willful broadcast of active sonar by other vessels in their own search for various underwater objects.

Other categories of sound generators or influences can be both nonhuman and nonbiological in origin, as in the case of geologic events such as submarine earthquakes, landslides, and volcanic eruptions. In addition, underwater sound effects can be created by the marine environment itself to varying degrees. Among these environmental influences are such phenomena as currents, tides, waves, and differences in temperature, salinity, and density. Environmental influences often do not directly generate sound as such, instead distorting, redirecting, or channeling sounds created by other sources. The marine environment complicates the efficacy of sonar systems by being a factor that is often impossibly complex to predict. This is because the farther sound waves travel in the ocean, the more distortion or redirection they encounter. After a certain point, numerous underwater sounds having myriad origins blend together into a composite of nondescript background sounds, sometimes referred to as ocean noise or sonar noise. This added factor makes effective, long-distance sonar either very difficult or impossible to achieve.

Active sonar, like passive sonar, is greatly affected by the vagaries of the marine environment. Nevertheless, active sonar has the advantage of not being dependent on sound-generating sources for signals. Instead, it dynamically broadcasts its own signals and subsequently receives and analyzes the returning reflections for range, bearing, and sometimes object identification. To do this, an active system uses hardware elements that include both the reception equipment used in passive sonar systems and elements that produce outgoing sound pulses. The actual system components involved in a total working circuit are onboard electronic control devices for signal selection and initiation; a pulsing switch; a transmitting amplifier; an outputting transducer (a device that converts energy from one form to another, in this case from electronic pulses to mechanical sound waves); a receiving transducer (essentially a very sensitive hydrophone, or underwater microphone, that converts the returning underwater sound waves back into electronic signals); a receiving amplifier; and a display device or unit. In modern sonars, the same unit often functions as both outputting and receiving transducer. The unit in most cases is a synthetic piezoelectric crystal made from a special ceramic material. Piezoelectric materials possess the property of either changing their shape when voltage is applied to them or emitting electrical energy (voltage) when their shape is altered mechanically. Such transducers are an indispensable part of modern sonar systems because they transmit a sound pulse outward into the water when a current is applied across them (the dramatic sonar "ping") or, in a reciprocal function, emit voltage that is analogous to the received pressure waves in water (sound) for analysis in the reception part of the sonar cycle of both passive and active sonars.

Reception of sonar signals, whether in active or passive sonars, is ultimately a processing function of the display unit. The display unit itself can be one of several types: In its simplest form, it can be a basic analog device, such as a paper graphing display with an electromechanically actuated stylus that inscribes output on a rotating drum feeding a paper sheet. This device is commonly termed a paper sonar recorder. It is typically used in economical fishery-type sonars and is similar in appearance to some analog seismic recorders used for earth-tremor display.

The most familiar class of sonar display device is the aural type seen in numerous submarine war films. Amplified sonar signals are passed from the receiving transducer and converted into hearable sound, which trained technicians use to supply useful information to ships' executive officers. This type of display unit had inherent limitations that were overcome in the third class of output device: cathode-ray tube (CRT)–type visual display units that use television technology to show sonar-signal characteristics as graphic displays. As the technology became more widely available, LCD and plasma displays were used as well. Visual displays are ideal because the ability of humans to distinguish sounds from one another is less reliable and more limited than the human visual sensory ability, and purely electronically time-based equipment is inherently faster than more mechanically time-based equipment (that is, paper graphing and aural outputs). Thus, on the human-user level, electronically inputted and digitally processed displays for sonar output devices are intrinsically superior to previous display systems because of speed and a quantum improvement in resolution.

An important advance in sonar systems was the advent of side-scan sonar, which allows simulated three-dimensional images of underwater objects to be constructed graphically by computer equipment. To grasp how this type of sonar functions, it is necessary to understand how all types of sonar fundamentally function. No sonar system actually measures or ascertains the depth, the distance, the bearing, or any other quality of any submerged object. Rather, what sonars do is measure and display the travel time of underwater sound waves. Transmitted sound pulses travel under water at approximately 1482 meters per second in freshwater, with some variation depending on water temperature, pressure, and salinity. Computerized equipment calculates how long it takes for the sonar pulse to travel to an underwater object and back. The resulting data is then processed for the known characteristics of the ocean medium, the signal strength, and the direction used to generate sonar display output. In the case of side-scan sonar, additional features and techniques are employed, such as sideways-slanting sonar signals, multiple signal channels aimed in different directions, narrow rather than wide sonar beams, and a towed body, which is a trailing, submerged sonar-transducer platform that increases device stability and image resolution. The final results are often amazingly detailed graphic images of ocean-bottom topography and sunken objects such as ships.

Applications

One of the most productive ways to obtain information on many aspects of the ocean environment has been the employment of sonar technology. This specialized offspring of the science of sonics has expanded into an extensive array of underwater sensing technology. Originally perfected for military purposes, sonar came to enjoy many applications in the civilian sector as well. The post–World War II era even witnessed valuable spinoff technologies for nonmarine uses, such as in medicine and industry.

Sonar research and development has traditionally been most closely tied to military surface and subsurface force deployment and strategy, a by-product of rival powers' naval competition in past world wars. Thus, sonar systems' sophistication and range of use have accelerated in relation to their importance to the world's advanced navies. The principal applications of military sonar can be listed along functional lines, such as detection of potential targets, classification of a positively detected target, spatial localization, navigation, communication via sonar pulses rather than other media such as radio or laser, and use as a naval countermeasure tool—that is, foiling enemy plans or actions by broadcasting false or misleading signals.

A number of naval applications of sonar have found ready application within the realm of commercial shipping, particularly in the form of navigation and dangerous obstacle detection and classification. Among these uses are reef detection and distancing, iceberg detection and distancing, and general ocean-, lake-, and river-bottom ranging.

As a welcome addition to the toolkit of scientific marine research and exploration, sonar applications have been expanded or adapted for oceanographic purposes. Among their uses are the detection and location of marine life under study, such as cetaceans (marine mammals such as dolphins and whales) and fish; the recording, simulation, and reproduction of cetacean language; ocean-, lake-, and river-bottom topographic mapping; subsurface geologic ocean mapping; chemical and physical oceanographic research into water-mass movements, salinities, densities, temperatures, and patterns; and simulation of submarine seismic events.

Military developments have also led to industrial sonar applications. Among industrial uses are the many applications found within the fishing industry, such as detection, location, and herding of fish schools. Because sonar is able to penetrate softer bottom sediments to some degree, mining concerns often use it for oil and mineral exploration. Water-resource agencies and consultants use sonar as a high-technology water-flow meter.

On land, applications of sonics technology include the use of ultrasonic projectors as grinding and drilling tools, ultrasonic cleaners, viscosity meters, and industrial process-control sensors. The technology is used in the medical field as well; noteworthy among a growing number of applications is the use of ultrasound scanning equipment to detect internal injuries, abnormal growths, and the development and sex of human fetuses.

One application of sonics that has seen use in various industries is the Hypersonic Sound Beam, also called the HyperSound System (HSS), a parametric sound generator that can "project" sounds to a point some distance away. This is possible because air is nonlinear, meaning that its mass does not change in a linear fashion when pressure is applied, such as by a sound wave. As a result, the wave travels at variable speeds and thus distorts, which creates new frequencies within the sound wave. Parametric sound generators work by projecting a tightly focused ultrasonic (above the range of human hearing) beam at a target so that only the frequencies created by the distortion can be heard; this causes the sound to seem as though it is originating from the target and only be audible to people near the target. The HSS was originally patented by the American Technology Corporation, which in 2010 created the Parametric Sound Corporation to focus on developing and distributing hypersonic technology.

Sound waves can also be used to transmit information to smartphones and similar devices. Sonic Notify, a company founded in 2011, developed a new method of embedding data in high-frequency sound waves. Any mobile device with a microphone can pick up the audio signal and, with the proper software, decode it to provide the user with targeted content. The product is aimed at television broadcasters and retail stores as a way to send users information about specific products and promotions as they watch television or shop, but it has wider applications as well.

Context

The roots of sonics research and its resultant technological applications can be found within the history of the field of acoustics. Acoustics has been defined as the physical study of mechanical waveforms and their properties, whether those waveforms are observed to propagate within liquids, solids, or gases. The earliest observers and commentators on natural phenomena, such as the early Greeks, noted the easily definable and predictable nature of simple surface waves seen in bodies of water of all sizes. Nevertheless, it took a number of centuries for a more involved understanding of waveforms to evolve. A basic roadblock to a fuller comprehension of wave phenomena was the erroneous thought, once widespread, that some physical mediums, even water, were noncompressible. Early investigators in the general field include Leonardo da Vinci in fifteenth-century Italy, Francis Bacon in sixteenth- and seventeenth-century England, and the eighteenth-century French clergyman and researcher Abbé Nollet, who confirmed through direct experimentation that various noises could be perceived underwater. These early attempts laid the theoretical groundwork necessary for later progress in acoustics. The brilliant Dutch Renaissance experimenter and philosopher Christiaan Huygens formulated an insightful general principle, applicable to all waves, that described the general characteristics of wave propagation, along with an approach to logical, mathematical prediction.

Progress in the field took a great leap in 1826, when a research team successfully measured the speed of sound in freshwater in Lake Geneva, Switzerland. Finally, practical experiments were conducted by the United States Navy in 1894 on submarine signaling. These experiments accelerated research efforts that culminated in the US invention by Reginald Aubrey Fessenden in 1914 of an oscillator device, which allowed audio-frequency operation. The accidental destruction by iceberg of the supership Titanic helped spur increased research into submarine acoustics and resulted in the submarine echolocation of an iceberg at a range of several kilometers using a frequency of 1,100 hertz. Two years later, the Frenchman Paul Langevin succeeded in producing the first ultrasonic frequency echo. In England in 1917, Langevin and an associate used ultrasonic echoing equipment to successfully locate a submerged submarine. World Wars I and II generated tremendous interest in the development of sonar equipment and theory, along with the necessary funding and facilities for rapid strides in sonics research. The post–World War II technological environment of worldwide research and development in all the sciences maintained this accelerated pace and allowed the development of the large spectrum of sonics-related devices so widely used in naval, industrial, and medical applications.

Principal terms

ACTIVE SONAR: a sonar system designed to radiate sound pulses at will in order to receive sonar echoes from targets for analysis

HYDROPHONE: a receiving transducer used in both active and passive sonar systems that converts waterborne sound waves into equivalent electronic waves for analysis

PASSIVE SONAR: a sonar system designed for nonactive listening; only sounds generated by sonar targets themselves can be received for analysis

RESOLUTION: the ability of sonar equipment to distinguish between two distinct but closely spaced submerged objects; not synonymous with detectability, the basic ability of sonar to "see" objects

REVERBERATION: a class of unwanted extraneous underwater background noise from either natural or artificial sources that confuses active sonar

SIDE-SCAN SONAR: a type of sonar technology that allows simulated three-dimensional images of underwater objects to be constructed graphically by computer equipment

SONAR: an underwater sensing technology, originally designed for military purposes, that uses sound pulses in water; name derived from SOund Navigation And Ranging

TRANSDUCER: any device or instrument that converts one type of energy into another

WAVE PATH: a natural path that serves to channel sound underwater, caused by, among other factors, water masses or layers having different temperatures or salinities

WAVE PROPAGATION: the various physical properties of movement that waveform phenomena possess

Bibliography

Albers, Vernon M. Underwater Acoustics Handbook. Lancaster: Intelligencer, 1960. Print. A very thorough but highly technical treatment of the entire field of sonar technology and the physics behind it. An extremely detailed book that includes not only acoustic but also electronic theory. Describes the various electronic and mechanical hardware involved in sonar devices. Intended for readers at the college level or above with a strong grasp of physics, mathematics, and technology.

Albers, Vernon M, ed. Underwater Sound. Stroudsburg: Dowden, 1972. Print. A classic reference work on the subjects of submarine acoustics, sonar theory, and hardware. A collection of highly influential articles written by authorities on the subject and spanning several decades. Numerous equations and diagrams explain the intricacies of sonar and underwater acoustics and the various problems faced in the application of the technology. Suitable reading for college students and above with a very solid foundation in physics and mathematics. The formal introduction to each article serves as a readable synopsis of aspects of sonar for all readers.

Carlin, Benson. Ultrasonics. New York: McGraw, 1949. Print. Serves as a useful overview of the field of practical ultrasonic applications, including sonar. Chapter 7, "Pulsed Ultrasonic Systems," contains an illuminating discussion of sonar technology and theory. A useful book on the subject of sonar and related technologies for all readers at an advanced high school level or above with a good foundation in physical science.

Cheeke, J. David N. Fundamentals and Applications of Ultrasonic Waves. 2nd ed. Boca Raton: CRC, 2012. Print.

Cox, Albert W. Sonar and Underwater Sound. Lexington: Lexington, 1974. Print. Highly informative book that explains sonar technology use in great detail. The perspective is that of military users, in either surface or subsurface vessels, having submarine or antisubmarine tactical roles. All interested readers, civilian or military, can gain a wealth of useful knowledge on the subject, as Cox presents both theoretical and applied aspects in an extremely lucid, logical manner. Many well-drawn diagrams and graphs enhance the text.

Everest, F. Alton, and Ken C. Pohlmann. Master Handbook of Acoustics. 5th ed. New York: McGraw, 2009. Print.

Excell, Jon. "Sound Ideas." Engineer 23 Apr. 2007: 22–25. Print.

Kock, Winston E. Sound Waves and Light Waves. Garden City: Doubleday, 1965. Print. A highly readable, instructive, and enjoyable treatment of the physics of wave phenomena and theory. Kock uses visible wave phenomena such as light to illustrate the principles behind technical, nonvisible phenomena such as audible and nonaudible sound. Chapter 7, "Other Sound Patterns," discusses submarine sound sources, both artificial and natural, as well as sonar. Recommended for readers at the high school level and above.

Lawler, Ryan. "Sonic Notify Launches Platform for Broadcasters and Retailers to Connect with Mobile Apps through Audio Signals." TechCrunch. AOL, 11 Feb. 2013. Web. 16 Dec. 2013.

Li, Qihu. Digital Sonar Design in Underwater Acoustics: Principles and Applications. Hangzhou: Zhejiang UP, 2012. Print.

Macaulay, David. The Way Things Work. Boston: Houghton, 1988. Print. An easy-to-understand treatment of a wide spectrum of electronic and mechanical devices, including the history and evolution of each type of underlying technology. Excellent, original explanatory drawings illustrate the concepts and hardware involved. Sections on sonar and related types of equipment are of interest. A good basic introductory book for readers without any foundation in the physical sciences.

Tucker, D. G. Underwater Observation Using Sonar. London: Fishing News, 1966. Print. A good basic introduction to the fundamental principles of sonar systems and their applications, both civilian and military. Numerous diagrams explain complex concepts such as acoustic waves, beams, and echoes. Tucker compares sonar use to older, conventional submarine observational techniques. Useful informational source for high school readers and above.

Producing and Detecting Sound