Langevin continued his activities in ultrasonics well into the 1920s. He successfully patented his quartzUltrasonicsquartz transducer sandwich transducer in Europe, although failed in the USA as a result of prior patentsPatentUSA by NicolsonNicolson, Alexander McLean and PupinPupin, Michael, based on the trans-Atlantic wartime transfer of knowledge. Langevin remained an external advisor to the Naval laboratory in ToulonToulon, which focused attention on ultrasonic transducer design and measurement. He advised on hydrophone design, and a novel power meter with IshimotoIshimoto, Mishio, visiting from JapanJapan. He patented the “Langevin stackLangevin stack” transducer. He gave a course on ultrasonics at the Collège de FranceCollège de Francecourses atultrasonics in 1924. Driven in part by Chilowsky, echo sounding was developed commercially by SCAMSociété de condensation et d’applications mécaniques (SCAM), their two patents assigned under license. Langevin failed to gain significant remuneration on behalf of ChilowskyChilowsky, Constantin from the British Admiralty. The ultrasonic investigations of WoodWood, Robert, BoyleBoyle, Robert, BrillouinBrillouin, Léon and BiquardBiquard, Pierre were all inspired by Langevin.