On the Frequency Response of Nanostructured Thermoacoustic Loudspeakers

Abstract

In this work, we investigate the thermal and acoustic frequency responses of nanostructured thermoacoustic loudspeakers. An opposite frequency dependence of thermal and acoustic responses was found independently of the device substrate (Kapton and glass) and the nanometric active film (silver nanowires and nm-thick metal films). The experimental results are interpreted with the support of a comprehensive electro-thermo-acoustic model, allowing for the separation of the purely thermal effects from the proper thermoacoustic (TA) transduction. The thermal interactions causing the reported opposite trends are understood, providing useful insights for the further development of the TA loudspeaker technology.

High Efficiency Thermoacoustic Loudspeaker Made with a Silica Aerogel Substrate

“…Historically, the optimization of the TA loudspeaker technology focused primarily on the reduction of the heat capacity per unit area of the active film, widening the bandwidth of the TA transduction up to 100 kHz and above, well beyond the requirements of the audio applications. So far, the research for efficient active films led to the development of TA loudspeaker made of suspended CNTs, single‐layer and multilayer graphene, reduced graphene oxide, graphene oxide aerogels, suspended metal wires such as aluminum and gold, Indium‐Tin Oxide (ITO), poly(3,4‐ethylenedioxythiophene) polystyrene sulfonate (PEDOT:PSS), random networks of metal nanowires such as silver (AgNW) and copper (CuNW), and nanometric metal films of titanium and gold …”

“…Recent studies suggest that the TA loudspeaker technology could be further improved engineering the substrate (see Figure S1 in the Supporting Information). In fact, if fabricated using high thermal effusivity materials, such as glass or polymers, the substrate would drain most of the heat generated by the active film, reducing the heat injection into the air.…”

“…Below the high frequency cutoff, the generated sound pressure is determined by the effusivity ratio . To maximize the efficiency of the TA loudspeaker, the substrate material must have the lowest thermal effusivity possible.…”

“…Figure shows the effect of the thermal effusivity of the substrate on the theoretical sensitivity of a TA loudspeaker (dB SPL/W at 1 m) at different frequencies, including different materials for reference. The simulations are performed using the model proposed in, using the material parameters reported in Table S1 (Supporting Information). As shown in Figure , among the solid materials the aerogels, i.e., silica aerogel, and the polymeric foams, i.e., polyurethane foam, exhibit extremely low thermal effusivity values, making them good candidates for being used as efficient substrate materials.…”

“…The tested sample is depicted in Figure e. The resulting heat capacity per unit area of the 100 nm gold film is estimated as 251.2 10 −3 J m −2 K −1 , while its electrical resistance is 0.1 Ω.…”