Page 49 - Winter2018

Nathanael Mayo
Address:
Naval Undersea Warfare Center Sensors and Sonar 1176 Howell Street Newport, Rhode Island 02841 USA
Email:
nathanael.mayo@navy.mil
Advancements in Thermophones: Sound Generation from Nanoscopic Heaters
Researchers adapt solid-state sound-generation techniques discovered shortly after the invention of the telephone.
Introduction
It is often stated that “nothing is new” to portray the idea that everything that we do and learn is, in one way or another, a recycled version of something that someone else has done before. I find this statement to be true everywhere I look, and in the instances in which it didn’t appear to be true, it was only because I hadn’t looked hard enough. Of course, there is nothing wrong with this. It seems only natural that we learn through imitation before we can then adapt what we’ve learned to something else by “standing on the shoulders of giants,” to borrow a metaphor. Even, at times, when our insights seem serendipitous or of our own accord, another has already come across the same concept before. Such was the case when, in 2008, a group from Tsinghua University in Beijing, China, passed an alternating electric current through a thin, transparent sheet of carbon nano- tubes (CNTs) and discovered that it produced sound (Barras, 2008). With a bit of investigation, they found that such a device, called a thermophone, had existed for over a century but that the modern nanomaterial had simply made it much more efficient than those of previous generations. In fact, there is currently an entire field of physics called thermoacoustics that is undergoing a revitalization and progressing because of advancements in modern technologies such as lasers, computing, and very large scale integration (VLSI) that enables the manufacture and patterning of nanoscale materials. Thermoacoustic research shows promise for new devices as well as alternative takes on existing devices. Such devices range from biomedical imaging tools and sonar transmitters to engines to refrigerators.
Thermoacoustics
Thermoacoustics, or study of the interaction between heat and sound, has been a curiosity of individuals long before the “self-explanatory” term was made popu- lar by Rott (1980). Recorded observations of sound generation by heat date as far back as 1568 when a Buddhist monk described tones generated by a ceremo- nial Japanese rice cooker (Noda and Ueda, 2013). A video demonstration using a handmade variant of these rice cookers is can be seen at bit.ly/2NNGCHv.
The discovery of “singing flames,” which is flame heat inducing air motion along tubes or jars to produce sound, is attributed to Higgins (1802). This article re- ported the effect, along with a letter from Higgins claiming the initial discovery in 1777, which had become somewhat of a novelty demonstration by his students (Higgins, 1802). The article describes placing the neck of various sized jars at some distance over a hydrogen gas flame that produced “several sweet tones.” Various publications surfaced attempting to explain the mechanism of sound production in terms of water vapor evaporation and condensation or a series of small com-
Winter 2018 | Acoustics Today | 47 https://doi.org/10.1121/AT.2018.14.4.49