Thermoacoustic Loudspeakers Research Articles

A Large-Scale and Low-Cost Thermoacoustic Loudspeaker Based on Three-Dimensional Graphene Foam.

Graphene is a promising material for thermoacoustic sources due to its extremely low heat capacity per unit area and high thermal conductivity. However, current graphene thermoacoustic devices have limited device area and relatively high cost, which limit their applications of daily use. Here, we adopt a dip-coating method to fabricate a large-scale and cost-effective graphene sound source. This sound source has the three-dimensional (3D) porous structure that can increase the contact area between graphene and air, thus assisting heat to release into the air. In this method, polyurethane (PU) is used as a support, and graphene nanoplates are attached onto the PU skeleton so that a highly flexible graphene foam (GrF) device is obtained. At a measuring distance of 1 mm, it can emit sound at up to 70 dB under the normalized input power of 1 W. Considering its unique porous structure, we establish a thermoacoustic analysis model to simulate the acoustic performance of GrF. Furthermore, the obtained GrF can be made up to 44 in. (100 cm × 50 cm) in size, and it has good flexibility and processability, which broadens the application fields of GrF loudspeakers. It can be attached to the surfaces of objects with different shapes, making it suitable to be used as a large-area speaker in automobiles, houses, and other application scenarios, such as neck mounted speaker. In addition, it can also be widely used as a fully flexible in-ear earphone.

Shape-Configurable MXene-Based Thermoacoustic Loudspeakers with Tunable Sound Directivity.

Film-type shape-configurable speakers with tunable sound directivity are in high demand for wearable electronics. Flexible, thin thermoacoustic (TA) loudspeakers-which are free from bulky vibrating diaphragms-show promise in this regard. However, configuring thin TA loudspeakers into arbitrary shapes is challenging because of their low sound pressure level (SPL) under mechanical deformations and low conformability to other surfaces. By carefully controlling the heat-capacity-per-unit-area and thermal effusivity of an MXene conductor and substrates, respectively, we fabricated an ultrathin MXene-based TA loudspeaker exhibiting high SPL output (74.5dB at 15kHz) and stable sound performance for 14 days. Loudspeakers with the parylene substrate, whose thickness was less than the thermal penetration depth, generated bidirectional and deformation-independent sound in bent, twisted, cylindrical, and stretched-kirigami configurations. Furthermore, we constructed parabolic and spherical versions of ultrathin, large-area (20cm × 20cm) MXene-based TA loudspeakers, which displayed sound-focusing and 3D omnidirectional-sound-generating attributes, respectively. This article is protected by copyright. All rights reserved.

New high fidelity (hi-fi) three-dimensional thermophone CNT sponge

A novel, high-fidelity (hi-fi) thermophone based on three-dimensional porous CNT sponge is designed for attachable and wearable sound generation devices. In addition to its simple structure and easy fabrication, CNT sponge thermophone exhibits excellent thermoacoustic and mechanical characteristics, particularly its wideband frequency response, due to its highly efficient energy conversion capability through the CNT nano-structural materials. In this paper, the hi-fi sound response of suspended CNT sponge thermophones is analyzed by theory and experiment. A theoretical thermoacoustic model for CNT sponge thermophone is proposed and the corresponding analytical solutions are established. To explore applications of CNT sponges, a flexible thermophone and a hi-fi thermoacoustic loudspeaker are developed and investigated by experiment. The flexible CNT sponge thermophone presents stable sound response before and after it is attached to some complicated surfaces. The CNT sponge thermoacoustic loudspeaker shows a wide bandwidth, high-quality audio output, and stable sound response. Such premium characteristics ensure its feasibility as a potential replacement for the conventional voice coil and piezoelectric loudspeakers.

Highly Transparent, Flexible, and Self-Healable Thermoacoustic Loudspeakers.

Thermoacoustic (TA) loudspeakers have garnered significant attention in recent times as a novel film speaker that utilizes temperature oscillation to vibrate the surrounding air. Conventional film-type TA loudspeakers are known to experience problems when external environments damage their conductive networks, causing them to malfunction. Therefore, introducing self-healing polymers in TA loudspeakers could be an effective way to restore the surface damage of conductive networks. In this study, we present transparent, flexible, and self-healable TA loudspeakers based on silver nanowire (AgNW)-poly(urethane-hindered urea) (PUHU) conductive electrodes. Our self-healable AgNW/PUHU electrodes exhibit significant self-healing for repairing the surface damages that are caused due to the dynamic reconstruction of reversible bulky urea bonds in PUHU. The fabricated self-healable TA loudspeakers generate a sound pressure level of 61 dB at 10 kHz frequency (alternating current (AC) 7 V/direct current (DC) 1 V). In particular, the TA speakers are able to recover the original sound after healing the surface damages of electrodes at 95 °C and 80% relative humidity within 5 min. We believe that the technique proposed in this study provides a robust and powerful platform for the fabrication of transparent and flexible TA loudspeakers with excellent self-healing, which can be applied in flexible and wearable acoustic electronics.

Frequency response calculations of carbon nanotube based nanothermophones

Due to the extraordinary high thermal conductivity and low heat capacity, carbon nanotube (CNT) has shown a great potential as a thermo-acoustic (TA) material recently. It can be used to fabricate novel TA loudspeaker free of magnets and moving parts, a promising device that directly converts thermal energy into acoustic energy. In this paper, we have systemically analysed the frequency responses of the dot/wire/film TA emission from CNTs by using the equations we derived for acoustic field of TA emission from point source and arbitrary source. Formulas used for TA calculation from near-field to far-field can be the same. The dependence of sound pressure level (SPL)-frequcecy response of all kinds of CNT based nanothermophones on distance and angle are investigated in detail. It is found that TA sound pressure fluctuates in near-field, while increases linearly with logarithmic frequency in far-field. The amplitude of near-field fluctuation increases with increasing distance, but the frequency cycle of the fluctuation decreases and tends to be a constant depending on angle. These studies lay the foundation for further development of CNT based nanothermophones.

Linearization of thermoacoustic loudspeakers by adaptive predistortion

In this work we present a novel driving technique for thermoacoustic (TA) loudspeakers. The proposed technique allows linearizing the pressure response of TA loudspeakers while reducing the average power dissipation on the device, and thus its working temperature. This is achieved exploiting an adaptive predistortion algorithm, implemented through digital signal processing. The controlled TA loudspeakers show exceptionally low values of total harmonic distortion and intermodulation distortion in their pressure response, exceeding the performance of previously proposed techniques, and a significantly reduced working temperature.

Study on the thermo-acoustic emission from a carbon nanotube stripe

Carbon nanotube (CNT) was demonstrated to show a great potential as a thermoacoustic (TA) material. It can be used to fabricate novel TA loudspeaker free of magnets and moving parts, a promising device that directly converts thermal energy into acoustic energy. In this paper, the characteristics of TA emission from a CNT stripe are explored by means of parametric analysis. The impacts of the length of stripe, detecting distance and detecting angle on the sound pressure level (SPL) of emitted TA sound are discussed seperately. In addition, the directivity of TA emission from stripe is also investigated. Hence, some phenomena and principles are found and summarized, which would be of guiding significance for the further invegation and development of nanothermophones.

Fabrication and investigation of a thermoacoustic loudspeaker based on carbon nanotube coated laser-scribed graphene

In spite of being known since a century ago, high operational voltage and low sound pressure level have limited wide applications of thermophones. In recent years, utilizing graphene in thermophones has been proposed as an attractive solution, which is benefiting from high electrical and thermal conductivity and inherent high surface area, to enhance the heat transfer and lowering the required electrical input power. In this paper, a laser-scribed graphene (LSG) layer is utilized to realize graphene-based thermophones, which provide a considerably high sound pressure level of about 80 dB (Pin = 0.6 W). Furthermore, a layer of carbon nanotubes (CNTs) is incorporated on the prepared LSG sheets, which benefits from both randomly oriented CNTs over the surface of reduced graphene oxide (rGO) sheets and suspended CNTs, which act like heat exchanger nano-fins on rGO sheets and efficient heat generators, respectively. The input voltage for CNT/LSG based thermophones is measured 7 times lower than the prepared LSG samples, in order to generate the same sound pressure level (75 dB). The achieved sound pressure, normalized to the input power, has increased about 80 dB by incorporating CNTs which entitles the proposed structure as a promising candidate for novel thermophones.

On the Frequency Response of Nanostructured Thermoacoustic Loudspeakers.

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.

Solution-Processing of Copper Nanowires for Transparent Heaters and Thermo-Acoustic Loudspeakers

In this study, we present a copper nanowires (CuNWs) based spray deposition process for the fabrication of transparent heaters and thermo-acoustic loudspeakers. We developed a scalable and solution-based synthesis process for CuNWs, which allows to fabricate spray deposited transparent electrodes that show performances comparable to indium tin oxide based TEs, at much lower material and deposition costs. Without any post-processing, the CuNWs films exhibit a sheet resistance as low as 12.6 ${\Omega}/\square $ at a high transparency of 77%. CuNW-based transparent heaters and thermo-acoustic loudspeakers are accurately characterized and modeled in both the thermal and the acoustic domain, showing performances aligned with the state-of-the art.

High Efficiency Thermoacoustic Loudspeaker Made with a Silica Aerogel Substrate

AbstractThe extremely low thermal effusivity of the silica aerogel is exploited to develop a high efficiency thermoacoustic (TA) loudspeaker with solid substrate. The deposition of the electrically conductive, low heat capacity active film on the silica aerogel surface is achieved with both the spray coating of silver nanowires and the sputter coating of gold films. The uniform spray coating of the hydrophobic silica aerogel is enabled by a low pressure plasma treatment, which however impairs its robustness. The spray‐coated samples prove to be fragile when subjected to elevated temperatures and thus not suitable for TA applications. Sputter coating, not requiring any treatment of the aerogel surface, allows the fabrication of working TA loudspeaker samples with a 100 nm gold active film. The electroacoustic response of the gold‐sputtered silica aerogel TA loudspeaker is characterized at different input power levels. The experimental results are compared with those present in literature, showing an improved efficiency with respect to the other TA loudspeakers with solid substrate.

Construction of a Multiway Carbon Nanotube Loudspeaker with Finely Tunable Resonance Frequencies

AbstractThe sound pressure generated by thermo‐acoustic (TA) loudspeakers is proportional to the frequency of the sound. This characteristic yields a low sound pressure level (SPL) in the low‐frequency region, hindering the practical application of TA loudspeakers. The limitation of low SPL is approached by using a resonating structure, in which a TA sound generator, such as multiwalled carbon nanotube sheet, is enclosed within a cavity covered by a vibrating plate. However, the cavity‐plate resonator produces an amplified SPL only around the resonating frequency, so its operating frequency bandwidth is narrow. Hence, a multiway TA loudspeaker consisting of multiple resonators that can amplify sound over a wide frequency range is designed. An accurate mechanical model is presented for predicting the resonance frequency and it shows that the behavior of resonators designed from the proposed model matches the theory well. The model enables the fine‐tuning of resonances through the application of different materials for a resonator plate and by changing the structural dimensions. A crossover network for combining multiple TA resonators is designed, and the total frequency response shows a flat frequency response with amplified SPL. These improvements open up possibilities for the use of TA loudspeakers as a practical audio device.

Influential factors on thermoacoustic efficiency of multilayered graphene film loudspeakers for optimal design

Graphene thermoacoustic loudspeakers, composed of a graphene film on a substrate, generate sound with heat. Improving thermoacoustic efficiency of graphene speakers is a goal for optimal design. In this work, we first modified the existing TA model with respect to small thermal wavelengths, and then built an acoustic platform for model validation. Additionally, sensitivity analyses for influential factors on thermoacoustic efficiency were performed, including the thickness of multilayered graphene films, the thermal effusivity of substrates, and the characteristics of inserted gases. The higher sensitivity coefficients result in the stronger effects on thermoacoustic efficiency. We find that the thickness (5 nm–15 nm) of graphene films plays a trivial role in efficiency, resulting in the sensitivity coefficient less than 0.02. The substrate thermal effusivity, however, has significant effects on efficiency, with the sensitivity coefficient around 1.7. Moreover, substrates with a lower thermal effusivity show better acoustic performances. For influences of ambient gases, the sensitivity coefficients of density ρg, thermal conductivity κg, and specific heat cp,g are 2.7, 0.98, and 0.8, respectively. Furthermore, large magnitudes of both ρg and κg lead to a higher efficiency and the sound pressure level generated by graphene films is approximately proportional to the inverse of cp,g. These findings can refer to the optimal design for graphene thermoacoustic speakers.

Physical modeling and characterization of thermo-acoustic loudspeakers made of silver nano-wire films

Recent developments of ultra-low heat capacity nanostructured materials revived the interest in the thermo-acoustic (TA) loudspeaker technology, which shows important advantages compared to the classical dynamic loudspeakers as they feature a lower cost and weight, flexibility, conformability to the surface of various shapes, and transparency. The development of the TA loudspeaker technology requires accurate physical models connecting the material properties to the thermal and acoustic speaker's performance. We present here a combined theoretical and experimental analysis of TA loudspeakers, where the electro-thermal and the thermo-acoustic transductions are handled separately, thus allowing an in-depth description of both the pressure and temperature dynamics. The electro-thermal transduction is analyzed by accounting for all the heat flow processes taking place between the TA loudspeaker and the surrounding environment, with focus on their frequency dependence. The thermo-acoustic conversion is studied by solving the coupled thermo-acoustic equations, derived from the Navier-Stokes equations, and by exploiting the Huygens-Fresnel principle to decompose the TA loudspeaker surface into a dense set of TA point sources. A general formulation of the 3D pressure field is derived summing up the TA point source contributions via a Rayleigh integral. The model is validated against temperature and sound pressure level measured on the TA loudspeaker sample made of a Silver Nanowire random network deposited on a polyimide substrate. A good agreement is found between measurements and simulations, demonstrating that the model is capable of connecting material properties to the thermo-acoustic performance of the device, thus providing a valuable tool for the design and optimization of TA loudspeakers.

Ultrathin thermoacoustic nanobridge loudspeakers from ALD on polyimide

The recent development of low-temperature (<200 °C) atomic layer deposition (ALD) for fabrication of freestanding nanostructures has enabled consideration of active device design based on engineered ultrathin films. This paper explores audible sound production from thermoacoustic loudspeakers fabricated from suspended tungsten nanobridges formed by ALD. Additionally, this paper develops an approach to lumped-element modeling for design of thermoacoustic nanodevices and relates the near-field plane wave model of individual transducer beams to the far-field spherical wave sound pressure that can be measured with standard experimental techniques. Arrays of suspended nanobridges with 25.8 nm thickness and sizes as small as 17 μm × 2 μm have been fabricated and demonstrated to produce audible sound using the thermoacoustic effect. The nanobridges were fabricated by ALD of 6.5 nm Al2O3 and 19.3 nm tungsten on sacrificial polyimide, with ALD performed at 130 °C and patterned by standard photolithography. The maximum observed loudspeaker sound pressure level (SPL) is 104 dB, measured at 20 kHz, 9.71 W input power, and 1 cm measurement distance, providing a loudspeaker sensitivity value of ∼64.6 dB SPL/1 mW. Sound production efficiency was measured to vary proportional to frequency f 3 and was directly proportional to input power. The devices in this paper demonstrate industrially feasible nanofabrication of thermoacoustic transducers and a sound production mechanism pertinent to submicron-scale device engineering.

Directivity control of a graphene loudspeaker array using elementwise switching technique

Graphene is a promising material for thermoacoustic transducers because of its low heat capacity per unit area and easy fabrication process. In contrast to the conventional voice-coil loudspeaker, a small thermoacoustic loudspeaker can produce omnidirectional radiation even without enclosures. In addition, the sound level from a thermoacoustic loudspeaker is less sensitive to its radiating surface area, which makes it an ideal omnidirectional sound source in high frequency region. In this work, we present an elementwise switching control of a graphene loudspeaker array utilizing the advantages of thermoacoustic loudspeakers. With an array of 3D graphene transducers, a directivity is controlled by polarity switching of each element according to a pre-determined pattern. A single channel input signal is modulated into two different channel signals using a polarity inverter. Each graphene element is driven by the same signal with different sign, and the polarization pattern of the array forms a designated beam pattern. The proposed technique simplifies the control of beam patterns without using large number of independent channels, and hence, enables the construction of a high frequency driver with variable-directivity.

Multi-physics modeling of conformal, solid-state, and thin-film thermophones

Thin-film thermophones are thermoacoustic loudspeakers made from new materials such as aligned carbon nanotubes or graphene. They are solid state devices, in that there are no moving parts associated with the sound generation mechanism. Lumped parameter models have been developed for planar thermophones and agree well with experimental results. One benefit of thin-film thermophones is that they can be designed such that the transducer can be conformal to complex geometries. However, the lumped parameter models are not appropriate for complex geometries or electrical/thermal transients. An electrical-thermal-acoustic multiphysics model of these transducers has been created using COMSOL Multiphysics to address the issues present with the lumped parameter models. Development and validation of the model will be discussed.

Application of N-Doped Three-Dimensional Reduced Graphene Oxide Aerogel to Thin Film Loudspeaker.

We built a thermoacoustic loudspeaker employing N-doped three-dimensional reduced graphene oxide aerogel (N-rGOA) based on a simple template-free fabrication method. A two-step fabrication process, which includes freeze-drying and reduction/doping, was used to realize a three-dimensional, freestanding, and porous graphene-based loudspeaker, whose macroscopic structure can be easily modulated. The simplified fabrication process also allows the control of structural properties of the N-rGOAs, including density and area. Taking advantage of the facile fabrication process, we fabricated and analyzed thermoacoustic loudspeakers with different structural properties. The anlayses showed that a N-rGOA with lower density and larger area can produce a higher sound pressure level (SPL). Furthermore, the resistance of the proposed loudspeaker can be easily controlled through heteroatom doping, thereby helping to generate higher SPL per unit driving voltage. Our success in constructing an array of optimized N-rGOAs able to withstand input power as high as 40 W demonstrates that a practical thermoacoustic loudspeaker can be fabricated using the proposed mass-producible solution-based process.

Free-Standing Graphene Thermophone on a Polymer-Mesh Substrate.

A graphene thermoacoustic loudspeaker with a thin polymer mesh is fabricated using screen-printing. An experiment with substrates of various free-standing areas shows that a higher sound pressure level can be achieved as compared to previously reported graphene thermoacoustic loudspeakers. Moreover, a modified equation to predict the sound pressure level of the thermoacoustic loudspeaker with a thin and patterned substrate is proposed and verified by experimental results.

Thermoacoustic Chips with Carbon Nanotube Thin Yarn Arrays

Aligned carbon nanotube (CNT) films drawn from CNT arrays have shown the potential as thermoacoustic loudspeakers. CNT thermoacoustic chips with robust structures are proposed to promote the applications. The silicon-based chips can play sound and fascinating rhythms by feeding alternating currents and audio signal to the suspending CNT thin yarn arrays across grooves in them. In additional to the thin yarns, experiments further revealed more essential elements of the chips, the groove depth and the interdigital electrodes. The sound pressure depends on the depth of the grooves, and the thermal wavelength can be introduced to define the influence-free depth. The interdigital fingers can effectively reduce the driving voltage, making the chips safe and easy to use. The chips were successfully assembled into earphones and have been working stably for about one year. The thermoacoustic chips can find many applications in consumer electronics and possibly improve the audiovisual experience.