Practical implementation of immersive gases for improved efficiency of thin-film carbon nanotube thermoacoustic loudspeakers

Researchers have recently been investigating the use of carbon nanotube (CNT) thin-films as thermoacoustic loudspeakers due to their light weight, solid state reliability, compact size, low cost and independence from rare earth materials. It has been shown that immersing CNT films in certain gases can increase the film's sound generation efficiency up to 15 dB relative to air. However, a number of challenges must be overcome in order to practically implement gaseous immersion of the CNT film in a loudspeaker. Most obviously, the CNT film must be contained within an enclosure in order to constrain the immersive gas. Past research investigated the response of CNT films to gaseous immersion by measuring the pressure response within the immersive gas itself. This paper presents measurements of the in-air radiated sound pressure and input power for prototype CNT loudspeakers where the CNT film was immersed in various gases in aluminum enclosures with a near acoustically transparent diaphragm forming one side of the enclosure. The measurements showed that gaseous immersion could not increase the CNT loudspeaker's sound generation efficiency more than 4 dB relative to air but it was capable of attenuating it up to 21 dB. A theoretical basis for these results is also discussed.