Electrodynamic loudspeakers use a small coil moving in permanent magnet to drive the large cone. Cones have three chief properties – being lightweight, stiff and having a lack of "colouration" – which makes it difficult to combine the matching of acoustical impedance with them. In a flat TA loudspeaker, resistively heated free-standing CNT sheets may be used as a nanoscale heater. The whole surface of the CNT heater drives the surrounding air or the vibrating membrane placed in close vicinity. Therefore, the problem of matching the acoustical impedance is less severe for an encapsulated TA loudspeaker.

Designing low-frequency transducers

On the other hand, TA transduction can provide an additional degree of freedom. This is through the separation of internal pressure-modulated forces from the peculiarities of vibrating plates interacting with an external medium. This is extremely important for the design of low-frequency transducers that work in air or underwater.

Understanding low-frequency operation

Combating this, the team of researchers from the University of Texas at Dallas, Virginia Tech and the Naval Undersea Warfare Center study the power limit of CNT sheets in different environments and under optimal thermal conditions for efficient TA transduction. The thermal management of a nanoscaled TA heater at high applied ac power is discussed and exploited to construct an efficient and tunable underwater sound projector. This operates at the relatively low frequencies of 10 Hz – 10 kHz.

The researchers find that in a vacuum and ultrahigh purity argon gas, the 1 cm wide single-layer CNT sheet withstands 22 W/cm2 and 28 W/cm2 input electrical powers, respectively. They control the heat dissipation from the interior of the encapsulated device and match the acoustical impedance. From this, the researchers are able to design a TA projector radiating 115 dB (re 20 µPa) of sound pressure level at ƒ=1.5 kHz with an energy conversion efficiency of 2%.

The researchers presented their work in the journal Nanotechnology 25 405704.

Further reading

Encapsulation boosts output from carbon nanotube thermoacoustic sound projector (June 2013)
CNT networks make better flexible supercapacitors (dec 2013)
Graphene-based remote controlled actuators put to the test (Jan 2012)