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

Highlights

• A novel thermophone is designed based on 3D CNT sponge with extremely low heat capacity per unit volume.

• Sound response characteristics of CNT sponge thermophone are revealed.

• A flexible thermophone is developed for attachable and wearable acoustic device.

• A wide-band and high-fidelity thermoacoustic loudspeaker is fabricated.

• A thermoacoustic model for 3D thermophone is proposed and analytical solutions are obtained.

Abstract

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.

Introduction

With the recently fast development of nanotechnology and nanomaterials, thermophones are found as new and efficient sound generation elements and they are promising application candidates for high-power sonar arrays, flexible loudspeakers, and noise cancellation devices [1]. Unlike conventional resonant sound emitters such as voice coil speakers and piezoelectric (PZT) transducers which excite sound by mechanical vibration, thermophones generate sound waves based on thermoacoustic effect and they have no vibrating components and no vibration mechanism [2] When a thermophone is connected with an alternating current, temperature oscillations will occur within the heating element. The thermophone in turn heats up its surrounding medium, which results in periodic air volume expansion and subsequent sound wave generation [3]. Due to their non-resonant characteristics, thermophones usually show a wide bandwidth and stable sound response [4]. Furthermore, thermophones can generate sound wave without any matching layers or magnets, therefore they usually have the advantages of simple structure and low production cost [5]. To achieve high thermoacoustic efficiency, thermoacoustic elements with extremely low heat capacity per unit area (HCPUA) are usually chosen for making thermophones, such as nanowires [6], porous silicon [7], graphene [8], and carbon nanotube (CNT) films [9]. Compared with other thermoacoustic materials, CNT-based thermophones usually work most efficiently and they usually are able to generate the highest sound pressure level [10]. In addition, CNT-based thermophone also has many other advantages, they are free-standing [11], flexible [5] and transparent [12]. Thus such superior characteristics make them the most fascinating thermoacoustic generator today.

In recent decades, the thermoacoustic effect of CNT-based thermophone has attracted extensive attention and interest from many researchers. Xiao et al. [9] successfully drew a kind of super-aligned CNT thin films directly out of CNT arrays and found that the CNT thin film can generate loud sound when it is applied with sound frequency electric currents. Aliev et al. [2], [12] performed a series of experiments to study acoustic characteristics of CNT aerogel sheets. They found that CNT sheets work at low frequency much more efficiently than the classical PZT transducer. They also reported that the sound generation efficiency could be greatly improved by encapsulating CNT sheets in inert gases. Suzuki et al. [13] conducted a comparison study of sound pressure response of CNT thin films and CNT webs, and they concluded that CNT web exhibits the higher thermoacoustic efficiency due to its high heat-diffusion structures. Using an experimental approach, Wei et al. [14] studied thermoacoustic effects of CNT thin yarn arrays and they successfully fabricated CNT earphones. Barnard et al. [15] discussed the feasibility of a high-powered audio CNT speaker, and they studied the thermoacoustic effect of stacked films. Lim and Tong presented a series of rigorous theoretical models and they obtained sound pressure solutions for suspended CNT thin film [16], CNT thin film encapsulated in inert gas [17], CNT thin film suspended on an insulator substrate [18], curled cylindrical shapes [19], and carbon nanotube opto-acoustic lens [20]. Based on the theoretical models of Lim et al. [16], Mao et al. further considered the viscidity of surrounding medium [21] and applied static [22] and periodic [23] magnetic fields in their thermoacoustic model. The corresponding analytical solutions were also derived and reported [21], [22], [23]. Zhou et al. investigated the acoustic response characterization of a multi-layer carbon nanotube thin film transducer [24] and a planar CNT thin film array [25].

In view of the literature survey reported above, it is concluded that most of the researches was focused on thermoacoustic effects of two-dimensional (2D) CNT-based thermophones. However, the 2D CNT-based thermophones face two major problems in application: (i) 2D CNT-based thermophones usually cannot be self-supporting and they need to be placed on electrodes or substrate in order to remain planar [2]; (ii) 2D CNT-based thermophones are easily damaged due to their low mechanical strength through-thickness [2]. Recently, a new kind of three-dimensional (3D) CNT-based materials with porous structure (CNT sponge) was fabricated. 3D CNT sponge attracted the interest of many researchers because of its excellent self-supporting performance [26], high mechanical stability [27], and high-energy conversion efficiency. Obviously, it is an excellent alternative material to overcome the problems faced by 2D CNT-based thermophones. From the literature survey reported above, it is found that the study on the thermoacoustic effect of 3D CNT sponge thermophone is still very limited [10], [28], [29]. In theory, Guiraud [28], [29] firstly proposed two-temperature thermoacoustic models for the sound generation of porous 3D thermophones. In the experimental aspect, Alive¹⁰ measured the frequency response and sound pressure at different input power of CNT sponge. Because of scarcity in theoretical studies but high potential application values, therefore, it is of great importance to develop a theoretical model and to explore the applications of 3D CNT sponge thermophones.

In this paper, the hi-fi thermoacoustic effect of CNT sponge thermophones is investigated. A 3D CNT sponge thermophone is first presented and the corresponding analytical solutions of thermoacoustic fields are derived. Subsequently, the one-dimensional (1D) analytical solution of a point source is obtained first and then the 3D analytical solution is obtained by Rayleigh’s integral. The details of derivation are presented in the supplementary material. Next, the sound response characteristics of CNT sponge thermophone are analyzed by theory and experiment. Lastly, a flexible CNT sponge thermophone and wide-band hi-fi thermoacoustic loudspeaker are designed to demonstrate and promote the potential applications of CNT sponges.