As ultrasonics in general follows the principles delineated in acoustics, its development, particularly in the early years, is
to some extent embedded in the broad developments in acoustics. The study of acoustics probably had its beginning with the Greek philosopher Pythagorus (6th Century B.C.), whose experiments on the properties of vibrating strings were so popular that they led to a tuning system that bears his name (the Sonometer). Aristotle (4th century BC) assumed (correctly) that a sound wave resonates in air through motion of the air; a philosphy-based hypothesis more than one of experimental physics. Vitruvius (1st century BC), determined the correct mechanism for the movement of sound waves, and he contributed substantially to the acoustic design of theatres, because he was an architect. Boethius (6th century AD), the Roman philosopher, documented several ideas relating science to music, including a suggestion that the human perception of pitch is related to the physical property of frequency.
Galileo Galilei (1564-1642) is said to have started to modern studies of acoustics. He elevated the study of vibrations and the correlation between pitch and frequency of the sound source to scientific standards. His interest in sound was inspired in part by his father, who was a mathematician, musician, and composer. Following Galileo's foundation work, progress in acoustics came relatively quickly. The French mathematician Marin Mersenne studied the vibration of stretched strings; the results of these studies were summarized in the three Mersenne's laws. Mersenne's Harmonicorum Libri (1636) provided the basis for modern musical acoustics. Later in the century Robert Hooke, the English physicist, first produced a sound wave of known frequency, using a rotating cog wheel as a measuring device.
Further developed in the 19th century by the French physicist Félix Savart, and now commonly called Savart's disk, this device is often used today for demonstrations during physics lectures. In 1822, Swiss physicist Daniel Colladen used an underwater bell and successfully estimated the speed of sound in the waters of Lake Geneva. In the late 17th and early 18th centuries, detailed studies of the relationship between frequency and pitch and of waves in stretched strings were carried out by the French physicist Joseph Sauveur, who provided a legacy of acoustic terms used to this day and first suggested the name acoustics for the study of sound. In the early 1900's, lightships used a ranging system combination of an underwater gong and a foghorn on deck. The crew on approaching ships could hear both. The underwater sound was received by the use of a hydrophone on the hull. By timing the difference of the two sounds, they could determine their approximate distance from the lightship.
Investigations of high-frequency waves did not originate until the 19th century. The era of modern ultrasonics
started about 1917, with Langevin's use of high-frequency acoustic waves and quartz resonators
for submarine detection. Since that time, the field has grown enormously, with applications
found in science, industry, medicine and other areas.
Underwater detection systems were developed for the purpose of underwater navigation by submarines in World war I and in particular after the Titanic sank in 1912. Alexander Belm in Vienna, described an underwater echo-sounding device in the same year. Within a week of the Titanic tragedy, Lewis Richardson filed a patent with the British patent office for echo ranging with airborne sound, following a month later with a patent application for the underwater equivalent. The first functioning echo ranger, however, was patented in the United States in 1914 by Canadian Reginald A. Fessenden, who worked for the Submarine Signal Company. Fessenden's device was an electric oscillator that emitted a low-frequency noise and then switched to a receiver to listen for echoes; it was able to detect an iceberg underwater from 2 miles away, although it could not precisely determine its direction.
Sergei Y. Sokolov at the V.I. Ulyanov (Lenin) Electrotechnical Institute proposed in 1928, and a few years later demonstrated a through-transmission technique for flaw detection in metals. He advanced his idea in the late 1920s, at a time when the required technology did not exist. He proposed that such technique could be used to detect irreguarities in solids such as metals. The resolution of the experimental devices which he fabricated was however poor and could not be used at a practical level.
Sokolov subsequently described a different and obviously the more important concept in ultrasonic applications. He demonstrated that sound waves could be used as a new form of microscope, basing on a reflective principle. Sokolov recognized that a 'microscope' using sound waves with a frequency of 3,000 megahertz (MHz) would have a resolution equal to that of the optical microscope. It was nevertheless not until the late 1930s that the technology for such devices was progressively developed, and the high frequencies required for Sokolov's microscope are found in microwave and ultrasonic systems used for radar and underwater navigation.
In the reflection technique, a pulsed sound wave is transmitted from one side of the sample, reflected off the far side, and returned to a receiver located at the starting point. Upon impinging on a flaw or crack in the material, the signal is reflected and its traveling time altered. The actual delay becomes a measure of the flaw's location; a map of the material can be generated to illustrate the location and geometry of the flaws. In the through-transmission method, the transmitter and receiver are located on opposite sides of the material; interruptions in the passage of sound waves are used to locate and measure flaws. Usually a water medium is employed in which transmitter, sample, and receiver are immersed.
The equipment suggested by Sokolov which could generate very short pulses necessary to measure the brief propagation time of their returning echoes was not available until the 1940s. Industrial use of ultrasonic testing apparently started spontaneously at around similar times in the United States and Great Britain. Such technology had also been in place in Germany and Japan, but developments had been curtailed because of the second world war.
The key-persons, Floyd Firestone, Donald Sproule and Adolf Trost had no knowledge of each other as they worked strictly in secret. Not even their patent-applications were published. Sproule and Trost used transmission-technique with seperate transmitter- and receiver-probes. Trost invented the so-called "Trost-Tonge". The 2 probes were contacted on opposite sides of a plate, held in same axis by a mechanical device - the tonge - and coupled to both surfaces by continuously flowing water. Sproule placed the 2 probes on the same side of the workpiece. So he invented double-crystal probes. He used this combination also with variing distance from each other. Firestone was the first to realize the reflection-technique. He modified a radar instrument and developed a transmitter with short pulses and an amplifier with short dead-zone. Sproule eventually gave up the transmission method and filed a patent in 1952 entitled "the improvements in/ or relating to apparatus for flaw detection and velocity measurement by ultrasonic echo methods".
Firestone had made the following presentations and/or registered the following patents between 1940 and 1951:
- 1940 Flaw detecting device and measuring instrument
- 1943 Resonance inspection method; Surface and shear wave method and apparatus
- 1945 Supersonic reflectoscope, an instrument for inspecting the interior of solid parts by means of sound waves
- 1946 Refinements in supersonic reflectscope, Polarized sound
- 1948 Tricks with supersonic reflectscope
- 1951 Method and means for generating and utilizing vibrational wave in plate
Sproule and Firestone found industrial partners for their instruments: Kelvin-Hughes and Sperry Inc. Kelvin-Hughes produced their first commercial machine in 1952. Several years later, in Germany in 1949 two persons received information about the Firestone-Sperry-Reflectoscope by publications in technical papers: Josef Krautkrämer in Cologne and Karl Deutsch in Wuppertal. Both started developments - without knowledge of each other. Josef Krautkrämer and his brother Herbert were physicists, working in the field of oscilloscopes. They could develop ultrasonic instruments alone. Karl Deutsch, a mechanical engineer needed a partner for the electronics and found him with Hans-Werner Branscheid who had got some technical experience in radar-technique during the war. Within only one year both young and tiny companies could present their Ultrasonic testing-flaw-detectors, starting a competition still existing today.
