Editing Sonar (section)

=== Later developments in transducers ===
Magnetostrictive transducers were pursued after World War II as an alternative to piezoelectric ones. Nickel scroll-wound ring transducers were used for high-power low-frequency operations, with size up to {{convert|13|ft}} in diameter, probably the largest individual sonar transducers ever. The advantage of metals is their high tensile strength and low input electrical impedance, but they have electrical losses and lower coupling coefficient than PZT, whose tensile strength can be increased by [[prestressing]]. Other materials were also tried; nonmetallic [[Ferrite (magnet)|ferrites]] were promising for their low electrical conductivity resulting in low [[eddy current]] losses, [[Metglas]] offered high coupling coefficient, but they were inferior to PZT overall. In the 1970s, compounds of [[rare earths]] and iron were discovered with superior magnetomechanic properties, namely the [[Terfenol-D]] alloy. This made possible new designs, e.g. a hybrid magnetostrictive-piezoelectric transducer. The most recent of these improved magnetostrictive materials is [[Galfenol]].

Other types of transducers include variable-reluctance (or moving-armature, or electromagnetic) transducers, where magnetic force acts on the surfaces of gaps, and moving coil (or electrodynamic) transducers, similar to conventional speakers; the latter are used in underwater sound calibration, due to their very low resonance frequencies and flat broadband characteristics above them.<ref>{{Cite journal |url=https://books.google.com/books?id=srREi-ScbFcC&pg=PA45 |title=Transducers and Arrays for Underwater Sound |journal=[[The Journal of the Acoustical Society of America]] |volume=124 |issue=3 |pages=1385 |url-status=live |archive-url=https://web.archive.org/web/20180426001103/https://books.google.com/books?id=srREi-ScbFcC&pg=PA45 |archive-date=2018-04-26 |bibcode=2008ASAJ..124.1385S |last1=Sherman |first1=Charles H |last2=Butler |first2=John L |last3=Brown |first3=David A |year=2008 |doi=10.1121/1.2956476 |isbn=9780387331393|doi-access=free }}</ref>