“Nanotube speakers” made from carbon nanotube sheets have been found to be able can both generate sound and cancel out noise — properties ideal for submarine sonar to probe the ocean depths and make subs invisible to enemies, according to a report in ACS’ Nano Letters.

Ali Aliev of MacDiarmid NanoTech Institute, University of Texas at Dallas and colleagues explain that thermoacoustic sound generation results from temperature variation in a carbon nanotube sheet that is produced by heating the sheet using an applied alternating voltage. A hot CNT sheet heats up surrounding air in the loudspeaker application, thereby inducing volume expansion and subsequent pressure waves over a wide frequency range, 1−100,000 Hz. Chinese scientists first discovered that effect in 2008, and applied it in building flexible speakers.

Aliev’s group took the next step, showing that nanotube sheets produce the kind of low-frequency sound waves that enable sonar to determine the location, depth, and speed of underwater objects. They also verified that the speakers can be tuned to specific frequencies to cancel out noise, such as the sound of a submarine moving through the depths.

FULL TEXT ARTICLE: Underwater Sound Generation Using Carbon Nanotube Projectors, Ali E. Aliev, Marcio D. Lima, Shaoli Fang and Ray H. Baughman, Nano Lett., May 27, 2010