Favorite Scientists

Paul Langevin: The Originator of Ultrasonics 

William Cady claimed in 1946, that Langevin was ‘the originator of the modern science and art of ultrasonics’. If he were alive today, he would be amazed at the number of modern industries and applications which have been derived from his quartz piezoelectric ultrasound transducer.

When war broke out in 1914, he was a rising star of European physics, recognised for his theoretical work on  gaseous ion mobility, diamagnetism, relativity  and electron physics. At the urging of Marie Curie and Jean Perrin, Langevin directed his research towards  finding enemy submarines underwater with ultrasound.  After two years of initial work using a singing capacitor, he realised that a better practical solution was to use the piezoelectricity of natural quartz crystals, discovered by the Curie brothers  and with which he was familiar. Specifically, he selected a crystal orientation which aligned the electrical with the stress axes. In his patented ‘sandwich transducer’ (1918) he increased overall gain by replacing the half-wavelength slice of  X-cut quartz with a design that separated the electromechanical from the resonant properties, the latter being set by two quarter-wave steel plates,  Lowering the resonance frequency to about 40 kHz and increasing the diameter to be greater than ten wavelengths satisfied the objective of a directive beam (rather than other omnidirectional acoustic systems) He used high powered transmitters and stages of triode amplifiers to increase detection sensitivity. Langevin succeeded in generating a kilowatt of acoustic power which resulted in killing fish, one of the first observations of ultrasound-induced bioeffects. 

After the First World War, Langevin, maintained his interest in ultrasound until his anti-fascist activism limited his scientific work. In 1923, he presented an extensive course on ultrasonics at the Collége de France. He explored beam formation, including apodisation and dimensional asymmetry. Langevin’s work led directly to Wood and Loomis’ contributions on bioeffects, Boyle’s on cavitation, Florisson’s on commercial echo-sounding, Cady’s on frequency control. His team, including Japanese physicist Ishimoto, established the foundation for ultrasonic metrology, using piezoelectric probe hydrophones and radiation force. His work created the impetus for  magneto-strictive transducers and other piezoelectric materials for underwater applications. Nevertheless, quartz remained the transducer material of choice in ultrasound laboratories.

Much of what we take for granted in ultrasound, the use of piezoelectric  materials in transducers  in half wave resonances, directive  beams and pulse echo ranging and detection of distant or hidden objects, the characterization of  transducer fields,   started with Langevin’s ingenuity. The ubiquitous applications of the pulse echo principle to SONAR which was very successful at submarine detection in the second  world  war, to sound ranging, to diagnostic ultrasound,  and to nondestructive ultrasound testing of metals and structures sprung from his original pioneering work.  From F . A. Duck and  T.L. Szabo, Paul Langevin: Originator of  Ultrasonics , 2022  IEEE Ultrasonics Symposium, October 13, 2022.